Thermochemistry of organic and heteroorganic species. Part XIX. Structural

aspects and thermochemical approach to isomerization and fragmentation of

negative ions: Skeletal rearrangements, molecular ion and artifacts*

Dmitry Ponomarev a,*, Viatcheslav Takhistov b

a Department of Chemical Engineering, St Petersburg Forest Technical Academy, 194021 St Petersburg, Russian Federationb Center of Ecological Safety RAS, 197042 St Petersburg, Korpusnaja ul. 18, Russian Federation

Received 17 February 2005; received in revised form 1 July 2005; accepted 5 July 2005

Available online 17 November 2005

Abstract

Many hundreds of isomerization and fragmentation processes for negatively charged ions obtained in electron attachment dissociation,

collision induced dissociation and negative chemical ionization have been analyzed to find out certain regularities in gas-phase monomolecular

chemistry of negative ions like it was earlier performed for positive ions (Eur.J.Mass Spectrom., 8 (2002) 409). This analysis combined with

systematic application of the data on energies of the processes (appearance energies, isolation of stable resonance states on energetical scale in the

range 0–15 eV, enthalpies of formation of even negative ions and electron affinities of corresponding free radicals) allowed to come to quite

definite conclusion of validity of Eyring’s quasi-equilibrium theory (QET) to mass spectrometry of negative ions. However, it is valid only in the

limits of a stable resonance state rather than in a whole range of energies being applied. The validity of QET was illustrated by many diverse

simple bond cleavage and rearrangement processes.

Those hundreds of known processes for negative ions dissociation were reduced to a few isomerization and fragmentation types for odd-and

even-charged negative ions—eight types for simple bond cleavage and seven—for H-migration, all being in certain aspect common with those

suggested earlier for positive ions. The isomerization and fragmentation types were assigned to about 350 processes of hydrogen and skeletal

rearrangements with presentation of possible structures for isomeric molecular and fragment ions. Possibility of such specification of the processes

for negative ions, partial thermochemical approach for their description, general and specific application of QET make unnecessary the appeal to

participation of high-energetical electron excitation, nonergodic fragmentation, to any specifity in charge-remote processes—all these used in

literature to explain occurrence of some unusual processes. The latter could be described by the introduced reaction types or their combination, by

formation of many (instead of a single one) neutrals in one-step process, H-wandering over ionized benzene ring, ring opening as a routine event

(e.g. to explain the appearance of molecular ions in several domains of energies for aromatic compounds.).

q 2005 Elsevier B.V. All rights reserved.

Keywords: Negative ions; Fragmentation types; Thermochemistry; Artifacts

Introduction. In the first part of this work dedicated to mass

spectrometry of negative ions [1] we showed the availability of

quasi-equilibrium theory (QET) of Eyring et al. [2,3] to

monomolecular ion isomerization and fragmentation of

negative ions and introduced several types of simple bond

cleavage each of the types being characterized by its specific

way of stabilization of ionic and/or neutral fragments.

Similarly, for hydrogen rearrangements several types were

0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2005.07.035

* For the part XVIII, see Ref. [1].* Corresponding author.

E-mail addresses: juniper@mailbox.alkor.ru (D. Ponomarev), tahistov@

vt10515.spb.edu (V. Takhistov).

presented each of them being characterized by its specific

mechanism. Both kinds of processes, simple bond cleavage and

hydrogen rearrangements are based on thermochemical

grounds with application of literature and calculated ions’

enthalpies of formation, electron affinities (EAs) of corre-

sponding free radicals, molecules deprotonation enthalpies

DHacid, ions appearance energies (AEs), experimental maxima

of resonance states of ions in domains of their existence.

Here, we continue our thermochemical approach to

description of behavior of negative ions this time addressing

skeletal rearrangements, problems round existence and

stability of molecular ion and artifacts encountered in MS of

negative ions.

The following types of processes of simple bond cleavage

and hydrogen rearrangements were presented in the first part of

Journal of Molecular Structure 784 (2006) 198–213

www.elsevier.com/locate/molstruc

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 199

our work [1] which are also systematically used in present

work: A1-s-bond cleavage in a-position to an atom with high

EA value; A2-s-bond cleavage in b-position to an atom with

lone pair; A3-s-bond cleavage in g-position to an atom with

high EA value; Ap-s-bond cleavage with stabilization of the

negative charge through polarizability effect; Ap -rupture of

b-bond to p-system and resonance stabilization of the fragment

ion by p-system; App -rupture of b-bond to a system of

conjugated multiple bonds, including aromatic compounds;

Aar-cleavage of b-bond relative to p-system leading to

production of Huckel aromatic ions, like c–C5H5K; AR-type—

the formation of a stable free radical as a substantial part of the

driving force for formation of abundant fragment ions; B1-

type-b-bond cleavage to the free radical center; B2-type in

which free radical center is at atom with double bond, C-type-

bond rupture in a-(C1) and b-(C2) position relative to anionic

center with formation, respectively, stable carbene (:CX2)-like

species or neutral molecules; D-elimination of a two-atom

fragment from the ring; D1-type is a version of D-type that

takes place in ionized six-membered cycles containing a

double bond, it represents a retro-Diels-Alder reaction.

The following hydrogen rearrangement processes were

exercised in Ref. [1]: K-type: 1,2-H-migration; L-type: 1,3-H-

shift to free radical or neutral part or its interaction with

negative charge through four-membered transition state with

concerted a-bond cleavage; M-type: H-shift to ionized or

neutral multiple bond through six-membered transition state

with concerted b-bond cleavage: McLafferty rearrangement for

carbonyl compounds; N-type isomerization of a cyclic radical-

ion comprising two steps: (a) cleavage of a cycle’s bond,

leading to an isomeric distonic ion, followed by (b) H-shift

from the position adjacent to the anionic center to free radical

center (or vice versa); O-type: migration of two H-atoms to the

same function; P-type: neutral molecule HX elimination from

either odd or even-electron ions; R-type: non-specific

migrations including remote shift and randomization.

Skeletal rearrangements. For skeletal rearrangements in

positive ions MS earlier there were suggested the following

types [4–6]: SC-type (migration of an atom or a group to

cationic center); SR-type (migration to or interaction with free-

radical center); Sp-(interaction of two p-systems); SS-

(anchimeric assistance to bond rupture from p-system or

remote atoms and groups); ‘internal solvation’; S-type

rearrangements (these with unidentified or very complex

mechanisms).

With exception of SC-type which for negative ions is

replaced by SA-type (interaction with anionic center) all other

types are common for positive and negative ions. The skeletal

rearrangements with negative ions are discussed in many

publications [7,9–23,26,27,29–33,40,41,43–55,57–60,62,63,

68]. In our quotations we follow author’s schemes but in

some cases we introduce our own mechanisms (if they were not

given in the literature source) or change the authors’ schemes

usually giving the arguments for such correction. Of course, in

all cases we do not insist that only our schemes are correct

(without reserve). The reactions in mass-spectrometer are

extremely complex and it is very difficult (both for us and other

authors) to suggest the schemes especially in cases where the

ions’ AEs have not been measured.

We give here several reactions of certain groups (SA-type)

with anionic center (Eqs. (1)–(3)) [7–11] (M* represents quasi-

molecular ion generated in NICI and CID processes).

(1)

Ph2N– CK

HPh����/SA

Ph NK

CHPh2 (2)

(3)

Some interactions with anionic center have their analogy

with reactions in solutions like Wittig [9,12,13] (Eqs. (4) and

(5)), Smiles [14] (Eqs. (6) and (7)), Wolf [15] (Eqs. (8) and

(9)), Beckmann [16] (Eq. (10)), Claisen-rearrangement [8,17]

(Eq. (11)).

PhO CK

CPh2 ����/SA

PhCOK (4)

(5)

(6)

(7)

(8)

(9)

(10)

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213200

(11)

Hereafter, the processes comprising several types operating

in concerted mechanism are put in square brackets while those

without such brackets indicate several consecutive steps. The

notions like C(Ap) indicate the mixed type, i.e. several ways of

system’s stabilization.

Let us consider the SR-type process beginning with well

known nitro-nitrite rearrangement (Eqs. (12) and (13)) [18–19]

.

(12)

(13)

The elimination of %NO is an extremely favorable process

with AEs for [M–NO]Kion even lower than 0 eV (Eq. (13)).

