C. S. Giam

Energy dependence of H-exchange versus CC bond formation in the complex [CH3OH CH2CH+2]: unimolecular and bimolecular studies

The reactions of the ion—neutral complex [CH3OH CH2CH+2] have been examined by unimolecular and bimolecular approaches. We conclude that the distonic ion CH3OH+ CH2CH2 (1) isomerizes to CH3CH2CH2OH+ (5) via the internal ion—molecule reaction [CH3OH CH2CH+2] → [+HOCH2CH2CH3] followed by CC bond formation. Metastable H and H2O losses follow this isomerization. About 60 kJ mol−1 higher (attained by collision-induced dissociation and reaction of CH2CH+2 with CH3OH) CH2OH+ and CH3OH+2 are formed. Hydrogen exchange between oxygen and ethene occurs at this energy, but CC bond formation does not. Hydrogen atoms exchange by interconversion of ion-neutral complexes rather than by isomerizations involving formation of distonic ions. At 33 kJ mol−1 higher still (reaction of CD3OD+ with CH2CH2), H-transfer without exchange occurs. The reactions of [CH3OH CH2CH+2] are thus strongly energy dependent.

Competing dissociation of and bond formation between CH3CHOH+ and .CH2CH3 formed by bimolecular processes

Abstract The C4H10O.+ potential energy surface was accessed at several energies through different ion/molecule reactions. Reaction of CH3CH.+3 with CH3CHO and CH3CHO.+ with CH3CH3 gave predominantly CH3CHOH+ +. CH2CH3 and small amounts of CH3CH2CHOH+ +. CH3. CH3CH.+3 also produced a small amount of CH3CHO+CH3 +. CH3 upon reaction with CH3CHO. CH2 = CHOH. + did not react with CH3CH3. CH3CH2OH. + reacted CH2 = CH2 and CH2 = CH.+2 with CH3CH2OH to produce CH3CH2OH+2 and CH3CHOH+, but only the second pair of reactants produced detectable C3H7O+ ions. CH3CH2CHO.++CH4 produced only CH3CH2CHOH+. In all of the reactions examined, initial proton or H-transfer was much more often followed by simple dissociation than by C C bond formation or multiple H-transfers. This contrasts with the metastable decompositions of ionized 2-butanol, in which elimination of ethane and methane through the complexes [CH3CHOH+.CH2CH3] and [CH3CH2CHOH+.CH3] are important processes. This contrast is attributed to the ion/molecule reactions taking place in a higher energy regime than the metastable decompositions.

Why are alkane eliminations from ionized alkanes so abundant

Eliminations of alkanes consisting of the side chain plus a hydrogen from ionized alkylcycloalkanes are unusually abundant among such processes. For example, ethane is eliminated from ionized ethylcyclopentane more than 10 times more often than it is from its acyclic isomers. To explore why, we characterized the eliminations of ethane from ionized ethylcyclopentane and of butane, 2-methylpropane, and cyclohexane from isomeric butylcyclohexane ions. We hypothesized that one reason these alkane eliminations are particularly favored is that the partners in the complex do not readily escape from reactive configurations. Supporting this, hydrogens are transferred to butyl partners from around cyclohexyl rings, demonstrating that the partners in cycloalkyl-containing complexes do react with each other through several configurations. A very prominent cyclohexane elimination from ionized tert-butylcyclohexane demonstrates that alkane elimination is abundant no matter which partner in the intermediate ion-neutral complex bears the charge. C4H8+ is the dominant dissociation product of ionized tert-butylcyclohexane, even though the formation of the cyclohexene ion plus 2-methylpro-pane is thermochemically favored, a highly unusual ordering among mass spectral fragmentations. This is attributed to H-atom transfer from a tret-butyl ion to a cyclohexyl radical being preferred over transfer of hydride in the opposite direction. The effect of energy on the magnitude of alkane eliminations and the associated simple dissociations was elucidated utilizing photoionization mass spectrometry. Appearance energies show that forces of attraction between the partners are less than 30 kJ mol−1, no stronger than when both partners are acyclic. However, the shapes of photoionization efficiency curves demonstrate that these alkane eliminations are significant over a wide energy range, in contrast to most other alkane eliminations. Thus, ionized cycloalkanes generate unusually stable ion-neutral complexes; this is probably the reason alkane eliminations through them are so abundant. Alkane eliminations from acyclic alkane ions are also very abundant, suggesting that ion-neutral complexes formed from alkylcycloalkane and alkane ions have a common feature which makes energy relatively ineffective in driving the partners apart.

The reaction coordinate for forming c-C3H+6 from CH2CH2CH2OH+2 and the energetics of related reactions

Abstract Several techniques are used to characterize the dissociation of CH 2 CH 2 CH 2 OH + 2 (2) to c-C 3 H + 6 + H 2 O and the reverse reaction of H 2 O with c-C 3 H + 6 . Ionized propene and c-C 3 H + 6 both underwent substantial H/D exchange with D 2 O. The products of exchange between c-C 3 H + 6 and D 2 O did not react with NH 3 or CS 2 in a fashion characteristic of c-C 3 H + 6 , demonstrating ring opening before exchange. MP2/6-31G(d)//6-31G(d) + ZPVE ab initio calculations place the xomplex [c-C 3 H + 6 H 2 O], with the water aligned along the long CC bond, about 60 kJ mol −1 below the dissociated partners, and indicate that only 5-6 kJ mol −1 is required to convert the complex to CH 2 CH 2 CH 2 OH + 2 . Thus 2 should dissociate to c-C 3 H + 6 + H 2 O at thermochemical threshold. UHF 3-21G ab initio MO computations suggest that front-side attack of water on the long CC bond can also form 2 below the threshold for dissociation of the reactants. Thus, there is probably not a strong directional constraint on 2 → c-C 3 H + 6 + H 2 O. Appearance energies for forming c-C 3 H + 6 from ionized tetrahydrofuran, cyclobutanone and γ-butyrolactone demonstrate that each of these ions decomposes to c-C 3 H + 6 close to thermochemical threshold. Thus anchimerically-assisted cyclization-fragmentation reaction of γ-distonic ions are generally facile.

Dependency of H-transfer on the initial configuration of the partners in ion-neutral complexes: motion of the partners as a function of the initial geometry of complexes

Abstract The hypothesis that partners in ion-neutral complexes in the gas phase react with each other more readily when the smaller partner originates near the center of mass of the larger partner than when the smaller partner originates away from that center of mass is further explored by additional characterization of the losses of ethyl and ethane from a series of substituted ethylnonane ions. Formation of a complex by cleaving ethyl from near the end (2-position) of the nonyl chain results in subsequent H-abstraction predominantly from non-adjacent positions in the nonyl partner, while cleavage from the middle (5-position) results in specific abstraction from the positions adjacent to that of the original ethyl, demonstrating migration away from the end but not from the middle of the chain. In addition H-transfer to eliminate ethane increases in importance as the origin of the ethyl is shifted toward the center of the chain, and ethyl escapes more readily from near the end of the chain. The greater tendency for ethyl to escape without abstracting a hydrogen when C-C cleavage occurs near the end of the nonyl chain increases further with increasing internal energy in the ion. Differences in photoionization appearance energies for ethyl and ethane losses show no systematic variation as a function of the position of origin of the ethyl. The dissociation patterns and energy dependencies of the dissociations of ionized ethylnonanes are consistent with greater migration of ethyl away from its origin starting near the end of the nonyl chain relative to near the middle, supporting the previous hypothesis.