Fragmentation of deuteronated aromatic derivatives: The role of ion-neutral complexes

ELSEVIER International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166

Mass Spectrometry and Ion Processes

Fragmentation of deuteronated aromatic derivatives: ion-neutral complexes

Alex G. Harrison*, Jian-Yao Wang 1

Department of Chemistry, University of Toronto, Toronto, Ont. M5S 3H6, Canada

Received 20 March 1996; revised 10 May 1996; accepted 25 May 1996

the role of

Abstract

The low-energy collision-induced dissociation reactions of the MD ÷ ions of a number of alkyl phenyl ethers, alkylbenzenes, acetophenones and benzaldehyde have been studied as a function of collision energy to establish qualitatively the dependence of the fragmentation reactions observed on internal energy. Deuteronated alkyl phenyl ethers (ROC6Hs.D +, R = C3H7, C4H9) fragment at low collision energies to form C6H5OHD ÷ + (R-H), the thermochemically favoured products; with increasing collision energy (and, hence, internal energy) formation of the alkyl ion R ÷ increases significantly in importance. Deuteronated alkylbenzenes (RC6Hs, RC6H4 R', R = C2H5, C3H7) similarly form the deuteronated benzene (the thermochemically favoured product) at low collision energies with formation of the alkyl ion R ÷ being observed at higher collision energies. The results for both systems are consistent with a fragmentation mechanism involving initial formation of an R+/aromatic ion/neutral complex. At low internal energies proton transfer occurs within this complex to form an ion/neutral complex consisting of the deuteronated aromatic and a neutral olefin; this complex fragments to the thermochemically favoured products. Since the transition state leading to these products is a "tight" transition state involving loss of rotational degrees of freedom, the proton transfer reaction is unfavourable entropically with respect to simple dissociation of the R+/aromatic complex to R ÷ + ArD. Conse- quently, these products increase in importance as the internal energy is increased. The fragmentation of deuteronated aromatic carbonyl compounds can also be rationalized by similar mechanisms involving the intermediacy of ion/neutral complexes. Deuteronated acetophenone forms only CH3CO + at all collision energies; this is both the thermochemically and entropically favoured product. However, deuteronated p-aminoacetophenone forms deuteronated aniline, the thermochemically favoured product at low collision energies with formation of CH3CO +, the entropically favoured product increasing in importance with increasing collision energy. Deuteronated benzaldehyde forms C6HsD + + CO at low collision energies but HCO ÷, the entropically favoured product, is observed at higher collision energies. © 1997 Elsevier Science B.V.

Keywords: Ion-neutral complexes; Deuteronated aromatic derivatives; Fragmentation

1. Introduction

It is well known that a large number of gas- phase bimolecular ion/molecule reactions,

* Corresponding author. a Present address: Department of Pharmacology, Medical Univer-

sity of South Carolina, Charleston, SC, USA.

particularly those involving proton transfer, occur in a stepwise fashion and involve the formation of two intermediate ion/neutral complexes [1-15]. This is illustrated in Fig. 1 (solid curve) for exothermic proton transfer reactions involving either positive or negative ions where a and b represent the intermediate complexes stabilized by ion/dipole and/or ion/induced

0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved t:'11 S0168-1176(96)04482-5

158 A.G. Harrison, J.-Y. Wang~International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166

R-+HY ~--'~"R---HY "" - RH---Y- ~ R H + Y -

R++ M " ~ R++---M " - (R-H)--triM " - (R-H)+MH +

RHY-

RM +

Fig. 1. Potential energy surface for proton transfer reactions.

dipole interactions. It is possible that in specific cases complex b might be better represented as a proton-bound complex of two moieties (R- and Y-, (R-H) and M); such species are most likely when the two moieties involved have similar proton affinities.