Now we give a few examples of hidden fragmentations which

are preceded by SR-process (Eqs. (14) and (15)) [20,21] (the

value in brackets signifies the position of resonance on

energetical scale in which the ion’s abundance was measured).

(14)

(15)

Eq. (14) represents the rare process of elimination of neutral

carbenes. The ion [M–OMe]K (Ia) might have a high AE

owing to the presence of O-atom in adjacent position, while the

structure of rearranged, a very stable ion Ib fits its high

abundance with AE expected to be !0. An interesting

isomerization due to SR-process was suggested to take place

in M%K ion of 1-Cl-dibenzo-p-dioxin [22] (Eq. (16))

(16)

From analysis of literature mass spectrum of 2,4,6-

trinitrotoluene [7] we suggest the following isomerization

and fragmentation scheme (Eq. (17)). This comprises three SR-

processes: nitro-nitrite rearrangement and migration of CH3-

and HO- groups to free radical center on nitro-group:

(17)

Formation of 3,5-dinitrophenyl anion II could proceed with

formation of either %CH2–O–NO radical or [CH2aOC%NO]

pair. DH0f of the former w32 kcal molK1 was established from

known values for %CH2OH, CH3OH, CH3ONO species, K3.2,

K48.16, K15.84 respectively, and correction term wC2 kcal molK1 due to expected destabilization for H/NO

replacement [23] and K4.2 kcal molK1 for latter pair from

DH0f CH2aO and NO K26.0 and 21.8 kcal molK1, respect-

ively [24]. Thus, formation of two species instead of one

appeared to be energetically much more profitable. Isomeriza-

tion of the initial M0%K ion by R-and SR-types gives rise to

formation of three novel molecular ions M1%K–M3%K.

We also changed the scheme of Dua et al. (Eq. (18)) [25] in

favor of mechanism including SR-type fragmentation (Eq.

(19)). We add to Eq. (18) [25] the notions of our fragmentation

types

(18)

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 201

(19)

Dua et al. [24], by comparison of CID spectra of several

C6H9O2Kisomers, found that [M*-C3H6]Kion possesses the

structure of IIIc ion and not other structures including IIId

isomer. We calculated the enthalpies of formation for quasi-

molecular ion M* (III) and for three [M*-C3H6]Kisomers

[23]. We do not see any driving force for formation of IIIb

isomer with so high AE value while the isomer IIIc is by

w60 kcal molK1 more stable. EA of the free radical

corresponding to IIIb ion we estimated as 0.2 eV or less.

Moreover it is possible that its EA !0 (see Ref. [23]).

An abundant S%Kion is formed from thiophene at three

energies. We tried to restore the energetical picture of its

formation [23] (Eqs. (20)–(24)).

Only the experimentally found AEs of S%Kion and their

comparison with calculated values [23] revealed that the latter is

formed in three resonance states by diverse fragmentation

schemes (Eqs. (22)–(24)) one of them including SR-process. In

addition to [C4H4] neutral species presented in Eqs. (24–(28)

other possible combinations might be considered [H(CbC)2HCH2] (AEcalc4.4 eV); [HCbCC(%)aCH2CH%] (6.5 eV);

[HCbCCHaCH%CH%] (7.3); [%CbCCHaCH2CH%] (7.75);

[HCbC%CCH2aCH%] (7.95); [HCbC%CH%CC2H2] (9.53)

[23]. Although neither of these AEcalc fits the AEsexp values but

this becomes evident only after such calculations have been

performed. Still contribution of some of these high energetical

processes remains possible if the wide domains of existence for

some of the S%Kions are taken into consideration.

Sp-type skeletal rearrangement, representing the through-

space interaction between two p-systems, is very popular in

fragmentation of positive ions. Several examples are known for

negatively charged ions [12] (Eqs. (25) and (26)).

(25)

(26)

The driving force for such process is quite obvious:

formation of substituted aromatic cyclopentadienyl anions.

Earlier a different scheme for elimination of [C4H4] neutral was

suggested (Eq. (27)) (here we added the fragmentation types)

[8,26].

(27)

We do not insist on mechanism reproduced in Eq. (25), i.e.

the structure of [C4H4] is not necessarily to be two C2H2

molecules (see Eq. (24)). Another point is that the exact

structure of ion (IV or VI) and its neutral counterpart [C4H4]

could be found only from AE measurements. From the limited

information available we can argue that H% and H2 elimination

which gives very intense peaks and leads to very stable ions

(Eq. (26)) could hardly proceed from the ion like V. It is also

possible that the mechanism of C4H4 elimination can differ

depending on energy of CA-process.

Thus we think that scheme (Eq. (25)) is more reliable since

it finishes by formation of a very stable aromatic C9H7Kion (IV)

compared with its less stable isomer PhC(K)HCbCH (VI).

Moreover, in suggestion of the validity of Eq. (27) (without

participation of the second benzene ring) one would expect

formation of intermediate Dewar benzene (like V) in many

other compounds including substituted phenols or dibenzo-

dioxins. But, say, in dibenzodioxin and its monochloro (1-or

2-) derivatives the peak of [M-C4H4]%Kand [M-C4H3Cl]

%Kions was not registered [22]. Elimination of toluene and

xylene molecules from carotenoids was observed in their mass

spectra [27]. For these processes we apply the mechanisms

Table 1

Elimination of XY molecules in S-type process (values in brackets signify the

position of maximum of resonance state, in eV) [4,7,20,32,35]

Molecule Fragment

1. CF3COOMe [M-MeF]%K3.3% (0.8)

2. CF3COOCMe3 [M-Me3CF]%K 12% (1.5)

3. CF3COOCH2CbCH [M-FCH2CbCH]%K 16% (0.7)

4. CF3COOPh [M-PhF]%K 44% (0.9)

5. CF3COOSiMe3 [M-FSiMe3]%K100% (1.7)

6. C2F5COOMe [M-MeF]%K 15% (1.0)

7. C2F5COOCMe3 [M-Me3CF]%K 50% (1.1)

8. C2F5COOSiMe3 [M-FSiMe3]%K 100% (0.3)

[M-FSiMe3–CO2]%K 14% (0.9)

9. FCH2COOMe [M-MeF]%K 25% (1.2)

10. CF3CHFCOOMe [M-MeF]%K 0.28% (0.7)

11. Cl2CHCOOMe [[M-MeCl]%-100% (1.5)

12. ICH2COOMe [M-MeI] 39% (0.9)

13. m-C2F5PhCOOMe [M-MeF]%K 43% (2.8)

[M-MeCF3]%K 5.3% (4.5)

14. p-C2F5PhCOOMe [M-MeF]%K 43% (2.8)

15. p-C3F7PhCOOMe [M-MeF]%K 8.3% (2.3)

16. p-FCH2PhCOOMe [M-MeF]%K 74% (0.9)

17. p-Cl3CPhCOOMe [M-MeCl]%K 11% (0.6)

18. CF2Cl2 [FCl]%K; [Cl2]%K

19. F(CFaCF)2F [M-F2]%K

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213202

known for positive ions [4–6,20,28] (Eq. (28)).

(28)

[M-X]K ion (XZF, Cl, Br) appeared to be the most specific

ion in p-XC6H4NaNC6H5 (VII) molecules being registered in a

wide range of energies 3.3–5.5 eV (XZF), 4.1–8 eV (Cl) and

3.2–7.8 eV (Br). Only for XZF the AE1 corresponded to the

formation of KC6H4NaNC6H5 structure. The existence of [M-

X]K ion in wide range of energies indicated the involvement of

many other structures for these ions [29]. Isomerization and

fragmentation scheme was elaborated based on primary Sp-

process followed by a sequence of isomerization processes of the

initial molecular ions. The enthalpies of formation for 17

[C12H9N2]Kisomers were calculated. For [M-Cl]Kions the either

structures VIIa or VIIb best fit the AEexp but for [M-Br]Kions

such structure was not found. Many structures might contribute to

the process of X-elimination at higher than AEexp values [29].

The further opening of the second benzene ring might give the

structures of [M-X]Kions with higher enthalpies of formation

than that for VIIc ion ðDH0f 210 kcal molK1Þ with AEscalc 7.78

(XZF), 6.51 (Cl) and 5.92 eV (Br). The quoted work [29] is an

example of systematic calculation or estimation for DH0f values

of corresponding [C12H9N2]Kanions. Some of the latter values

were gained from estimation of the DHacid (deprotonation

enthalpies) values of [C12H9N2]-H molecules.

