ORGANIC MASS SPECTROMETRY, VOL. 22, 637-641 (1987)

Hydrogen Interchange Prior to the Fragmentation of Protonated Molecules

Alex. G. Harrison? Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada

Protonated ethanol shows extensive interchange of hydrogen between the carbons and oxygen prior to fragmentation to either [C2HS]++ H,O or to [H,O]++ CzH4. Protonated isopropanol shows only a small amount of interchange of hydrogens between carbon and oxygen prior to fragmentation, while protonated alkyl amines show practically no interchange of hydrogens between carbon and nitrogen prior to fragmentation. These results, and earlier results (Terlouw et al., Org. Mass Spectrom. 21, 665 (1986)) showing hydrogen interchange between carbon and oxygen in protonated methyl isopropyl ether, are discussed in relation to the energetics for fragmentation of the protonated species [RHXH]+. It is concluded that as long as the proton affinity of XH is greater than the proton affinity of the olefin R by no more than -50 kJ mol-' hydrogen interchange between the alkyl group and the heteroatom will occur.

INTRODUCTION

Terlouw et a/.' have recently reported that, prior to or during unimolecular fragmentation to [CH30H2]+, pro- tonated isopropyl methyl ether undergoes considerable interchange of the added (oxygen-bonded) hydrogen with the isopropyl hydrogens. Thus, protonation of (CD3)2CHOCH3 leads to formation of [CH3D20]" and [CH,DO]+ in the ratio 100:30. If the hydrogen added to the oxygen remained bonded to oxygen one would expect to see [CH,DO]+ and, possibly, [CH,O]+ but no [CH3D20]+. More surprisingly, they observed a low yield (7.4%) of [CH3D20]+ when protonated (CH3)2CHOCD3 underwent fragmentation, indicating some exchange of the methyl and isopropyl hydrogens.

The present work reports a similar study of the extent of interchange of hydrogen between the heteroatom and the carbons in a number of protonated alkyl alcohols and amines prior to or during unimolecular or low- energy collision-induced dissociation (CID). To sum- marize briefly, protonated ethanol shows extensive inter- change of the oxygen-bonded hydrogens with the ethyl hydrogens prior to formation of either [H30]+ or [C2Hs]+, protonated isopropanol shows only a small extent of interchange prior to formation of [C3H7]+ and protonated alkyl amines show essentially no interchange of the nitrogen-bonded hydrogens with carbon-bonded hydrogens prior to fragmentation. These results, and the results for protonated methyl isopropyl ether,' are rationalized in terms of the mechanism of fragmentation of the protonated species and the relative energetics or potential energy profiles for these fragmentation reac- tions. Such potential energy profiles have been f o ~ n d ~ - ~ to be particularly useful for the interpretation of the

t Killam Research Fellow 1985-87.

0030-493)3/81/ 090637 -05$05 .OO @ 1987 by John Wiley & Sons, Ltd.

low-energy fragmentation reactions of isolated organic ions.

EXPERIMENTAL

All experiments were carried out using a VG Analytical ZAB-2FQ mass spectrometer, which has been described in detail.' Briefly, the instrument is a reversed-geometry (BE) double-focusing mass spectrometer with a third stage consisting of a deceleration lens system, an r.f.-only quadrupole collision cell and a quadrupole mass analyzer. In the study of unimolecular fragmentation reactions the appropriate ion was selected by adjusting the magnetic field and underwent fragmentation in the field-free region between the magnetic and electric sec- tors with the ionic products of fragmentation being analyzed by scanning the electric sector voltage.6 In low-energy CID experiments the ion of interest was selected by the double-focusing (BE) mass spectrometer, decelerated to the required energy and introduced into the quadrupole collision cell which contained N2 at a pressure of - 1 x loF7 Torr, as indicated on the ionization gauge attached to the pumping line for the quadrupole stage. This pressure attenuated the main beam by -40%. The ionic fragments were analysed by scanning the final quadrupole. When reactive collisions were examined the N2 collision gas was replaced by the appropriate neutral reaction partner.

