Structure and isomerization of C7H7+ ions formed in the charge-transfer-induced fragmentation of ethylbenzene, toluene, and norbornadiene

Abstract

C7H7+ ions with internal energies varying from ~0.5 to 6.5 eV above their ground-state energy have been prepared in an ion cyclotron resonance spectrometer (ICR) by charge-transfer-induced fragmentation of ethylbenzene, toluene, and norbornadiene. It is shown that in the case of (ethyl-a,a-¿2)benzene and toluene-a,a,a-if3 the abundance of unscrambled C6H5CD2+ ions produced by direct bond cleavage increases with energy to reach respectively 65 and 10% of the total C7(H,D)7+ population at 3 eV above the appearance energy. C7H5D2+ ions are also produced after the occurrence of ring expansion and contraction in the parent ion. However, because these ions produced from (ethyl-a,a-<f2)benzene are not statistically scrambled, especially at high internal energies, it is suggested that only a few passages over the energy barrier separating the six-membered ring from the seven-membered ring structure occur during the dissociative lifetime of the ethylbenzene ion. In contrast, the hydrogens in benzyl-d2 and benzyW3 ions produced from toluene-a,a,a-<f3 after ring expansion are statistically scrambled. At higher energies, the benzyl ions undergo extensive scrambling, possibly as a result of a fast equilibrium between the benzyl and tropylium ion structures, as proposed in a collisional activation electron-impact study of C7H8 compounds by McLafferty and Bockhoff. However, the internal energy at which the C7H7+ undergoes isomerization (3-4 eV) in the ICR is higher than that derived from the CA study (~2 eV). With regard to the population of benzyl (B+) and tropylium (T1") ions from the three title compounds,

Hydride transfer reactions via ion–neutral complex: fragmentation of protonated N‐benzylpiperidines and protonated N‐benzylpiperazines in mass spectrometry

“…Therefore, the benzyl cation is able to react with saturated and unsaturated hydrocarbons and ethylbenzene via gas-phase ionmolecular reaction including the hydride transfer reaction. [63,68,69] www.interscience.wiley.com/journal/jms The observation of a hydride transfer reaction in the fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines indicated that the corresponding product ions have benzylium structures. Compounds 7 and 9 are a pair of positional isomers.…”

Gas-Phase Chemistry of Benzyl Cations in Dissociation ofN-Benzylammonium andN-Benzyliminium Ions Studied by Mass Spectrometry

“…They are highly electron-deficient so that they can initiate electrophilic attack on electron-rich species. The best investigated reaction is the methylene transfer from the benzyl cations to the neutral toluene [11,15,17,18]. The benzyl cation is also an ideal Lewis acid in that it is able to capture a hydride ion from a saturated hydrocarbon group but it does not donate a proton-it does not act as a Brønsted acid [19].…”

Formation of C7H7+ ions from ethylbenzene and o-xylene ions: Fragmentation versus isomerization

“…In the photodissociation of toluene ions the fraction of benzylium ions increases monotonically with increasing excitation energy in the precursor [9,13]. The actual numbers vary between about 20% Bz + close to the threshold and about 70% at higher internal energies.…”

“…ethylbenzene [9] leaving only the Trop + ion unchanged. Information on the ratio of Bz + to Trop + being formed has also been derived from the analysis of kinetic energy release (KER) [3,14].…”

“…In general the following trend appears to apply. Toluene ion fragmentation leads to a mixture of Trop + and Bz + ions, with the ratio depending on the internal energy of the toluene ions, whereas larger alkyl benzenes (ethyl- [1,9], propyl- [10,11] and butyl-benzene [12]) predominantly lead to Bz + ions (>80%).…”