We give a few examples on SS-skeletal rearrangements [14,

30] (Eqs. (29) and (30).

(29)

The Ss mechanism is suggested in process of ROH elimination

from CH2aCHCH(K)(CH2)nCH2OR ions [10] and is rejected for

charge remote elimination of alkanes or water in anions of

carboxylic acids [9]. We suggest the following scheme for [M*-

H]%Kion formation from M* p-MeOC6H4CH(K)OCHO ion [9]

(Eq. (31)). This is a good example of free radicals (H% and Me%)

elimination from even-electron anions contrary to early

suggestions that such processes are more typical for odd-electron

anions [32].

(31)

To S-type isomerization and fragmentation we refer the

processes with very complex or unclear mechanism [8,20,33]

(Eqs. (37)–(41)).

(32)

(33)

Some of these processes can be united under the common

features that is elimination of stable molecules formed by

unknown mechanism through migration (formation of a bond) of

atom or group to another one, e.g. elimination of CF4, F2, F2CaO

(Table 1). In some aspect these have common features with

P-type rearrangement when H-X whereas in S-type X-Y

molecules are eliminated (Table 1). From our experience we

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 203

would stress that such processes are very rare in MS of positive

ions. The reasons of such drastic difference are not clear.

We specially selected for Table 1 the processes going on at low

energies to avoid the suspicion on possible formation of two

separate species XCY in successive elimination of each of them

instead of a single molecule XY. For example, M-[C2H6]%K ion

in p-MePhCOOMe appears at 8.0 eV (2.6%) [32] indicating

formation of two Me. free radicals rather than of C2H6 molecule.

Further, confirmation for formation of XY molecule rather than of

two radicals comes from the increase of abundance of

[M-XF]%Kion due to increase of F-affinity of the X-group,

compare XZMe, CMe3, Ph and especially SiMe3 (items 1, 2, 4, 5

or 6–8). We also stress that the enthalpy of formation of MeF is

K54.3 [1] while of [MeCF] is 35.1C19ZC54.1 kcal molK1

[4], i.e. DDH0f is 54.1K(K54.3)Z108.4 kcal molK1 or 4.7 eV.

Similar large differences in DH0f values are for other XY and

[XCY] species. Thus, in the processes occurring at low energies

(items 1–12, 15, 16) the neutral XY molecules are formed.

Certain increase in energy of resonance state may arise from the

difference in the enthalpies of formation of ‘isomeric’ distonic

radical-anions. For example, for para-isomers KC6H4COO%and.C6H4COOKspecies the calculated enthalpies of formation

are 12.8 and K37.7 kcal molK1, respectively, thus indicating that

the difference in their AEs will exhibit 50.5 kcal molK1 or 2.2 eV

[23]. So, we see that formation of XY molecules is energetically

the favorable process and their occurence could be treated as the

driving force of the process.

However, the question arises how this XY molecule

formation can occur in m-and p-substituted benzenes? Anyhow

the X- and Y-atoms and groups should appear in close vicinity

to each other to form a new X–Y bond. One possibility to do

this is to open the ring. We performed all necessary

calculations on example of p-FCH2PhCOOMe (Table 1, item

16) in which [M-MeF]%K exhibits an intense peak of 74% at

Emax 0.9 eV (Eq. (34)).

(34)

Using the additive scheme [4,34] we estimated the

enthalpy of formation of the parent molecule VIII

K115 kcal molK1 and the parent molecule for ion IX

[CH2aCHC(CH2F)aCHCHaCHCOOMe] as K99.3 kcal

molK1. Applying isodesmic reactions for free radicals

[35–38] we estimated the enthalpy of formation of

%CHaCHC(CH2F)aCHCHaC(%)COOMe biradical

11 kcal molK1 [from DHfo CH2aCH% 70 and CH2

aC(%)COOMe -25 (estimated from nCH in CH2aCH–H and

CH2aC(–H)COOMe [38])]. Estimating EA of the latter about

2.7 eV {from EA 0.667 and 1.096 eV for C2H3 and Ph radicals,

respectively, and some other data [39]} we finally obtained the

enthalpy of formation for IX -51 kcal molK1 [23]. Hereafter we

use Ph% free radical as a model for estimation of EA values of

conjugated free radicals of vinylic type. From known data on

the EAs values of substituted benzenes [28], e.g. 1.0 eV for

PhNO2 [39] and EN values 9.5 and 7.5 units for NO2 and

COOMe groups, respectively [35], we think that EA of

compound VIII is close to zero or is slightly negative. We

take the random value K0.2 eV while not expecting it to be

lower and thus the estimated enthalpy of formation for VIII%K

radical-anion is about K110 kcal molK1. Thus the enthalpy of

reaction (Eq. (42)) appears to be highly endothermic about

60 kcal molK1 or 2.6 eV and in no way the ion IX can be the

source of the ion [M-MeF]K.

Another way to let two groups FCH2 and Me to meet is the

migration of the latter to the benzene ring followed then by the

next Me-migrations along the ring to produce either of ions

VIIIc or VIIId which can equally be the precursors of the

fragment ion VIIIg.

(35)

The calculated enthalpies of formation for VIIIa, VIIIe, VIIIf

(with its EA to give VIIIa being about 3.4 eV like with EA for

PhCOO% radical [39]), VIIIg species are K116, K120.5, K38

and K54.5 kcal molK1, respectively. What is important that the

enthalpy of formation of isomer VIIIa (K116) is close to that of

the parent molecular ion VIII (K110 kcal molK1) (see above).

So, the scheme in Eq. (35) seems to be quite reasonable in both

energetical and mechanistic aspects and fits well the experimental

findings. The idea of substituent ‘wandering’ over the ring system

has been attracted earlier in mass spectrometry of positive ions

[4–6,20,28].

Following the results of elaboration of the mechanism of

S-type fragmentation for compound VIII we have revised the

original mechanism of Taylor et al. on fragmentation of

compound X. The authors [9] suggest the involvement of 1,2-

Wittig rearrangement with consequent formation of ion-neutral

complex, quasi-molecular ion Xa which, according to authors

statement, is the precursor of all fragment ions (Eq. (36)).

(36)

For example, the authors explain formation of [M*-

CH2O]K(CHDO, CD2O) ions of three diverse structures by

elimination of CH2aO from both 1-and 4-positions of benzene

ring from a single precursor Xa ion. We suggest another

scheme strictly following the experimental results of Taylor et

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213204

al. [9]. For example, since the benzene ring was not labeled by

deuterium we included its hydrogens in our scheme (Eqs. (37)

and (38).

(37)

Thus, we present four routes of CH2O molecule elimination

from M0* (X) ion with production of Xb-e ions while it is doubtful

to imagine that all these are formed from a single precursor Xa

with non-clear and not evident structure (Eq. (36)). To explain the

formation of [M*-CD3CHO]Kfragment we suggested CHO-

group migration from side chain to negative charge at the benzene

ring and its consequent migration over ring to encounter CD3-

group similar to the process in Eq. (35). While CH3-migration is a

routine event, migration of CHO-group established in Ref. [9] and

in this work is a novel and an interesting fact. Further support for

CH3CHO molecule instead of [CH3CCHO] formation comes

from comparison of the enthalpies of formation for the former

K39.7 and 35.1C10.1Z45.2 kcal molK1 for the latter pair, with

DDH0f 45.2K(K39.7)Z84.9 kcal molK1 or 3.68 eV [24] with

obvious priority for production of CH3CHO molecule. Since

many options could be suggested for [C2H2O2] species CHOCHO

ðDH0f K50:7Þ, 2CHO (20.2), [COCCH2O](K52.2), [HCCOC

CHO] (K0.1 kcal molK1) and others [24] the structure of

[C2H2O2] neutrals (Eq. (37)) could be established only by AE

measurements.

Some other rearrangement processes with unknown mech-

anism are common for negative ions MS in diverse classes of

organic compounds, this is production of H-bridged cluster

ions [32,33,36].

(38)

Finally, we suggest the fragmentation scheme for a very

simple compound furane XI which comprises the S-type

isomerization with final formation of C3H3K isomer XIa with

high enthalpy of formation instead of more stable isomers

either XIb or XIc [41] (Eq. (39)).