The ions of interest were prepared by the reactions discussed below in a chemical ionization (CI) source operating at -150 "C with an ionizing electron energy of 50 eV. Typically, in CI operation, the pressure indi- cated by the ionization gauge located on the source region pumping line was 1 to 5 x lo-' Torr. The ion- accelerating voltage was 8 keV.

All samples used were commercially available and were of the highest purity.

Received 28 January 1987 Accepted 8 May 1987

638 A. G. HARRISON

RESULTS AND DISCUSSION

Protonated ethanol

[C2H50H2]' was prepared in the ion source by i-C,H,, CI of ethanol and subjected to CID in the quadrupole cell at collision energies from 2.5 to 100 eV (laboratory scale). The results are shown in Fig. 1 in terms of a breakdown graph expressing the relative abundances of the fragment ions as a function of collision energy. Both [C2H,]+ and [H,O]+ are observed as primary fragmenta- tion products, with minor fragmentation of the former to [C2H3]+ at higher collision energies. Formation of both [C2H5]+ and [H30]+ has been observed previously in both low-energy' and high-energy',' CID of proton- ated ethanol. Formation of [H30]+ has a slightly lower reaction enthalpy and begins to dominate over formation of [C2H,]+ (eqn ( 2 ) ) at the lowest collision energies.

[C2H,0H2]+ --t [H,0]++C2H4 AH=118 kJmol-' (1)

-+ [C2H5]++ H,O AH = 174 kJ mol-' (2)

The following isotopically labelled ions were prepared and subjected to CID studies at 10 eV collision energy.

(i) [C2H50D2]+, prepared in a C2H50H/D20 mixture, both being introduced through the heated inlet system with D20 in excess.

(ii) [C2D50H2]+, prepared in a C2D50D/H20 mixture, both being introduced through the heated inlet system, with H 2 0 in excess.

(iii) [CH3CD2OH2lf and [CH3CD20HD]+, prepared by self-CI of CH3CD20H introduced through the heated inlet system.

(iv) [C,H,]++D20 adduct, prepared in an ethylbromide/D,O mixture, both being introduced through the heated inlet system. Under the experi- mental conditions used [(C2H5)2Br]+ was a major ion in the spectrum, and the most likely reaction leading to formation of the observed product of

I I I I I ' I I I I I 1

8o t 1 N2 C I D

2ot Collision energy (eV, lab)

Figure 1. Breakdown graph for protonated ethanol

m / z 49 is

[(C,H,),Br]+ +D,O --t [C2H50D2]++C,H,Br ( 3 )

The results of these collisional experiments are presented in Tables 1 and 2 in terms of the relative percentages of [(H, D),O]+ and [C2(H, D)J+ ions observed. Also included in the tables are the ionic distributions calcu- lated on the assumption of complete equilibration of all H /D prior to formation of the fragment ions. Clearly, there is extensive interchange of oxygen-bonded and carbon-bonded hydrogens, the extent of interchange being greater prior to formation of the hydronium ion than it is prior to formation of the ethyl ions. However, in no case is there complete equilibration of all H/D; rather there is a distinct preference for formation of

Table 1. Relative abundances of [(H, D),O]+ in fragmentation of protonated ethanols"

[C,H5OD,l+ IC2D5OH21' (CH,CD,OH,]+ (CH,CD,OHD]* [C,H5-D201' expt calc expt calc expt calc expt calc expt calc

[H3Ol+ 21 29 35 29 16 11 18 29 [HZDOl+ 48 51 28 14 52 57 49 51 41 51 [H DZOI + 32 14 48 57 12 14 32 34 35 14 [D301+ 24 29 3 3

a 10-eV CID

Table 2. Relative abundances of [(C,(H, D)$ ions in fragmentation of protonated ethanols"

lC,H,OD,l [C,D5OH,I+ [CH,CD,OH,]+ [CH,CD,OHD]' IC$'5-D201' expt calc expt calc expt calc expt calc expt calc