An alternative method of accessing the potential energy surface applicable to the proton transfer reactions of Fig. 1 is by suitable activation of covalently bonded ions as illustrated by the dashed curve of Fig. 1. 2 This approach has been most widely applied in the negative ion field. There is ample evidence that the fragmentation reactions of alkoxide [20-24] and enolate [25- 27] ions upon activation involve the initial formation of an ion/dipole complex which may undergo either simple bond fission or, more often, an internal proton transfer reaction followed by decomposition. Thus, activation of RC(CH3)20- leads to R----(CH3)20, the complex a in the reaction of R- with acetone (HY), while activation of RCOCH2 leads to R- ' - -CH2CO, the complex a in the reaction of R- with ketene (HY). Brauman and co-workers [20,21,24-27] have activated a variety of

2 While the dashed curve implies an energy barrier for collapse of the complex a to the covalently bonded species, it is clear that in a number of cases there is no energy barrier for collapse of a but rather an entropic barrier to this collapse [16-19].

alkoxide and enolate ions by IR multiphoton absorption and observed the fragmentation products by ion cyclotron resonance (ICR) mass spectrometry. Such experiments generally explore only the lowest energy fragmentation pathways since the rate of fragmentation of an ion is rapid relative to the rate of photon absorption once the fragmentation threshold is reached. In general, the Y- ion (the product of proton transfer) was the only product observed although activation of CF3COCH2 led to formation of both CF3 (R-) and HCCO- (Y-), with the yield of the former product increasing with increasing laser fluence.

An alternative method of activating gaseous ions is by collisional activation [28,29]. By using low, variable energy collisional activation, the so-called energy-resolved mass spectrometric (ERMS) technique [30-32], the energy dependence of the ionic fragmentation reactions can be explored at least in a qualitative fashion. Using this approach, a number of studies from this laboratory [33-37] have explored the energy dependence of the fragmentation of a variety of alkoxide and enolate ions. At the lowest collision energy (and, thus, internal energy) the thermochemically favoured products (RH + Y-) result- ing from proton transfer were the dominant fragmentation products. However, as the collision energy (and, thus, the internal energy of the fragmenting ion) was increased the thermochemically disfavoured products (R- + HY) increased greatly in importance. These results can be rationalized by noting that the transition state leading from a to b, and, thus, to proton transfer, is a "tight" transition state in which some of the rotational degrees of freedom will either be totally frozen or seriously hindered. By contrast, the transition state leading from a to (R- + HY) is a " loose" transition state resembling the separated species R- + HY. In such a situation, once the internal energy of the ion/neutral complex a reaches the threshold for production of R- + HY, a significant frac- tion of the complexes will exit by this channel

A.G. Harrison, .L-Y. Wang~International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166 159

because it is entropically favoured although thermochemically disfavoured.

There is substantial evidence [38-44] that the fragmentation reactions of covalently bonded positive ions also proceed by way of ion/neutral complexes such as a. In many cases the thermochemically favoured channel involves proton transfer from R ÷ to M. For example, protonated phenyl n-propyl ether fragments primarily by elimination of C3H6 [45] and evidence has been presented [46] that this reaction occurs by initial formation of a C3H~-'-HOC6H5 complex. Simi- larly, fragmentation of protonated phenyl isopropyl ether occurs in a stepwise fashion to give protonated benzene and propene at low internal energies [17]. Fragmentation of protonated alkylbenzenes, where the alkyl group is ethyl or larger, proceeds, in part, by elimination of an alkene [47-50], and evidence has been presented [51-54] that this reaction proceeds by way of the ion/neutral complex R+...C6H6, with subsequent proton transfer to give (R-H) + C6H~ at low internal energies. If these initial ion/neutral complexes are the species a of Fig. 1 and proton transfer occurs over an energy barrier involving a tight transition state to give the complex b, one would expect, by analogy with the negative ion systems, that, as the internal energy of the ions is increased, the reaction channel to form the products of simple bond fission (R ÷ + M of Fig. 1) should increase in importance, as has been pointed out earlier by McAdoo [39]. Since most studies have been restricted to metastable ion reactions which sample only the lowest energy fragmentation channel, it appeared of interest to explore the energy dependence of the fragmentation reactions to obtain further evidence for the intermediacy of ion/neutral complexes. Accordingly, we have studied the energy dependence of the fragmentation reactions of the MD + ions of a number of phenyl alkyl ethers, alkylbenzenes and aromatic carbonyl compounds where ion/neutral complexes have already been implicated from low-energy (metastable ion) fragmentation reactions.