(39)

Molecular Ion. In positive ion MS the presence or the absence

and the abundance of M%C ions are directly governed by

energetics of the most energetically favorable simple bond

cleavage. If AEcalc for such process is smaller than IE of a

molecule then the peak of molecular ion is not registered (alcohols,

acetals, ketals, ortho-esters, carbohydrates and their acetoxy

derivatives, per-and polyhalogenated non-aromatic compounds

and individual compounds from other classes [4–6,28,36,42]). The

larger is EcrZAEKIE for the process with lowest AE the more

abundant is molecular ion. Chemical derivatization is quite

effective in increasing this Ecr value or to make it at least O0

and has been systematically applied in practice [4,28,42]. If the

peak of M%C has even a minor intensity at 70eV then reduction of

ionizing energy will increase its intensity [4,28,36].

Since, we insistently advocate the validity of QET to

isomerization and fragmentation processes of negative ions we

transferred some of the principles of stabilization or

destabilization of molecular ions from positive to negative

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 205

ions MS. The reader noticed that most of the molecules in the

schemes do not give detectable peak of molecular ion. The

single positive conclusion about the reasons why certain

compounds do not give molecular ions is that it happens with

compounds which have their EA values !0 like alkanes,

alkenes, benzene aromatics and many of their derivatives

excluding those comprising strong electron-withdrawing

substituents. If this is remembered then we can proceed to

possible explanation of the absence of molecular ions in

compounds possessing EAO0. Carette et al. claim that for

large polyatomic molecules, the excess of internal energy

(gained from reacting electron) in molecular anion can be

absorbed by a great number of vibrational modes resulting in

ions with long lifetime and thus being registered. Similarly,

transfer of electronic into vibrational energy can delay electron

autodetachment (see above) to an extent allowing a mass

spectrometric detection of the parent ion directly [43]. In a

plausible and straightforward picture the lifetime of the

transition negative ion should increase as the energy of the

primary electron decreases and as the number of vibrational

modes and hence the size of the molecule increases. Parent

anions are in fact are observed in large molecules at electron

energies near 0 eV, with exception of p-benzoquinone [44].

Similar conclusions were declared in Refs. [45,46].

We do not agree with this position. The peaks of molecular

ions have been registered in very small molecules (not only in

p-benzoquinone) like MeCOCOMe, HOOCCH2COOH,

MeNO2 (seven atoms), (NC)2CaC(CN)2, CF3CbCCF3 (98%

S!) [32] but are absent in very large molecules like

polyacetylated carbohydrates (we expect that these have

positive EA like other compounds comprising MeCO-group),

m-and p-(CF3)3CCH2PhCOOMe and many other compounds

[32] (see also text).

We think that the reason for absence of molecular ions is

their instability in relation to fragmentation (like in positive

ions–see above) when the ground state of a certain [fragment

ionCneutral] system lies below that of molecular ion, i.e. when

the latter being produced it immediately decomposes. Compare

the abundances of molecular ions of structurally close

compounds p-RPhCOOMe (XIIa-f) whose EA values are

obviously positive [32]Table 2

R CF3 XIIa C2F5 XIIb n-C3F7 XIIc

M%K 100(0) 89% S 68(0) 16.7% S 100 (0) 53.3% S

Base peak M%K C2F5C6H4K M%K

R (CF3)2CF XIId (CF3)3C XIIe (CF3)3CCH2

XIIf

M%K 2.5!10K3(0) 7!10K5(0) 0

Base peak [M-HF]%K 94.

6%S

[M-CF3]%K 99.

3%S

(CF3)3CK 98.

2% S

The extreme stability of fragment ions (CF3)3CK (in XIIf),

XIIg (together with its partner HF) and XIIh (in XIIe) causes

the practical absence of molecular anions in these compounds.

Moreover, the peaks of such low intensities as in these

compounds 10K3 or 10K4% are rarely paid attention and

registered. For positive ions we assumed involvement of non-

ergodic processes and other ways for violation of QET for ions

with abundances not more than 1–2% S [36]. Here we are

ready to admit that molecular ions in compounds XIId,e

(possibly but not necessarily) do not completely randomize the

excess energy.

Another example is the compounds XIIIa,b [32]. Compound

XIIIa gives a long-lived (see later) molecular ion with high

abundance whereas compound XIIIb—does not, although the

EA of the latter increases owing to introduction of strong

electron-withdrawing Cl-atom. We explain this fact by

appearance in compound XIIIb a new, energetically favorable

fragmentation channel with formation of two very stable

species ClK ion and XIIIc free radical. It proceeds in a

concerted A1 (ClK formation) and ring opening with cleavage

of the b-bond to free radical center (B1-type) formed after Cl-

loss. This b-bond cleavage is accompanied by production of a

very stable free radical XIIIc (AR-type) (Eq. (40)).

(40)

The way to avoid the loss of molecular ion if one or more

fragment ions have AEs close to zero or even negative ones is

to open the ring in cyclic compounds, i.e. to perform the

‘internal fragmentation’. For example, in compound XIVa in

the presence of a set of low energetical ions at 0 eV with loss of

NH2, Cl, HCl, CH2O, HBr, and [M-Cl]K and BrK (100% at

Emax 0.1 eV) ions the peak of molecular ion exhibits 7.3%. In

its isomer XIVb owing to intense formation of rearrangement

ion [M-HCl]K (100% or 84.2% S) the abundance of molecular

ion is only 0.5% or 0.41% S [47].

The absence of molecular ion in XV molecule despite the

expected high EA value (for example, EA of p-dinitrobenzene

is 2.0 eV [39]) can be explained by formation of several ions

with AE 0 eV: C2H2NOK, NO2K, NCOK, HC2OK and many

others due to loss of neutrals NO, 2NO, 2NOCH2, NO2CCO,

NO2CCO2 [23].

Another specifity arises with molecular negative ions being

considered is their unusually high abundance. Some molecules

practically do not fragment at all [4,20,32,48–50] e.g.

compounds XVI–XXIII. This rarely happens in MS of positive

ions.

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213206

The ion XVII%K4 PhC(OK)aC(O%)Ph has been known

since 1836. In this year, according to communication of Born et

al. [51] Laurent recognized this by the deep blue color that

appeared upon the addition of KOH to a solution of benzil

XVII (RZH) [52]. The idea of ring opening, which leads to

isomerization as a route to retain the molecular ion, comes

from our experience in positive ion MS. Owing to ring opening

the peak of molecular ion appears in mass spectra of many

natural products comprising HO–, HOO–, HOOC–, ketal-

moieties, tertiary groups with Br-atom and others [42]. One

might expect similar processes to occur in all cyclic (aromatic,

heteroaromatic) compounds while the molecular anions XIX

possess the essential potentialities for delocalization of

negative charge or to form several cyclic isomers incorporating

S and/or N atoms. The extremely high stability of molecular

ion for aliphatic compound XXII we explain by formation of

very stable ‘isomeric’ ions through p-bond cleavage and

stabilization of the negative charge by powerful electron-

withdrawing CF3-group in ions XXIIa,b. To add, one should

realize how much energy might be spent to ‘open’ triple bond.

A specific problem is how to explain the appearance of

several molecular ions on energetical scale in rather a wide

range of energies 0–3 eV or even higher [32,44,53]. Tobita et

al. give the following explanation of this fact [44]. The

resonances which are observed in the parent anion mass spectra

at energies higher than zero energies are attributed to processes

involving concomitant electronic excitation of M by a ‘fast’

electronic excitation and capture of the ‘thermolized’ electron

by the so-produced electronically excited molecule M*, viz.

Eq. (41):

eKðfastÞCM/M� CeKðthermalÞ/M�K (41)

This is called two-electron process of negative molecular ion

formation explaining, according to authors, the appearance of

two (in addition to that at zero energy) domains of existence of

molecular anion for acridine (XXIV), of three domains for

tetracene (XXV) and more than one for perylene (XXVI).

Actually, in the latter the large width of the parent ion

resonance indicates contribution of more than one resonance

(0–1.5 eV) [44]. Deconvolution technique by a nonlinear

algorithm, PeakFit, unfortunately being applied very rare,

permits to establish additional domains of existence of

molecular and fragment ions [7,53–55].

We have a different opinion on this occasion. Firstly, why

these events take place only in selected molecules producing

2–4 molecular anions while the most of the compounds giving

rise to molecular ion produce a single ion at low energy giving

very narrow peak. We paid attention, analysing data in Refs. [7,

32,44,53], that all the compounds giving molecular ion in two

or more regions possess the cyclic structure, e.g. XXIV–XXXII

[32,44,53].