[CZH,I' 48 5 4 5 42 5 [CZH,Dl* 29 48 21 48 8 14 35 48 [CzH3Dzlt 23 48 70 48 65 57 24 48 [CzHzD31+ 19 48 26 29 [CZHD,I + 21 48 [ C A I ' 55 5

a 10-eV CID

HYDROGEN INTERCHANGE PRIOR TO FRAGMENTATION 639

ethyl ions by simple bond cleavage and for formation of hydronium ions by simple transfer of one hydrogen from the ethyl group to oxygen followed or accompanied by bond rupture. The results obtained for the [C,Hs]+/D20 adduct are very similar to the results obtained for [C2HsOD2]+, implying the same structure for both species. Similar conclusions have been reached' from high-energy CID studies of protonated ethanol and the [C2H5]+/H20 adduct.

It should be noted that the CID results showed no significant dependence on the collision energy. Thus, the results at 50eV collision energy agreed with the 10-eV results, within experimental error. In addition, [C2H50D2]+ was subjected to 8-keV collisions with He and gave the distribution [H,O)+ : [ H,DO]+ : [HD,O]+ = 23 :53 :24, not dissimilar from the results in Table 1. In the high-energy CID experiment, isotopically mixed ethyl ions were observed; however, their relative abun- dances could not be determined because of interference in the same mass range. The ions formed by proton transfer CI showed no detectable unimolecular frag- mentation reactions. However, the adduct of [C,H,]+ with D 2 0 did undergo unimolecular fragmentation in the field-free region and gave [ H,O]+ : [ H2DO]+ : [ HD20]+ = 45 : 43 : 12. There is considerably more hydrogen interchange prior to unimolecular fragmenta- tion than for the same species subjected to collisional activation.

Attempts were made to form an adduct of [D,O]+ with C2H4 in the ion source; however, no significant ion signal was observed at m l z 50. Therefore, the following experiment was performed. [D,O]+ ions, formed in the ion source, were collided with C2H4 in the quadrupole collision cell at 0.5 eV (lab scale) collision energy and an indicated ethylene pressure of 5 x Torr. Although the major reaction observed was simple deuteron trans- fer forming [C2H4D]+ ( 6 6 % ) , significant yields of [C,H,D,]+ (3%), [HD,O]' (12%), [H,DO]+ (7%) and [H,O]+ (2%) were observed. The observation of these isotopically mixed products indicates that in at least some of the collision events a complex is formed between [D,O]+ and C2H4 which survives sufficiently long for hydrogen interchange between carbon and oxygen to occur.

Protonated isopropanol

Protonated i-propanol, on low-energy collisional activa- tion, fragments to give [C3H7]+, with minor yields of [C,H,]+ and [C2H3]+ at higher (>30 eV) collision ener- gies. In contrast to high-energy collisional activation,' no [H,O]+ product is observed. CID of [i-C3D70H2]+ (produced by i-C4HI0 CI of i-C3D70H) gave [C3D7]+:[C,D,H]+:[C,D,H2]+=88: 10:2 over the col- lision energy range 10-100 eV. Complete equilibration of the H/D would lead to [C3D7]+:[C3D6H]+: [C3DSH2]+ = 3 : 39: 58; clearly the extent of interchange is much smaller than for the protonated ethanol case.

Protonated amines

[RND,H]+ ions were prepared by i-C4HI0 CI of RNDz (prepared in the heated inlet system by exchange of

RNH2 with D,O) for i-propyl and the four butyl amines and the unimolecular fragmentation reactions occurring in the field-free region were monitored. For protonated i-propyl, s-butyl and t-butyl amines the dominant meta- stable ion fragmentation reaction forms the ammonium ion, while for protonated n-butyl and i-butyl amines formation of the butyl ion also is observed."

