Protonated Ethanol and Its Neutral Counterparts

Chrysostomos Wesdemiotis Department of Chemistry, The University of Akron, Akron, Ohio, USA Aberra Fura and Fred W. McLafferty Baker Chemistry Laboratory, Department of Chemistry, Cornell University. Ithaca, New York, USA

Collisionally activated dissociation and neutralization-reionization experiments reveal that protonation of ethanol leads to two distinct isomers, the classical ion CH 3CH20Hi and the proton-bound complex C2H4 ••• H+ ... OH2 • The neutral counterpart of the latter is unstable, whereas that of the former can be produced in a bound state if the CH 3CH20Hi precursor ion is formed under low ion source pressure conditions and, thus, with higher internal enerfes. This suggests that there are substantial differences in the geometries of CH 3CH20H2 and the hypervalent CH 3CH20H2 .. This provides only a partial explana­ tion for unusual isotope effects; C2HsOD 2 ', CH 3CD20D2 ', and CD3CH20D2 • are substantially more stable than C2DsOD 2 • and C2HsOH2 •• (J Am Soc Mass Spectrom 1991, 2,459-463)

he unimolecular reactions of protonated ethanol [3]. Neither group suggested that such a complex have been the subject of several investigations could represent a stable, bound C2H70+ isomer. In T[1-5]. Bowers and co-workers [4b] observed that the present study we provide evidence from CAD and the intensities of the fragments formed upon collision­ neutralization-reionization mass spectrometry (NRMS) ally activated dissociation (CAD) [6] vary with ion experiments [7] that the proton-bound complex source pressure, indicating dependence on internal C2H4 .•• H+ ... OH2 (2+) is also formed upon the energy or angular momentum. Extensive labeling ex­ protonation of ethanol and survives long enough (> periments by Harrison [3] and by Bowers and co­ 10-5 s) to yield characteristic mass spectra. High-level workers [4a] showed further that the major dissocia­ molecular orbital calculations by Radom and co­ tions into C2H t + H 20 and H 30++ C2H4 are pre­ workers [8a] and by Bouchoux and Hoppilliard [Bb], ceded by substantial H scrambling between the hy­ reported since the completion of this study, predict drogens of the carbons and oxygen. Although that 2+ represents an energy minimum lying 53 [8a] CH3CH20Hi (1+) isomerization to protonated di­ or 69 [Sb]kJ mol "! above 1+ and separated from it by methyl ether has been excluded by these groups, a barrier of 42 or 51 k] mol-1. rearrangement to other stable C2H70+ structures has The counterneutral of protonated ethanol, the hy­ not been considered. The scrambling has been as­ pervalent CH 3CH20H 2 • r has not yet been identified sumed to occur in the dissociation process, via a and is also characterized in this study. Porter and C2H4 ••. +OH3 [4a] or C2H4 ••• H+ ... OH2 [3] co-workers [9] found that the similar but smaller hy­ ion-molecule complex that is sampled several times pervalent homologs CH 30Hz ' and OH 3 • are not before fragmentation. This claim was substantiated by stable (lifetime < 10-9 s), but that the perdeuterated showing that the CAD spectra of CH 3CH20H! and species are stable and experimentally observable (life­ 6 of C2H70+ generated in the ion source by association time - 10- s). Even more unusual isotope effects of C2H4 and OHj are very similar [4a]. Similarly, the have been observed; 180D3 . is reported to be far less reaction of ODt with C2H4 in a quadrupole collision stable than 160D3 . [9]. A recent NR study by Holmes cell produced signifIcant quantities of C2H3Dt, and Sirois [10]of protonated dimethyl ether finds that OHDt, OH 2 D+, and OHj, indicating that a complex the isomeric CH30H2CH3 ' can be produced as a between ODt and C2H4 is formed "which survives bound neutral. sufficiently long for hydrogen interchange to occur" Experimental Presented at the Thirty-Eighth ASMS Conference on Mass Spec­ trometry and Allied Topics, Tucson, June 1990. Address reprint requests to Chrysostomos Wesdemiotis. Depart­ The experiments were performed with the Cornell ment of Chemistry, The University of Akron, Akron, OH 44325. tandem double-focusing mass spectrometer (EB-EB)