Compounds XXX and XXXI produce four peaks of

molecular anions in the range of 0–4.2 (XXX) and 0–6 eV

(XXXI), some other compounds form stable molecular ions in

two domains: quinoline, 1,4-naphtoquinone, azulene, nickel-

and cobaltocene, anhydride of pyromellitic acid [32]. To our

knowledge, all acyclic compounds (with a single, to our

knowledge, exclusion—see later) among them CH2-

aCHCOOH, (HOOC)2CH2, HOOCCH2CH2COOH, MeNO2,

C(NO2)4, EF4 (S,Ge,Si), SF6 and many other fluorinated

elements, C4F10, CF3CbCCF3 [32] have a molecular anion

registered only in one domain. Thus, we suggest that the

bond(s) cleavage in ring system performs formation of

isomeric molecular anions. We performed analysis of

formation of isomeric molecular ions from anthraquinone

XXVIII, which produces them in three energetical regions [53].

We suggest formation of three isomers XXVIIIa,b,c species

(Eq. (42)).

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 207

The enthalpy of formation for parent molecule XXVIII

(K22.8) was taken from Ref. [56] and for that of parent

molecule for XXVIIIb,c,d species o-CHOPhCOPh molecule

21.5 kcal molK1 was calculated by additive scheme with

introduction of correction term about C5 kcal molK1

(repulsion of two CO-groups due to polar and steric

factors). The enthalpy of formation for biradical XXVIIId

120 was estimated by additive scheme from DH0f Ph% 78.5

[23], PhC(%)aO 26.1 kcal molK1 [4] and a summary

correction term of about C5 kcal molK1 for destabilization

of both free radical centers by electron-withdrawing CO-

group. The EA values for free radical centers in XXVIIId

biradical were estimated as 1.1 and 1.4eV for OaC. and C.

(benzene) centers, respectively [23]. This gives the

enthalpies of formation 94.5 and 62.5 kcal molK1 for ions

XXVIIIb and XXVIIIc, respectively. The DH0f K62.2 for

XXVIIIa ion was calculated from DH0f anthraquinone

XXVIII K22.8 kcal molK1 and its EA 1.7 eV extrapolated

from known EA values for 1,4-benzoquinone (1.91) and

1,4-naphtoquinone (1.81 eV) [39]. All that being performed

the AE values for three isomeric C14H8O2K ions can be

calculated: AEcalc XXVIIIa (K1.71), AEexp 0 eV; AEcalc

XXVIIIb 1.38, Emax (exp) 1.7 eV and AEcalc XXVIIIc

0.01 eV, Emax (exp) 0.44 eV.

The conclusion is straightforward: registration of mol-

ecular ions in three resonance states with maxima at 0; 0.44

and 1.7 eV manifests formation of three isomeric molecular

ions XXVIIIa, XXVIIIc and XXVIIIb, respectively and no

any modification of physical picture of the ionisation

process, given in detail at the beginning of this work, is

necessary to explain registration of molecular anions at

several energies. A very important work with anthraquinone

[53] was continued by the same group on analysing mass

spectra of four halogen-substituted anthraquinones (AQ)

[55]. To two well resolved resonance states for molecular

ions of 2-F-AQ (XXVIIIe) the authors, applying the

convolution of a signal (see above), added the third one

and ascribed all these three to either of (a) nuclear excited

Feshbach resonance; (b) the first shape resonance and (c)

core-excited Feshbach resonance.

Our interpretation of the experimental results gained in

Ref. [55] is a different one. First of all, two experimental

curves are very wide about 2 eV (the first) and 1 eV (the

second). Thus, if the convolution peak techniques is applied

the authors would find 2–3 resonance states more for

molecular anions. All this is in full agreement with our

suggestion (see above) of formation of molecular anions in

several resonance states through bond cleavage(s) in the

ring system. In case of 1-F-AQ (XXVIIIe) the following

isomers of the initial molecular ion could be formed due to

asymmetric structure (compared with unsubstituted anthra-

quinone XXVIII) of X-substituted molecules (XZF):

Some of the isomers have close DH0f and AE values, e.g.

XXVIIIh and k or XXVIIIi and j thus forming common

unresolved peak. AEcalc for XKwith conservation of molecule,s

skeleton in free radical in 1-X-AQ (XZF,Cl,I) or 2-Br-AQ is

2.0 (F), 0.5 (Cl), 0.2 (Br) and K0.5 eV (I) [23]. Hence, for

F-analogue, the most energetically favorable way to spend the

excess energy is ‘internal fragmentation’ with cleavage(s) of

the ring bond(s). Coming to Cl-(XXVIIIe, XZCl) and Br-

analogues (XXVIIIl) the ions possessing negative charge at

carbonyl group (like XXVIIIg,h,k) are not formed since, AE

XK is lower than for these ions. Hence, the number of domains

of existence for molecular ions reduces compared with

F-analogue. Finally, for I-analogue only production of initially

formed molecular ion becomes accessible energetically.

We found a single non-cyclic compound which also gives

molecular anion in two energetical regions, this is MeC(Me)

aNOC(OH)aNMe pesticide [7] while 14 other pesticides (all

cyclic) give molecular anions in two domains [7]. Our

suggestion is that this compound could exist in two tautomeric

forms each giving its own molecular ion at different energies

because these forms possess different enthalpies of formation

and EA values. Another option is the formation of a second

molecular ion through enol/keto isomerization of initial

molecular ion. In our explanation of appearance of molecular

ions in several domains we appealed only to ring opening but

why not to involve the processes of H-migration? In this

respect tautomerization by L-type is the most reasonable and

attractive one among other known processes of H-migration.

We performed similar analysis of experimental data on

measured molecular anions lifetimes t [32,46]. Since, most

often the range of lifetime values is very large, the mean value

is presented as the final characteristics. In experiments with

Rydberg K-atom collisions Suess et al. [46] found that C6F6

molecules form their molecular anions with a broad range of

lifetimes that extend from 1 ns to 50 ms but produce them in

three quite clearly separated time domains. The assumed

lifetimes used in this fit of experimental data are 5, 35 and

245 ms with relative weightings 81, 14 and 5%, respectively,

indicating that electron capture leads to the population of a

number of different negative ions states [46]. We interpret

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213208

these data and the fact of broad range of lifetimes for molecular

anions by formation of several structures of molecular ions

rather than by formation of a single initial structure of

molecular anion but with different electronic states as

suggested by the authors [46]. Such several structures for

C6F6 are easily predicted (XXXIIIa–e):

Formation of XXXIIIb–d ions could proceed only through

skeletal rearrangement of SA-type and, possibly, supports our

cautious suggestion (see above) that formation of molecular

anions in several regions could proceed through rearrange-

ments including H-shifts. Such suggestion might not be

considered unrealistic since in description of fragmentation

types we, on hundreds of reactions, demonstrated isomeriza-

tion of initial molecular anion.

Vasil’ev et al. once more stress the extreme complexity of

the physical picture for formation of the negative ions [57].

According to their claim, the authors showed that the well

known process [RO-H]%K/ROKCH% in many compounds

proceeds at the thermochemical threshold of the RO–H bond

cleavage and is independent of the molecular symmetry and the

symmetry of the parent resonance state. It has been possible to

deduce that the formation of these ions at the threshold is not a

direct dissociation process but very likely the result of

predissociation through a tunneling mechanism [57]. This

conclusion together with suggestions made in Refs. [44,75] are

in disagreement with the physical picture presented at the

beginning of this work [1] and followed through the text.

Here, we briefly consider some other aspects related to

molecular ions. With rise of temperature the low energy Emax

of molecular ions shifts to yet lower energies since heating

contributes to the energy obtained from captured electron [55].

However, in most cases the variation of molecular ion’s peak

intensity with temperature and with variation of other

instrumental parameters might be referred to artifacts (see

later). Registration of molecular anion only at high energy

7.9 eV and its absence at low energy in naphthalene [7] we also

refer to artifact since we do not see any theoretical grounds for

such event. The absence of a peak of molecular ions in NI

chemical ionization of polycyclic aromatic hydrocarbons

(pyrene, anthracene and others) possessing EAO0 is explained

by interference of ion-molecular reactions [58] because in

electron capture mass spectra pyrene and anthracene give the

peaks of molecular ions at Emax 0.21 and 0.11 eV, respectively,

and the second one at 7.7 and 7.3 eV, respectively. However,

the latter two, possibly, also could be referred to artifacts.