2. Experimental

All experimental work was carried out using a VG Analytical ZAB-2FQ hybrid BEqQ mass spectrometer (VG Analytical Ltd., Wythen- shawe, Manchester, UK) which has been described in detail previously [55]. Briefly, this instrument is a reversed-geometry (BE) double-focusing mass spectrometer that is followed by a deceleration lens system, a radiofrequency (r.f.)-only quadrupole collision cell (q) and a quadrupole mass analyzer (Q). In the low-energy collisional experiments, the appropriate ion beam was mass selected by the double-focusing BE stage at 6 keV ion energy, decelerated to the appropriate energy (5-50 eV, laboratory scale) and under- went collision with N 2 in the quadrupole collision cell (q) with the ionic fragmentation products being analyzed by the final quadrupole mass analyzer (Q). The N 2 pressure was 2x10 -7 tort as read by the ionization gauge attached to the pumping line for the quadrupole section. The CID results are presented in the following in the form of breakdown graphs expressing the per cent fragment ion abun- dance as a function of the centre-of-mass collision energy.

The MD ÷ ions of the relevant compounds were studied to avoid interference from fragmentation of the a3C isotopic peak of the residual molecular ion which is observed for aromatic compounds under CI conditions [56]. The MD + ions were prepared by chemical ionization using CD4 as the reagent gas. The EI/CI ion source was operated in the CI mode with an ionizing electron energy of 50 eV and a source temperature of 200°C. The samples of interest were introduced by way of a heated inlet system.

All compounds were commercially available and showed no impurities in their mass spectra; they were used as received. The CD4 reagent gas was obtained from CDN Isotopes, Vaudreuil, Quebec.


3. Results and discussion

3.1. Fragmentation of deuteronated phenyl alkyl ethers

Benoit and Harrison [45] first reported that protonated phenyl n-propyl ether fragmented by elimination of C3H6 . Using specific deuterium labelling of the propyl group they observed H- transfer from each position of the propyl group to the phenoxy oxygen in the fragmentation reaction (eqn (1)) observed under CI mass spectral conditions:

C6H5OC3Hy'H + ~ C6H5OH f + C3H 6 (1)

Kondrat and Morton [46] later examined the metastable ion fragmentation reactions of deuteronated phenyl n-propyl ethers which had been specifically deuterium labelled in the propyl group and reported results in agreement with those of Benoit and Harrison. Kondrat and Morton concluded that reaction (1) proceeded by the initial formation of a C3H~-..HOC6H5 ion/neutral complex in which the propyl group has rearranged to the more stable isopropyl structure. The reversible formation of a C3H6 "''H2OC6H~ complex was invoked to ratio- nalize the observation [45,46] that the H + added in the ionization step ended up in the C3H6 neutral with about 10% probability. Similar results have been reported for fragmentation of protonated phenyl isopropyl ether [17].

Fig. 2 shows the breakdown graph obtained in the present work for the MD + ion derived from phenyl isopropyl ether. In agreement with the metastable ion studies [17], C6HsOHD + (m/z 96) is the dominant product ion observed at the lowest collision energy. As the collision energy is increased the C3H~ ion signal increases rapidly in importance and, at the highest collision energy, is approximately equal in intensity to the C6HsOHD + ion signal. This is essentially the same behaviour as that observed [33-37] in analogous energy-resolved studies of negative ions which fragment by way of ion/neutral

I 1 I I I I I I IO0 -

@ CH3

8 0 N z C I D z < Q z

nm

< 6C z _o

Z u.I

h

0

I , , - - V , ' - O( I 2 5 4 5 6 7 B 9

COLLISION ENERGY (eV, centre of moss)

Fig. 2. Breakdown graph for deuteronated phenyl isopropyl ether.

complexes. Proton transfer within the C3H~.-.DOC6H5 complex (a of Fig. 1) to form C3H6 "'" +DHOC6Hs (b of Fig. 1) involves a tight transition state while fragmentation of the complex to form CsH~ + DOC6H5 involves a loose transition state. Thus, as the internal energy of the complex is increased, simple bond fission to form C3H~ becomes increasingly favoured. In effect, the reaction channel involving proton transfer is thermochemically favoured but entropically disfavoured.

Low intensity ion signals are observed for C6HsOH ~ and C3H6 D+ indicating the reversibility of the proton transfer between C3H~---HOC6H5 and C3H6--.+H2OC6H5 (a and b of Fig. 1). The observation of C6HsOH~ in the fragmentation of deuteronated phenyl propyl ether is in agreement with earlier observations [17,45,46]; however, the present work is the first observation of deuterium incorporation in the propyl ion product. This product ion was not observed in the metastable ion studies

A.G. Harrison, J. - Y. Wang~International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166 161

[17,46] and could not be identified in the chemical ionization study [45] because of interference from ions in the reagent gas plasma.