© 1991 American Society for Mass Spectrometry Received November 5, 1990 1044-0305/91/$3.50 Accepted April 25, 1991 460 WESDEMIOTIS ET AL. J Am Soc Mass Spectrom 1991, 2,459-463 described in detail elsewhere [11]. For NR spectra, lO-keV precursor ions are mass-selected by MS-I (EB) and enter the neutralization collision cell (CIs-I) where they undergo charge-exchange collisions with Hg va­ por. Any remaining ions are removed by electrostatic deflection and the resulting neutral beam enters the reionization cell (Cis-II) where it is collisionally reion­ ized by 02 targets into cations. The resulting ions are mass-analyzed through MS-TI (E-II). For CAD spectra, the precursor ions undergo dissociating collisions with 02 (70% transmittance) targets in Cis-II. The NR Hg/02 spectra [12] employed transmittance values of 90% for Hg and 70% for 02' corresponding to single­ collision conditions [13]. Neutral beam abundances are measured at a retractable in-line detector after Cis-II. The parent ions C 2H sOHL C 2H sODL CH 3CD20Hi, CD3CHpHi, and C2DsODi were produced by self-chemical ionization under high or low ion source pressure conditions. CH 3CD20Di and CD3CH20Di were generated in the ion source by mixing D20 with CH 3CD20H and CD3CH20H, re­ spectively. The [MH+]:[M+· ] ratio in the ion source was :2: 6 under high and :!> 0.5 under low ion source pressure. The contamination extent of NR MH+ (or MD+) with the BC (or 180) isotope of M+ was deter­ mined from [MH+]:[M+· J as well as the NR effi­ ciencies of MH+ (or MO+) and M+· measured under identical experimental settings [12]. All samples were obtained commercially and used without further Figure 1. (a-d) CAD and (e-h) NR spectra of (a, e) purification. CH 3CH20Hi, (b, f) CH 3CH20D;, (c, g) CH,CDpHr, and (d, h) CD 3CH 20 H ! under high ion source pressure conditions. Results and Discussion

C2H70+ Ions proton affinity of water in comparison to that of ethy­ lene (697 versus 680 k] mol- 1 [14]),2+ should resem­ High ion source pressure. C2HsOHi parent ions pro­ duced under thermalizing (high ion source pressure) ble more a complex of ethylene plus hydronium conditions lead upon CAD to two types of fragmenta­ (C2H 4 ••• OHj) than a complex of ethylium plus tion processes (Figure 1, left column, and Table 1 for water (C2Ht ... OH2). thermochemical values): (1) reactions preceded by The isomerization to 2 + indicates the initial form­ ation of energetic 1+ ions. The self-protonation substantial H exchange such as the formation of H 30+ C2HsOH+' + C2HsOH --. C2HsOH! + C2HsO· is and C2Ht by loss of C2H4 and H 20, respectively; 1 and (2) eliminations accompanied by no appreciable exothermic by 65 k] mol- [14]; the sum of this energy scrambling such as the specific losses of H . (methyl­ plus any internal energy of nondissociating C2H deposited upon electron impact (:!> 26 k] ene or methyl hydrogen) and CH 4 (methyl group and sOH+' one of the hydroxylic H atoms). These results can best mol" ') defines the maximum initial energy content of be rationalized by assuming the presence of two dis­ C2HsOHr (::!> 91 kJ mol " ')." This also is the upper tinct C isomers, namely, the initially formed limit of the barrier for 1+--' 2+ and compares favor­ 2H50H! 1 classical ion 1 + and the proton-bound complex 2 + to ably with one calculated value (95 k] mol- [8a]), but 1 which 1 + can partly isomerize after its formation less favorably with the other (120 k] mol- [Bb]). According to theory [8], ion 1+ is substantially (Scheme I). CH 20H+ and 'C2H40H! are then more stable than 2+, so that equilibration between formed from 1+ via specific 1,2-CH 4 and H. elimina­ these two forms with energies above the isomeriza­ tions, respectively, whereas C2Ht and H 30+ origi­ nate from 2+ in which extensive H exchange can take tion barrier should greatly favor 1+. Nevertheless, 2+ place prior to dissociation. The 2+ ions formed di­ rectly by association of C2H 4 with +OH3 should lose substantially less CH and H . , as indeed observed 4 'The upper limit of the internal energy of C 2 H sOH +, is 26 k] by Bowers and co-workers [4a). Because of the larger mol r ', the dissociation threshold to CH 3CH+OH + . H (14}. J Am Soc Mass Speclrom 1991, 2, 459-463 PROTONATED ETHANOL AND NEUTRAL COUNTERPARTS 461