The isotopic distribution observed in the metastable ion fragmentation reactions of [ RND2H]+ are summar- ized in Table 3. The ammonium ion is observed pre- dominantly as [NH,D,]+. For the protonated butyl amines complete equilibration of the H/D would lead to [ NH2D2]+ : [ NH,D]+ : [NH,]+ = 9 : 49 : 42, while for the protonated i-propyl amine complete equilibration would lead to the relative intensities 13 : 53 : 33. Clearly very little hydrogen interchange has occurred; rather, the ammonium ion results mainly from the transfer of one hydrogen from the alkyl group to nitrogen. For the n-butyl and i-butyl systems the butyl ions are observed primarily as [C4H,]+ with only a minor yield of [C4H8D]+, again indicating that practically no hydrogen interchange between nitrogen and carbon has occurred.

Mechanism of hydrogen interchange

The two fragmentation reactions of interest in the present discussion are the simple bond cleavage reaction (4), yielding an alkyl ion and a stable neutral molecule, and reaction ( 5 ) , involving bond cleavage accompanied by hydrogen rearrangement and yielding a protonated heteroatomic species plus an olefin.

[RHXH]+ -+ [RH]++X (4)

-P [XHZ]++R ( 5 )

There is ample evidence that the latter reaction does not occur in a direct concerted fashion but rather involves the intermediate formation of ion-molecule comDlexes.'0-'9 A reaction mechanism. incorDorating botc product channels, is proposed in Scheme 1.

RH+,+XH I

RHXH+ + RH+---XH + R---H+---XH -, R - - - x H ~

1 2 3 I

R + H2X+

Scheme 1.

The

detailed mechanism differs from those proposed earlier in that it includes the involvement of 2, a proton-bound complex of an olefin and a neutral molecule XH; in this complex the proton will be more strongly bonded to the

Table 3. Relative abundances of "(H, D)$ and [C,(H, D)$ in unimolecular fragmentation of protonated amines

" H P l + "H2D21t [C,Hsl+ IC,HaDl'

[i-C3H7ND2H]+ 1.7 98.3 [ n-C,H,ND,H]+ 1.8 98.2 96.7 3.3 [i-C4HgND2H]+ 5.2 94.8 95.5 4.5 [ s-C,H,ND,H]+ 3.4 96.6 [ f-C4HgND2H]+ 1.8 98.2

640 A. G. HARRISON

Table 4. Reaction enthalpies [RHXH]+ AH,(kJ rnol-') AH,(kJ mol-')

[C2H,OH21+ 139 99

[ i-C3H7N H3]+ 231 118

[i-C3H,0H2]+ 114 135 [i-C,H,OCH,. H]+ 163 1 34

[ s-C4H,NH3]' 21 1 114 [ t-C4H9NH3]+ 159 125

species of higher proton affinity. The inclusion of this complex is based on our recent ab initio calculations of the potential energy profile for fragmentation of proton- ated i-propyl amine," although such proton-bound com- plexes have been proposed earlier as intermediates in the fragmentation of protonated species.'710320-22

Hydrogen interchange between the alkyl group and the heteroatomic function will occur if the ion-induced dipole complex 3 can revert to the ion-dipole complex 1. The species 3 represents a complex of [XH2]+ and the olefin, which is sufficiently loosely bound that the hydrogen transferred in the reverse reaction can be any one of the hydrogens bonded to X. In this respect it differs from 2, which proposes a specific hydrogen in- volved in the interaction. Whether or not 3 can revert to 1 and result in hydrogen interchange depends on the relative energies of 1, 3 and the products of fragmenta- tion. If l and 3 are lower in energy than the products of either reaction (4) or ( 5 ) , hydrogen interchange should be observed.