Breakdown graphs similar to Fig. 2 were obtained for the MD ÷ ions of phenyl n-propyl ether and the phenyl butyl ethers. Fig. 3 shows the breakdown graph obtained for deuteronated phenyl n-butyl ether. The results for all the phenyl butyl ethers are consistent with fragmentation through a CaH~---DOC6H5 complex. No isotopic mixing was observed for the phenyl butyl ether systems; the isotopic mixing in the phenyl n-propyl case was similar to that observed for the isopropyl case.

3.2. Fragmentation of deuteronated alkylbenzenes

A common fragmentation reaction of protonated alkylbenzenes RC6Hs-H + is elimination of an olefin (R-H), when R --> ethyl, with formation of a protonated benzene [47-50];

alternatively, the protonated species may fragment to R ÷ + C6H 6 [47,50]. From a detailed study of the Bronsted acid chemical ionization of protonated alkylbenzenes, Herman and Harrison [50] proposed that these fragmentation reactions proceeded through the intermediacy of R+'"C6H6 ion/neutral complexes which either fragmented directly to give R + + C6H6 or under- went internal proton transfer to give (R-H) + C6H~. This proposal has received direct experimental support from the extensive mechanistic studies carried out by Audier and co-workers [16,51-54]. Very recently, Cacace and co-workers [57] have reported formation of protonated isopropylbenzene in the gas-phase reaction of protonated benzene with propene and have presented evidence that the reaction proceeds by formation of the complex b which subsequently forms a, the reverse of the pathway believed to be involved in the fragmentation of protonated isopropylbenzene.

Figs. 4 - 6 present the breakdown graphs for the MD + ions of p-ethyltoluene, p-isopropyltoluene

IOO

~ 8O z a z

on

Z 6C 0

I- Z l.iJ ~E

h O N 2(

I I I I I I I I

~O_CH2CHzCH,zCH3,D +

~ N2CT D

C4H;

o ° I I I I 2 3 4 5 6 7 8

COLLISION ENERGY (eV, centre of mass)

Fig. 3. B r e a k d o w n graph for deuteronated phenyl n-butyl ether.

I00

8O z

c'~ Z

Ix}

Z 60 0

l- Z LIJ

t.9 40

IZ la_

ii 0

N 20

I ' I ' I ' I

H3C--~C2H 5" D e

N 2C T D

0 +

O ~ 0 2 4 6 8

COLLISIO~ ENERGY (eV, centre of m a s s )

Fig. 4. B r e a k d o w n graph for deuteronated p-ethyl to luene .

162 A. G. Harrison, J. - Y. Wang~International Journal of Mass Spectrometry and 1on Processes 160 (1997) 157-166

I00

8O Z

Cl

Z

m .,~

60 Z 0

l.- z w ~E co 40

n.* u.

14= o

~ 20

J I I I I I 1 I

- - /CH3 \ H3C-~f~>-CH • D"

' = " "c~ N2CI D

~ CeHsD +

I I I I I I I I I 2 3 4 5 6 7 8

COLLISION ENERGY (eV, c e n t r e o f m a s s )

Fig. 5. Breakdown graph for deuteronated p-isopropyltoluene.

w 8O Z ,,~ (3 Z

m

6C Z o

I - Z bJ

~ 40 o:

o N 20

I I I I I I f I I

H~c~-~>-C~s.D +

NzCT D

D*

c6~

0 I F - I I I I I 1 I I 0 I 2 3 4 5 6 7 8 9

COLLISION ENERGY (eV0 centre o f m o s s )

Fig. 6. Breakdown graph for deuteronated p-diethylbenzene.

and p-diethylbenzene, respectively. The dominant fragment ion observed at low internal energies for both the ethyltoluene and the isopropyltoluene systems is CH3C6HsD +. This is as expected since PA(C6HsCH3) = 189.8 kcal mo1-1 [58] compared to PA(C2H4) -- 162.6 kcal mol -] [58] and PA(C3H6) -- 179.5 kcal mo1-1 [58]. However, with increasing collision (and, thus, internal) energy, fragmentation of the MD ÷ ions to form the alkyl ions C2H~ and C3H~, respectively, increases rapidly in importance. The MD + ion of p-diethylbenzene fragments in a similar way, the only additional feature being the loss of a second ethylene from C2HsC6HsD + to form C6H6 D÷. For the isopropyltoluene system minor yields (never exceeding 2% of the total ion signal) of CH3C6H~ and C3H6 D+ were observed; no isotopic mixing was observed for the other systems studied.