Although water and ethylene can also be formed by simple CAD of 2+, cogenerated with czHt and

H 30+ (Scheme I), this accounts for only a minor amount of HzO+· and CzH! . formed by NR. With ---- Hg targets the CAD fragment ion yield is only 0.2%, coproducing 0.2% neutrals. However, the total yield (no H scrambling) ---- of neutrals is 2.5%, indicating that most neutral prod­ ucts are formed after charge exchange. Furthermore, the absolute abundances of HzO+' and czHt . de­ crease to << 20% if Hg is replaced by He, which mainly dissociates parent ions instead of neutralizing them [15]. The CAD and NR spectra of the isomeric protonated dimethyl ether are distinctively different H2C from the spectra of Figure 1, indicating that this ion II" 'H+" 'OB2 does not interconvert easily to either 1+ or 2+ [4, 10]. B2C Low ion source pressure. Under low ion source pres­

sure conditions in which the newly formed CZH70+ (extensive H scrambling) ions are not thermalized, and thus have higher inter­ Scheme [ nal energies, the CAD abundances of H 30+ and CzHt decrease substantially (Figure 2), consistent with the abundance variability with change in internal can coexist with 1+ if its internal energy is dissipated energy for low energy CAD products (those dominant by collisions in the ion source, so that the thermody­ in the metastable ion spectrum) [4a). The fragment namically favored reverse rearrangement 2+-+ 1+ ions from H . and CH 3 . loss become more abundant; (which requires at least 42 kJ mol- 1 critical energy [8]) becomes impossible. In fact, under collision-free con­ ditions the isomerization 1+-+2+ is not observed (see below). Additional evidence for the coexistence of struc­ tures 1+ and 2+ is provided by the NR spectra (Figure 1, right column). Neutralization of the classical ion 1+ produces a hypervalent radical which should dissoci­ ate at the hypervalent center [9), generating mainly CH 3CHzOH + . Hand CzHs' + OH z. Neutraliza­ tion of 2+, on the other hand, should lead to a

mixture of C ZH 4, ·H, and OH z. The major dissocia­ tion products consistent with the NR spectra are ethanol (leading after reionization to its characteristic 30 31 a-cleavage fragments CHzOH+ at mf z 31 and CH 3CH+OH at m j z 45), ethylene (m ] z 24-28), and water (m I z 18). Ethanol originates from ion 1+ in which the H atoms retain their positional identity; however, ethylene and water are formed with their H c atoms scrambled (Figure 1), clear evidence that ion 2+ is present at neutralization because 1+ (or 1) should not form scrambled ethylene.

d Table 1. Reaction enthalpies Ll.H IkJ mol ')