Experiments with 2,4,6-trinitrotoluene have shown that

molecular ion abundance is increased by a factor of four

when collisionally quenched by a low methane gas pressure

introduced into the ion source housing [7].

In study of nine aliphatic and aromatic nitrocompounds [59]

in which electron is attached to nitrogroup, introduction of OH,

CR3, SiR3 groups has no effect on stability of molecular anion

while these groups lead very often to the absence of the peak of

molecular cation in positive ions mass spectra. Similarly,

double bond or Ph-ring usually stabilizing the positively

charged molecular ion have no any effect on stability of

molecular anion, at least in nitrocompounds [59].

In this work we do not analyze stability of quasi-molecular

anions M* (usually [M-H]K ions) obtained with CID, NI CI

MS, FAB and other ionisation methods.

Artifacts in negative ions mass spectrometry. Cross sections

of NI formation at EC process vary in a very broad range by

about four orders [32]. Hence, even an impurity with a small

concentration can distort the mass spectrum and the energetical

background of its description. That’s why the requirements to

the analyte purity have to be very rigid. But the impurity can be

formed in ion source as a result of thermal or thermocatalytic

destruction of a sample. Although this statement is a routine

and a well known in mass spectrometry, however, in practice,

is well-forgotten when a worker is investigating his (her)

substance. In analysis of mass spectrum of thiophene it was

found that the compound partly decomposes at cathode and

produces S-atom which after electron capture gives S%K ion

[32]. But the same ion is formed from molecular ion of

thiophene (Eqs. (20)–(24)). In dialkyl sulfides RSR the RSK

ions, formed at 7 eV are not sensitive to the temperature

increase while those registered at low energy go down to 0 eV

revealing that there is another source of their production. This

could be RS% free radicals which are produced in thermo-

catalytic process. Its concentration increases with temperature

and yet more contributes to formation of RSKions obtained

directly from molecular anion. Since, for such ionisation

process RS%Ce/RSK there is no stable resonance state at

7 eV the latter ions are not formed at this energy [60].

Rosa et al. found unusual temperature dependence in

dissociative electron detachment of 1,4-chlorobromobenzene

p-ClPhBr [61]. At room temperature BrK ion is by about a

factor of 70 more abundant than ClK ion while at the highest

temperature measured (about 270 8C) this factor decreases to

five. Increasing the temperature of the target molecule up to

270 C results in unexpected temperature dependence of the

cross-section in that it first increases and after passing a

maximum (130 8C for BrK and 190 C for ClK) decreases. This

behavior is qualitatively interpreted by the temperature

dependent population of the relevant normal modes containing

the C–Cl and C–Br stretch vibration [61]. We have the different

interpretation of these experimental facts. The increase of

cross-section for BrK and ClK ions formation at initial rise of

temperature we explain by the increase of contribution of

heating (additional energy) to ionization process since the latter

proceeds at low energies. With further rise of temperature the

thermocatalytic process of molecule destruction begins with

C–Br (at lower temperature owing to weaker C–Br bond) and

C–Cl bonds cleavage (at higher t0 owing to stronger C–Cl

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 209

bond). Thus the concentration of the target molecules decreases

with yet higher temperature. 1,4-Bromochlorobenzene is

thermally a stable molecule and its destruction at 130 8C

might be assigned to more catalytic than to temperature effect.

Similar temperature effects were noticed for substituted

anthraquinones X-AQ with XZ1-Cl and 1-I (compounds

XXVIIIe) and 2-Br-AQ (compound XXVIII) [55] (see above).

The molecular ion abundance begins to fall down in the first

resonance state at 230 8C (Cl) and at 110 8C (Br). In latter case

M%K and BrK ions exhibit 100 and 50% (808) while at 2908

these are 15 and 100%, respectively. With XZI the

abundances of M%K and IK ions are 100 and 90% (1058), 20

and 100% (1508), 10 and 100% (200 8C), respectively. All this

we explain with thermocatalytic destruction of the target

molecules at yet higher temperatures and yet more intense at

Cl/Br/I replacement while no temperature effect was

noticed for F-analogue [55] owing to a very strong C–F bond.

No physical arguments are necessary for interpretation of

experimental data in this case as it is done in Ref. [55].

The absence of (M-H)K ion and abundant (M-H2O)

%K(70%) with M%K (100%) [48] in phtalic acid we explain

by (at least partial) thermocatalytic reaction in ion source. The

occurrence of abundant molecular ions in aliphatic nitrocom-

pounds comprising HO-group and weak ones in aromatic

analogues [59] we explain by thermocatalytic elimination of

water. Such process leads to formation of conjugated p-system

and thus energetically is very favorable to such an extent that

the target molecule XXXIV completely decomposes in mass

spectrometer, as we think, without giving rise even to the traces

of molecular ion (Eq. (52)). Additional reason for this

instability of aromatic compounds compared with aliphatic

ones is the higher t0 necessary for evaporation of the former.

(43)

Analysing data on EC process for C2Cl4 molecule [7] we

calculated the AE values for ClK ions and found that those with

Emax 0.53 correspond to formation of [ClKCCl2CaCCl%] pair

with AEcalc 0.32 eV and of [ClKCCl%CClCbCCl] species

with AEcalc 0.81 eV [23], the latter reaction contributing to the

broad peak with Emax 0.53 eV [23]. For ClK ion in the lowest

resonance state with Emax 0.19 eV another precursor than

C2Cl4 should be looked for. Detailed thermochemical

arguments for fragment ion formation from a precursor other

than from the molecular ion (in compound IV—see above)

were given in Ref. [62].

Analysing EC mass spectrum of nitromethane MeNO2 the

authors, in contrast to previous studies, registered production of

NO2K, HOK, CNOKand NCK ions at low energies [63]. They

explained this in terms of dissociative electron attachment to

highly vibrationally excited molecules (hot bands). On the

example of HOKion several possible sources of its formation

were inspected: (a) from impurity; (b) via ion-molecular

reaction; (c) hot bands. Rejecting the first two after detailed

analysis they ascribed HOKproduction to hot bands. Their

calculation of AE for this ion with several possible partners

[CH2CNO], [HOCHCN] and [H2OCCN] gave too high

values being very far from the experimental energies close to

zero. For (MeNO2)%K/NO2KCMe% reaction they calculated

AE 0.37 eV while NO2K ion was registered at energies lower

than 0.3 eV. Thus, involvement of hot bands was again

attracted [63].

We included in calculation other, energetically more

favorable partners for HOKand other ‘unexpected’ ions and

found the processes going on at zero or negative energies

without appealing to hot bands and other possible artifacts.

According to our calculation AE values for HOK ion are C0.7,

K0.75 and K0.8 eV with neutral partners. CH2NO, CHONH%and H2NC(%)aO radicals, respectively. AEcalc values for

NbCK ion with [H2OCHO%], of NbCOK ion with [H2OCH.] or [H2CHO.] as neutral partners are K0.67, K1.17 or

K0.51 eV, respectively [23]. {The following DH0f values were

used for these calculations: K4.0 and K43.5 for H2NC(%)aO

and H2NCHO species, respectively [38], 44 and K2 kcal molK

1 for NbCO% and HOCN species, respectively [37]}. The

enthalpy of formation for CHONH% radical K2.5 kcal molK1

was estimated using series of isodesmic reactions [23,37]. EA

values of NbC% and NbCO% radicals 3.862 and 3.609 eV were

taken from Ref. [39]. Only for formation of NO2K ion at

energies (0.3 eV we have to suggest its production from NO2%free radical, obtained in thermocatalytic destruction of target

MeNO2 molecule. However, contribution of this process to

total NO2K ion current is very small [63]. We suggest the

following schemes for formation of the ions under discussion

(Eqs. (44) and (45)).