The CID results are entirely consistent with the conclusion [50-54] that protonated alkylbenzenes fragment by way of intermediate ion/ neutral complexes R+...ArH, where ArH is a neutral aromatic molecule. At low internal energies the thermochemically favoured but entropically disfavoured proton transfer reaction to form (R-H) + ArH~ occurs, while at high internal energies the entropically favoured reaction of simple bond cleavage to form R + + ArH becomes increasingly important.

3.3. Fragmentation of deuteronated benzaldehyde

Protonated benzaldehyde and protonated tolualdehyde fragment on the metastable ion time scale by elimination of CO to form the appropriate protonated aromatic ArH~ [59,60]. In a detailed study of the metastable ion fragmentation reactions of protonated methoxymethyl- substituted benzaldehydes [59] and naph- thaldehydes [61], Filges and Griitzmacher observed loss of HCOOCH3 which they interpreted in terms of formation of an intermediate

A. G. Harrison, J.- Y. Wang/International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166 163

HCO ÷..-ArCH2OCH 3 ion/neutral complex; interaction of HCO ÷ with the methoxymethyl substituent leads to elimination of HCOOCH3 and formation of ARCH,.

Fig. 7 shows the breakdown graph obtained for deuteronated benzaldehyde. As expected, C6H6 D÷ is the only product observed at low collision energies. However, with increasing collision energy, HCO ÷ is observed, rising to about 25% of the total fragment ion signal at 10 eV centre-of-mass collision energy. Since PA(C6H6) - - 1231.3 kcal mo1-1 [58] and PA(CO) = 142.4 kcal tool -1 [58], it is clear that fragmentation to form C6H6 D+ + CO is strongly favoured thermochemically over formation of HCO ÷ + C6HsD. The present results are consistent with the mechanism outlined in Fig. 1 and illustrate dramatically the effect of the entropic bottleneck in going from the complex a (HCO+.--C6HsD) to the complex b (C6H6D÷...CO); it is this entropic bottleneck which is responsible for formation of the products HCO ÷ + C6HsD at high internal energies.

I00

80 Z <~ n z

nn <

60 Z 0

Z W

0

2O

0

m,- 0 I ' I ' I ' I ' I

~ C 6 H 6 ~

/CHO- D"

NzCI D

H ÷

, I i " ~ - ' i - - " T I I I I I 2 4 6 8 I 0 COLLISION ENERGY (eV, (;entre of mQss)

Fig. 7. Breakdown graph for deuteronated benzaldehyde.

3.4. Fragmentation of deuteronated acetophenones

Protonated acetophenone and protonated p- methylacetophenone fragment exclusively to form CH3CO + + C6HsY (Y = H or CH3) both in metastable ion fragmentation reactions and under low-energy CID conditions [60]. Filges and Grfitzmacher have observed elimination of CH3COOCH3 from protonated methoxymethy- lacetophenones [62] and protonated methoxy- methylacetonaphthones [61], a reaction similar to the loss of HCOOCH3 from protonated methoxymethylbenzaldehydes and naphthalde- hydes. They interpreted the results in a similar manner involving the intermediacy of a CH3CO ÷...ArCH2OCH3 ion/neutral complex within which the acetyl ion migrates to interact with the methoxymethyl substituent.