The heats of formation of the protonated species [RHXH]+ and the products of fragmentation are sum- marized in the Appendix. These data lead to the values for AH, and AH, presented in Table 4 for the major species involved in this work. The ion-induced dipole complex 3 should be slightly lower in energy than the separated products [H2X]++ R. The recent ab initio calculations on the protonated i-propyl amine system" show that 2 is a minimum on the potential energy surface and is -40 kJ mol-' lower in energy than [H2X]+ + R. The same calculations show that the ion-dipole complex 1 is -57 kJ mol-' lower in energy than [RH]++XH. This stabilization energy is consistent with the inter- action of an ion with a dipole of 1.5-2.0 Debye at a separation of 0.25-0.30 nm.23

If we accept that 1 is stabilized by -50 kJ mol-' rela- tive to [RH]++XH and that 3 is only slightly lower in energy than [XH,]++R, interconversion of 3 and 1 will occur provided AH4- A H 5 s 50 kJ mo1-' and AH, < A H4. Examination of the thermochemical data in Table 4 shows that, for protonated ethanol, 1 should lie lower in energy than any of the products, and probably lower in energy than 3; consequently hydrogen interchange would be expected to occur. Similar comments apply to the protonated methyl i-propyl ether system. However, for protonated i-propyl and protonated s-butyl amines the present model predicts that the energy of 1 is con- siderably greater than that of [H2X]++ R, or of 3. Con- sequently any protonated species which achieve the configuration 1 will have more than sufficient energy to fragment to the products [XH,]++ R and likely will do so rapidly and without significant hydrogen interchange. On the other hand, the thermochemical data for [ t - C4H9NH3]+ suggest that hydrogen interchange should

Table 5. Thermochemical data (kJ mol-')

M AH,(M) PA(M)

H20 -242 724

NH3 -46.1 858 CH,OH -201 774

i-C,H,OH -273 808b i-C3H,0CH3 -252" 837b i-C3H,NH2 -83.8 91 8

i-C,H,NH, -104 91 8

C2H,0H -217 796

n-CaHsNH2 -92.0 916

s-C,H,NH, -104 923 t-C4H,NH2 -120 926 C2H4 52.3 684

2-C4H, -9.1 76 1 I-C,H, -16.9 824

Estimated from proton affinities of related compounds.

C3H6 20.4 745

a Estimated by group equivalent method (Ref. 27).

AHJMHI+)

565 627 556 51 8 450 442 529 523 510 504 485 899 806 761 690

be observed in this system; experimentally it is not observed and the reasons are not clear. Protonated n- butyl amine is considered2, to fragment by rate- controlling formation of an [ n-C4H,]+-NH3 ion dipole complex which collapses to [ s-C4H9]+-NH3 before frag- mentation. The [ n-C4H,lt-NH, complex will be even higher in energy than the [s-C4H9]+-NH3 complex and, hence, hydrogen interchange would not be expected. Similarly, [i-C,H,]+ is believed to form initially a [i- C4H9]+-NH3 complex; the initial complex is sufficiently high in energy to preclude significant hydrogen inter- change.

Protonated i-propanol is an interesting system in that the [RH]++XH products are lower in energy than [H,X]++ R; the low-energy CID of [i-C3H,0H2]+, in agreement, yields only [C3H7]+. In this case the energy of 1 is below that of either of the sets of products; however, the energy 3 will be above the energy of [RH]++XH. Hence, any ions which have sufficient energy to reach the structure 3 will have more than enough energy to fragment to [C3H7]++H20 and little hydrogen interchange would be expected.

The basic conclusion of the model proposed is that hydrogen interchange is likely to occur prior to frag- mentation of [ RHXH]+ provided A H4 - AH, 50 kJ mol-I. This difference can be expressed in terms of proton affinities (PA) as

AH4 - AH5 = PA(XH) - PA(R) ( 6 )

Thus, provided PA(XH) - PA(R) s 50 kJ mol-', hydro- gen interchange between the carbons of the alkyl group and XH in [RHXH]+ would be expected to occur. Examination of relative proton affinities indicates that hydrogen interchange is unlikely in protonated alkyl amines and for alcohols will occur extensively only for protonated ethanol. However, such interchange may be relatively common for a number of protonated alkyl ethers and esters.