(44)

(45)

Many authors applying EC or NI chemical ionization MS

have registered fragment anions being obtained from mol-

ecules or complexes possessing molecular weights higher than

that of the target molecule. Formation of [BrCN]%K (we

suggest [BrNC]%K isomer) was attributed to its production

from (BrCN)n clusters [64]. The molecular ion of dimer of

o-phtalic acid of unknown structure together with its fragments

was registered in Ref. [48]. In mass spectrum of 1-oxycarbo-

nylanthraquinone (XXVIIIe, XZCOOH) the peaks of several

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213210

ions originated from its dimer and [M-O]%K (3%) with M%K

(100%) ions were found [48]. Unusual fragmentation patterns

were found in mass spectra of 2,4-dinitrophenylhydrazine and

its propanone derivative which cannot be formed from their

molecular anions. The authors explain formation of such

‘foreign’ ions by reactions in ion trap giving rise to NH3, H2O,

MeOH and N2H4 molecules since, in atmospheric pressure CI

method the corona discharge has sufficient energy to generate

these molecules which then react with a fragment ion 151K

from the title compound [65].

Polycyclic aromatic hydrocarbons give abundant [MCCH2]

%K or [MCCH3–H]%K ions in a result of ion-molecular

reactions under NI Chemical Ionization [58]. EC MS of

s-triazine herbicides revealed formation of [MCX]K ions

(XZH, H2, CH, CH2, C2H4, Cl) [66]. Relative abundances of

these ions vary widely in a complex fashion depending on ion

source temperature, pressure and cleanness. We are not sure that

all the workers pay attention at such ions, especially at [MCH]K or [MCH2]%K ones treated as ‘isotopic’ ones while these

can be the precursors of many ions attributed to their production

from molecular ions. We performed thorough inspection of

thermochemistry of formation of [CH3O]K ions from CH3OH

molecule. AEcalc for CH3OK isomer is 2.9 eV while another

isomer does not exist because EA%CH2OH!0 [67]. However,

in addition to resonance state with onset about 2.9 eV there were

registered three peaks at energies higher than 5.5 eV [7]. The

suggestion that these might correspond to three MeOKions in

three diverse electron-excited states seems highly unprobable.

We doubt that all of them avoid electron autodetachment at such

high energies if EA MeO% free radical is only 1.57 eV [39].

Another suggestion is formation of ion-neutral or H-bridged

complexes like HC(-)/OH2, [cyclo-CH2OH]K or [HC(:)/H/OH]K species. But we doubt that such would survive at 5–

9 eV. So, the single suggestion left is formation of MeOK ions

from precursors alternative to MeOH molecules like (MeOH)n

or (MeOH)k/(H2O)l complexes. In this section (see above) we

demonstrated the existence of similar complexes. Compared

with, say, dimer of phtalic acid which possesses EAO0 and

hence could be registered those complexes including MeOH and

H2O in no way could produce stable molecular anions and

charged fragments with masses higher than MeOH. At the same

time, variation of neutral partners, as it was performed in many

cases earlier (see above) at, say, fragmentation [CH3OH]2%K/CH3OKC[CH5O] could easily fit the energetics of MeOKpro-

duction at high energies.

In mass spectrum of (CH3)2S molecules the peak of

[CH2S]%K ion with rather sharp onset at 7.0 eV was fixed

[32]. We estimated the enthalpies of formation of neutral

CH2aS and :CHSH isomers to be 24.5 and 50 kcal molK1

with their EA values 0.465 [39] and 1.0 eV(est), respectively

[23] (compare with EA :CH2 0.652 eV [28]). Thus, DH0f

values 13.8 and 30 kcal molK1 were calculated for

CH2aS%K and %KCHSH ions, respectively [23]. Taking

then several possible [CH4] neutrals, e.g. CH4; [CH3CH];

[CH2 (triplet)CH2]; [CHCHCH2] and [CH2C2H] we

obtained AEcalc for CH2aS%K ions 0.22; 4.77; 4.63; 9.41

and 9.15 eV, respectively. AEcalc for its isomer %KCHSH

with all these neutrals might be by 30K13.8Z16.2 or 0.7 eV

higher, i.e. 0.9; 5.5; 5.3; 10.1 and 9.85 eV, respectively [23].

Then we added two other possible sources for [CH2S]%K

isomers, MeS. and. CH2SH free radicals which can be

produced in thermocatalytic process from target (CH3)2S

molecule. Summarising all these calculations for 14 possible

routes of formation of either [CH2S]%K isomers we found

that neither of them can give these ions in the interval 5.5–

9.0 eV. Thus, we again have to look for source other than the

target molecule.

Calculation of the enthalpies of formation for 12 isomeric

[M-H]K ions from acetone [23] allowed us, as it seems, to find

the structure fitting the resonance state with Emax 6.5 eV [32]

(Eq. (46)):

(46)

However, we are not confident in the reality of such rather

strange mechanism and reserve our opinion that possibly [M-

H]K ion at high energy is formed from certain unknown

precursors rather than acetone. Since, as it was shown in this

section, formation of fragments from precursors other than

molecular ion (dimers, products of its reaction with the

species formed in ion source, products of pyrolysis and others)

is now a well established reality, it becomes uncertain whether

to look for diverse structures of an ion or a set of neutrals (or

both) in attempt to fit calculated thermochemical picture to

experiment (this is quite accessible for polyatomic molecules)

or to ascribe registration of ‘unusual’ ions or in ‘unexpected’

energetical domain to artifacts. In any case, the calculation of

thermochemistry for such ions varying their possible

structures and presentation then the reasonable isomerisation

and fragmentation schemes according to now known types is

highly desirable.

Very briefly we consider some other artifacts and possible

sources of errors in NI mass spectrometry. Many examples of

instrumental problems are given in Ref. [32] while the problems

of quantitative analysis are given in Refs. [7,68]. Ong and Nites

try to answer the question why EC NI mass spectra are not

reproducible coming to conclusion that this results from ion

source problem. For example, on one instrument (HP5985B) low-

mass ions enter the analyzer as a defocused beam while high-mass

ions enter the analyzer as a well-collimated beam. On the VG 30-

250 instrument low-and high-mass ions are transmitted to the

analyzer with equal efficiency by the ion extraction system [69].

The peak’s intensity of IKion (from C2F5I) recorded at different

ion draw out fields at an electron energy about 0 eV changes

essentially [70]. Samples introduced through solid inlet system

(a) and by GC (b) showed diverse results, e.g. the relative

abundances of NO2K ion in two resonances were about 1: 0.8(a)

but 0.3:1.0 (b). The differences are probably related to the

differences in the exit apertures, which in the case of the probe

samples (a) was 0.5 and for GC experiments (b) 1 mm [54].

The wealth of difficulties and artifacts encountered in NI

MS explains why we rather rare performed direct

comparison of the ions, abundances even in structurally

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213 211

close compounds what is often done in MS of positive ions

but just fixed the occurrence of a process related to the

particular type. We think that the physical picture coming

from the known events of interaction of an electron with a

molecule might not be directly applied to interpretation of

mass spectra since the chemical events taking place in mass

spectrometer appeared to be much more complicated than it

was realised before.

1. Conclusion

The general physical picture of non-dissociative and

dissociative interaction of an electron with a molecule is

presented being supplemented by physico-chemical approach

to description of these processes [1]. The validity of Quasi-

Equilibrium Theory (QET) of Eyring to negative ions mass

spectrometry was demonstrated on many examples coming

from practical interpretation of mass spectra of a large wealth

of compounds from diverse classes. The main conclusion,

coming from QET, is the complete randomization of excess

energy over the molecular radical-anion (whether it is

registered or not). This allows to apply the thermochemical

approach (preferential formation of thermodynamically more

stable charged and neutral fragments; [preferential cleavage of

the weakest bond(s) both in simple bond rupture, H-migration

and skeletal rearrangements] to description of isomerization

and fragmentation of negative ions but with one definite

limitation: the symmetry of an electronic state of a product ion

(fragment or rearrangement one) should correlate with the

symmetry of an electronic state of a molecular (in general, of a

parent) ion. Such correlation is vital for explanation of

dissociative electron capture processes when a set of fragment

ions being formed in the consecutive steps from the common

parent ion is observed in the same (usually narrow) range of

energies and not in the wide (0–15 eV) range of measured

energies. Direct experimental data in favor of QET are given in

Refs. [32,71,72] while the general concepts that are related to

energetics of both positive and negative ions fragmentation are

briefly reviewed in Ref. [68].

For large molecules (with molecular weights of tens and

hundreds of thousands of daltons) QET might be valid for

essentially gross sections of a molecule this being sufficient to

be valid, in summary, in the whole molecule owing to overlap

of such sections energetically. Additional argument is that most

of such large molecules possess the regular structure with

identical or, at least structurally close, centers of electron

capture (natural and synthetic polymers, dendrimers, polypep-

tides, proteins, nucleosides). Another field where incomplete

randomization of excess energy can happen is the early stage of

formation of molecular anion or temporary molecule/electron

complex. Here, the lifetime of either of these two is too short to

follow predictions of QET due to electron autodetachment.