The formation of only CH3CO +, the nominal product of simple bond fission, for protonated acetophenone and protonated methylaceto- phenones is not surprising since PA(CH2=CO) = 198 kcal mo1-1 [58] compared to PA(C6H6) = 181 kcal mo1-1 [58] and PA(CH3C6Hs) -- 190 kcal mo1-1 [58]. Since proton transfer from CH3CO + to the aromatic is substantially endothermic, a CH3CO+---ArH ion/neutral complex will always dissociate to CH3CO + + ArH since this exit channel is favoured both thermochemically and entropically. By appropriate choice of substituent on the aromatic ring one should be able to increase the proton affinity of the aromatic moiety sufficiently to reverse this behaviour. Fig. 8 shows the breakdown graph obtained for the MD + ion of p-aminoacetophenone. Since PA(HzNC6Hs) = 210 kcal mo1-1 [58] compared to PA(CH2=CO) -- 198 kcal mo1-1, it is not surprising that H2NC6HsD + is the dominant fragment ion at low collision energies; the observation that CH3CO + increases rapidly in importance with increasing collision energy is consistent with the intermediacy of a CH3CO ÷---C6H4DNH2 ion/neutral complex. The site of protonation/deuteronation of the aniline is


I00

~ 8 0 Z < r~

Z m <

6O Z ©

l-- Z l.J

~ 4c < n - i ,

i t

0

N 2C

I I I I [ [ I I

0

H2N--~}-- C C H3 D+

N2CI D

I

o I I I I I I I I I 0 I 2 3 4 5 6 7 8 9

COLLISION ENERGY (eV, centre of mess)

Fig. 8. B r e a k d o w n graph for deu te rona ted p - a m i n o a c e t o p h e n o n e .

not certain; the PA of 210 kcal mo1-1 refers to protonation at the amino group [63] but it is known [64] that the ring proton affinity is only slightly less than the amine proton affinity.

4. Conclusions

The present work has explored, by low-energy CID, the energy dependence of the fragmentation of the MD ÷ ions of a number of alkyl phenyl ethers, alkylbenzenes and aromatic carbonyl compounds. With the exception of the acetophenone and p-methylacetophenone systems, a common behaviour is observed. At the lowest collision energies the thermochemically favoured products are observed and involve hydrogen transfer from the alkyl or acyl moiety to the aromatic moiety producing deuteronated phenols for the ethers and deuteronated benzenes or substituted benzenes for the alkylbenzenes and carbonyl compounds. With increasing collision energy, apparent simple bond cleavage occurs

to form the thermochemically disfavoured products, an alkyl or acyl ion, and the appropriate aromatic neutral molecule.

These results are most readily interpreted in terms of the stepwise mechanism depicted sche- matically in Fig. 1. Activation of the deuteronated species produces initially a R+...M ion/ neutral complex a where R + is an alkyl or acyl ion. The thermochemically favoured route for fragmentation of this species is by proton transfer from R ÷ to M to give the complex (R-H)--- +HM, b, which dissociates to (R-H) + MH +. This proton transfer reaction proceeds over an energy barrier (for protonated isopropylbenzene the barrier has been estimated [16] to be about 6 kcal mol -]) and involves a tight or ordered transition state with the result that although the proton transfer is favoured thermochemically it is disfavoured entropically. As a result, with increasing collision energy (and, thus, internal energy) the R+...M complex dissociates by the entropically favoured reaction, simple bond cleavage, to give R + + M. We believe that formation of the R ÷ ion does occur through intermediacy of the complex a rather than by direct bond cleavage, although this is a view that is difficult to prove conclusively. Some support for the intermediacy of a in the formation of R ÷ comes from the observation of deuterium incorporation in the propyl ions formed in the fragmentation of deuteronated n-propyl phenyl ether and isopropyl phenyl ether; this result is most readily rationalized by assuming some reversibility in the formation of b from a. One might have expected a similar reversibility for the n-butyl phenyl ether system and the absence of deuterium incorporation in this system is rather surprising. The absence of deuterium incorporation in the alkyl ions in the alkyl- benzene systems is not particularly surprising since the proton affinities of the aromatic moiety are higher than that of the alkene. For deuteronated acetophenone and p-methylacetophenone formation of C H 3 C O ÷ is both the thermochemically and entropically favoured product

A. G. Harrison, J. - Y. Wang~International Journal of Mass Spectrometry and Ion Processes 160 (1997) 157-166 165

(proton transfer from CH3CO + to the aromatic is endothermic) with the result that the acetyl cation is the only product observed at all collision energies.

Acknowledgements

The authors are indebted to the Natural Sciences and Engineering Research Council of Canada for financial support and to Professors T.H. Morton and H.E. Audier for helpful discussions.

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Fragmentation of deuteronated aromatic derivatives: The role of ion-neutral complexes