Acknowledgements The author is indebted to the Natural Sciences and Engineering Research Council for a grant which made possible purchase of the ZAB-2FQ and for support of the present research. The award of a Killam Fellowship by the Canada Council is gratefully acknowledged.

HYDROGEN INTERCHANGE PRIOR TO FRAGMENTATION 64 1

REFERENCES

1. J. K. Terlouw, T. Weiske, H. Schwarz and J. L. Holmes, Org. Mass Spectrom. 21, 665 (1 986).

2. D. H. Williams, Acct Chem. Res. 10, 280 (1977). 3. R. D. Bowen. D. H. Williams and H. Schwarz, Angew. Chem.

lnt. Ed. fngl. 14, 451 (1979). 4. R. D. Bowen and D. H. Williams, in Rearrangements in Ground

and Excited States, ed. by P. DeMayo, Vol. 1, Ch. 2, Academic Press, New York (1980).

5. A. G. Harrison, R. S. Mercer, E. J. Reiner, A. B. Young, R. K. Boyd, R. E. March and C. J. Porter Int. J. Mass Spectrom. /on Processes 74, 13 (1 986).

6. R. G. Cooks, J. H. Beynon, R. M. Caprioli and G. R. Lester, Metastable lons, Elsevier, New York (1973).

7. P. H. Dawson, lnt. J. Mass Spectrom. /on Phys. 50, 287 (1983). 8. M. F. Jarrold, A. J. Illies, N. J. Kirchner and M. T. Bowers, Org.

9. M. Meot-ner (Mautner), M. M. Ross and J. E. Campana, J. Am.

10. E. J. Reiner, R. A. Poirier, M. R. Peterson, I. G. Csizmadia and

11. R. D. Bowen, B. J. Stapleton and D. H. Williams, J. Chem. SOC.

12. R. D. Bowen and D. H. Williams, J. Am. Chem. SOC. 102,2752

13. R. D. Bowen and D. H. Williams, J. Chem. SOC. Chem. Commun.

Mass Spectrom. 18, 388 (1983).

Chem. SOC. 107,4839 (1 985).

A. G. Harrison, Can. J. Chem. 64, 1652 (1986).

Chem. Common. 24 (1 978).

(1980).

836 (1981).

14. T. H. Morton, J. Am. Chem. SOC. 102, 1596 (1980). 15. R. D. Bowen, J. Chem. SOC., Perkin Trans 2 409 (1982). 16. T. H. Morton, Tetrahedron 38, 3195 (1982). 17. C. R. Moylan and J. I. Brauman, J. Am. Chem. SOC. 107, 761

18. P. Ausloos and S. G. Lias, J. Am. Chem. SOC. 108, 1792 (1986). 19. V. Filges and H.-F. Grutzrnacher, Org. Mass Spectrom. 21, 673

20. R. D. Bowen and D. H. Williams, J. Am. Chern. SOC. 100, 7454

21. J. J. Stamp, E. G. Siegmund, T. Cairns and K. K. Chan, Anal.

22. M. Weiss, R. A. Crombie and A. G. Harrison, Org. Mass Spec-

23. R. D. Bowen and D. H. Williams, lnt. J. Mass Spectrorn. lon

24. E. J. Reiner, A. G. Harrison and R. D. Bowen, to be submitted. 25. J. D. Cox and G. Pilcher, Thermochemistry of Organic and

Organometallic Compounds, Academic Press, New York (1 970). 26. D. H. Aue and M. T. Bowers, in Gas Phase /on Chemistry, ed

by M. T. Bowers, Vol. 2, Academic Press, New York (1979). 27. S. W. Benson, Thermochemical Kinetics, Wiley, New York

(1 976).

(1 965).

(1 986).

(1978).

Chem. 58,873 (1 986).

trom. 22, 21 6 (1 987).

Phys. 29, 47 (1979).

APPENDIX

The thermochemical data used in the present work are listed in Table 5. Unless otherwise noted the heats of formation of neutral species are taken from Cox and

Pilcher2’ while the proton affinities are from Aue and Bowers.26