C2HsOHi' ~CH20H++CH. 121

H30++ C2H. 136

C 2H;l" + OH 2 153 e C +. H 443 2H.OHi'· Figure 2. (a-e) CAD and (f-j) NR spectra of (a, f) CH20H!' +. CH 3 447 CH 3CH20H!, (b, g) CH 3CH20Di, (c, h) CH 3CD20Di, (d, i) C03CH200i, and (e, j) CD3CD200i under low ion source See ref. 14. pressure conditions. 462 WESDEMIOTIS ET AL. J Am Soc Mass Spectrom 1991,2,459-463 both are formed without noticeable H scrambling, spectra of the perdeutero sample, the losses of D . indicating that they proceed from the initial classical and C03 • which are diagnostic for structure 1+ are structure 1": their formation involves loss of a hydro­ significantly lower than the analogous losses in the gen from the original methyl or methylene (but not spectra of the other isomers (Figure 2). hydroxyl) groups and loss of the original methyl group, respectively. These same fragmentation proc­ esses are observed in the NR mass spectra, but these Conclusions spectra also show a dramatic new feature. Ion-neutral complexes play an important role in gas­ phase ion chemistry [18]. This study demonstrates CZH70' Neutrals that NR experiments can provide critical information The NR mass spectra indicate that the counterneu­ on whether such species are just transition states for tral is much more stable when formed from these isotopic exchange or are bound structures with finite isomerization and dissociation barriers. Consistent high energy CZH70+ ions (Figure 2, right column), leading to a sizable recovered parent ion (m / z 47 with independent theoretical calculations [8], we find the proton-bound complex 2+ to represent a for CZH70+). The contribution of the isobaric 13C12CH60+' in the NR spectrum of CzH;OHi is stable isomer of the conventional protonated ethanol 80+· ion, 1+. < 30% and the contribution of CzHl in the NR spectrum of CzHsODi is «10%, so that the recov­ For the C2H70 . counterneutrals, only the classical ered parent ions mainly arise by reionization of hy­ structure 1 represents an energy minimum. Its struc­ tural geometry must be distorted from that of 1+ to pervalent CZH70 . that survived intact. As found for other hypervalent neutrals [9], 1 account for the unusual observation that stable 1 is neutrals exhibit relatively high stability for the only formed from excited 1+. - ODt isomers; reionized abundances (relative to 7 that of the precursor ion, x 10- ) are CzHsOHz . , 7; Acknowledgments CzOsOOz', 10; C03CH zODz" 60; CH 3COzODz, We are gratefuJ to M. j. Polce, M.-Y. Zhang, F. Turecek, and D. 70; and CzHsODz, 90. Surprisingly, the perdeutero isomer shows an isotope effect similar to that of E. Drinkwater for assistance and helpful discussions, to L. Radom for a preprint of ref. 8a, to the National Science Founda­ CzHsOHz' . Upon reionization the surviving species lion (grants CHE-8712039 and CHE-9014883) for generous fi­ loses CH 3 • and H. without measurable H exchange, nancial support, and to the National Institutes of Health (grant consistent with the classical structure 1.t The absence GM16609) for partial instrument funding. of appreciable H 30+ and CzHt fragments after NR is also consistent with the lack of surviving CZH70 . radicals of structure 2, and that 1 ---> 2 does not take References place after reionization due to insufficient energy for 1. Dawson, P. H. In/. f. Mass Spec/rom. Ion Phys. 1983, 50, isomerization or/and lack of stabilizing collisions (see 287-297. above). The surprising increase in CH:lCHzOHz . sta­ 2. Meot-ner (Mautner). M.; Ross, M. M.; Campana, j. E. f. bility when formed from excited 1+ ions could arise Am. Chern. Soc. 1985. 107, 4839-4845. 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The sources of the additional observed Howe, I. ]. Am. Chern. Soc. 1973, 95, 2120-2129. Zwinsel­ isotope effects, especially those leading to the instabil­ man. J. j.; Nibbering, N. M. M.; Ciommer, B.; Schwarz. H. In Tandem Mass Spectrometry; McLafferty, F. W.• Ed.: Wiley: ity of C •, are harder to discern. A possible zDsOD 2 New York, 1983; pp 67-104. explanation is that the C2070+ ions contain a much 7. Wesdemiolis, c.. McLafferty, F. W. Chern. Rev. 1987, 87, higher proportion of isomer 2+, which produces an 485-500. Terlouw, j. K.; Schwarz, H. Angew. Chern. Int. Ed. unstable counterneutral. Indeed, in the CAD and NR Engl. 1987, 26, 805-815. Holmes, j. L. Mass Spec/rom. Rev. 1988, 8, 513-539. McLafferty, F. W. Science (Washington. D.C) 1990, 247, 925-929. tCH · and methylene-H. (or methyl-H· ) eliminations from neu­ 3 8. (a) Swanton, D. J.; Marsden, B. C. J.; Radom, L. OrK. Mass tral CH • would produce energy-rich ylides and are there­ 3CH20H2 Specirom., in press. (b) Bouchoux, G.; HoppiIIiard, Y. Am. fore extremely improbable. For example. CJ.H(CH3 . +. CII,OH, ") J. ~ + Z95 k] mol- 1 compared to values of -124 and -17 for Chern. Soc. 1990, 112, 9110-9115. CH,CH,' + OH, and CH 3CH,OII + . H, respectively [14, 16]. 9. Gellene, G. I.; Porter, R. F. Ace. Chern. Res. 1983, 16, Furthermore, ·CH • rejonization yields intense CHxO+ frag­ 20H2 200-207. Raksit, A. B.; Parler, R. F. Mass ments (x ~ 1-3) [17], which are not observed In the NR spectra of lnt. 1. Spectrom. Figure 2. Ion Processes 1987, 76, 299-306; Raksit, A. B.; Porter, R. F. JAm Soc Mass Spectrom 1991, 2, 459-463 PROTONATED ETHANOL AND NEUTRAL COUNTERPARTS 463

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