However, we came to a quite definite conclusion that

autodetachment of electron, discussed in many works on

physics of electron capture process, does not change essentially

the chemical routes of isomerization and fragmentation

processes. The loss of ions due to autodetachment of an

electron is only one of many sources of ions escape, typical for

MS methods and can be added to the list of ‘artifacts’ (see text).

Our position in favor of QET was further confirmed and

strengthened by finding the common features of the processes

of electron capture dissociation, collision induced dissociation,

negative ion chemical ionisation, radical-anions and anions

chemistry in solution. Importance of the latter has increased in

last years [8,31,73–76]. These common features are now

expressed in specific isomerisation and fragmentation types of

monomolecular reactions at diverse experimental conditions.

The introduction of these types now links physical and

chemical processes taking place in mass spectrometer. We

have to stress this because some recent publications in physics

of negative ions formation and dissociation seem if not to

revise but still to make the physical picture more complex and

less understandable. Among them the introduction of idea of

‘two-electron’ ionization process [44] (see above). Another one

is suggestion for occurrence of molecular radical-anion in two

p-and s-forms separated energetically by essential barrier for

p-to s-radical-anion transfer followed by formation of

molecular anion as an electrostatic complex also separated

by barrier from s-form of radical-anion. The final step of this

model is a fluctuative dissociation of the molecular ion

accompanied by the transition from p-into the s-form, so

called predissociation [55,75]. One more, unusual, idea comes

from Tomer [77] who suggested inversion of charge

(negative/positive) in RK ion in mass spectrometer working

in regime of NI MS if EA R%!0 like with alkyl free radicals.

Vasil’ev et al. once more stress the extreme complexity of

the physical picture for formation of the negative ions [57].

According to their claim, the authors showed that the well

known process [RO–H]%K/ROKCH% in many compounds

proceeds at the thermochemical threshold of the RO–H bond

cleavage and is independent of the molecular symmetry and the

symmetry of the parent resonance state. It has been possible to

deduce that the formation of these ions at the threshold is not a

direct dissociation process but very likely the result of

predissociation through a tunneling mechanism [57]. This

conclusion together with suggestions made in Refs. [44,75] are

in disagreement with the physical picture presented at the

beginning of this work [1] and followed through the text.

In the framework of the latter we have attempted to explain

the chemical behavior of negative ions introducing the less

known or confirming the well known statements some of them

coming from positive ions MS not forgetting about specifity of

negative ions. These statements are: (a) validity of QET to

isomerisation and fragmentation processes of negative ions

(see above); (b) validity of thermochemical approach

(immediately followed from validity of QET) to description

of the negative ions chemical behavior. For a moment we could

claim the introduction only of elements rather than of

systematic thermochemical approach because of the lack of

experimental data on thermochemistry of radical-anions and

even-electron anions (EA, DHacid and DH0f values). These

elements comprise formation of stable (with low values of

enthalpies of formation) neutral molecules and free radicals

(see text). Another element of thermochemical approach is the

D. Ponomarev, V. Takhistov / Journal of Molecular Structure 784 (2006) 198–213212

preferential bond cleavage (for both processes of simple bond

cleavage and H-migration) in b-position to anionic, free radical

centers or to p-system. However, for the most important

partner, the negative ion, less energetically favorable isomers

are also formed to fit stable resonance states. This distinguishes

the behavior of negative ions from positive ones. For the latter

formation of the most stable ions (and neutrals) in each step of

bond cleavage or rearrangement is currently in action. One

direct consequence of this principle is expressed in Stevenson-

Audier rule declaring that in process of simple bond cleavage

the positive charge retains with a radical possessing lower IE

(DIEO0.2–0.3 eV) (see Ref. [36]). We cannot apply with

confidence this rule to fragmentation of negative ions, i.e.

retaining of the negative charge at the radical with higher EA in

a process [A-B]%K bond cleavage. For example, in R–Cl

molecules usually both ClK and RK ions are registered.

Further fundamental work on this problem is necessary; (c)

contrary to earlier observations we declare the energetical and

mechanistic accessibility of profound isomerization and

rearrangement processes on the time scale of MS method

among them the ring opening, consecutive multi-step H-and R-

(atom, group) migration to adjacent or remote position; (d)

formation in one act (step) several (rather than a single one)

neutral species (molecules, radicals, biradicals) in simple bond

cleavages or these being combined with H-migrations or

skeletal rearrangements. To be decent, Collin as early as in

1968 to explain formation of HK(DK) ion from MeNH2

(MeND2) molecules in wide range of energies applied several

equations with formation of two neutrals [78]. Recently, Seiler

et al., to fit the experimental energetical data for formation of

fragments from MeNO2 molecules, also explored formation of

several neutrals [63]. However, since the resonances are very

wide, e.g. O%K(3–7 and 7–9.5 eV), NO2K(1–9 eV), HOK(0 and

several resonances in 3–9.5 eV range), [CNO]K(0–9.5 eV) we

added several more structures: NOOKand cyclo-OONKin

addition to NO2K ions or 5 [CNO]Kisomeric ions, in addition to

NbCOK species::CaNOK,N–O–C(%),:C(%)–ONK% and two

cyclic isomers. These should not be treated as fantastic ones,

e.g. five [HNCO] isomeric molecules were described in Ref.

[35]. Only with a help of these added structures one could try to

explain the complex energetical picture of such simple

molecule as MeNO2 studied in Ref. [63]; (e) formation of

molecular ions in several resonance states and with several

lifetimes we explain by isomerization of the initially formed

molecular ion rather than by involvement of several electronic

states like it is suggested in literature (see text). An extreme

example of registration of M%K ions in mass spectrum of C60

fullerene as an uninterrupted curve from 0 to 14 eV [79] we

explain by formation of many isomeric ions being formed by

cleavage of several C–C bonds; (f) the thermochemical

approach was systematically demonstrated in this and previous

works together with calculated schemes for AE, DH0f , DHacid

and EA values; in estimation schemes on thermochemistry the

polarizability effects are systematically taken into account; (g)

in many cases (see Refs. [7–15,18,19,21,25–

27,29,30,32,40,44,45,47,49,51,52,61,62,64] only the exper-

imental data were taken from the literature while the

isomerization and fragmentation schemes were created or

corrected by the authors of this work.

In our final remark we stress that without thermochemical

approach, including systematic measurement and estimation of

the enthalpies of formation for isomeric radical-anions (first of all

for molecular anions) and for charged and neutral fragments it is

doubtful to obtain correct mechanisms and fragmentation

schemes for isomerization and fragmentation of negatively

charged ions. Thermochemical approach is expressed in

introducing in this work of specific isomerization and fragmenta-

tion types (or rules) and this could become an effective aid to

practicing mass spectrometrists especially in situation when the

physical picture of the dissociative electron capture is still

uncertain. We do not see any limitations to apply these types to

any other classes of compounds studied by either EC, CID, NI CI,

FAB or other methods of negative ions production.

Implementation of isomerization and fragmentation types in

MS of both positive [51,58] and negative ions (present work)

might be useful for teaching all the beginners in such complex

method as MS. Applying these types (rules) we managed to

consider about 650 processes in the framework of two

publications (see Ref. [1]). This work is hardly possible to

perform if the traditional way of mass spectra analysis—by

classes—rather than by types has been applied.

Note added in proof. Recent publication of Voinov et al.

[80] presenting a new type of instrument, a gas chromatograph/

resonance ion capture TOF mass spectrometer for four

dimensions of negative ion analytical information, is really a

breakthrough in NI MS, especially in its analytical application.

However, when discussing the fragmentation of target

molecules (such discussion is inevitable in analysis of mass

spectra) we see again all the elements of physical picture of EC

process critisized in present work and in Ref. [1]. Thus, FK ion

formation at 9 eV (from n-C7F15COOH) is explained by its

formation from high-energy state, possibly a s* shape or a

core-excited Feshbach resonance contrary to our suggestion to

vary the structure of neutral part of the process (see text). No

explanation is given why BrK ions in cis-and transK1,2-

dibromoethylenes are found in two resonance states with AEs

close to zero and with two different Emax.

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