Structures of C6H7+ ions formed in unimolecular and bimolecular reactions

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Abstract
The structures, isomerization mechanisms, and reaction kinetics of C6H7+ ions formed in a variety of systems have been studied. The ions formed in the reactions (H2C=C=CH2++H2C=C=CH2) and (HC=CCH3++HC=CCH3), as well as the fragment C6H7+ ions in 1,3-cyclohexadiene, 1,4-cyclohexadiene, trans-1,3,5-hexatriene, 1-methylcyclopentene, 3-methylcyclopentene, and 4-methylcyclopentene exhibit at least two structures under the conditions of an ICR experiment. In each case, one isomer transfers a proton to bases with proton affinity higher than that of benzene demonstrating that this species has the benzenium (protonated benzene) structure. The remaining C6H7+ ions have in some systems (e.g., allene++allene, 1- and 4-methylcyclopentene) a conjugate base with a proton affinity of 202 kcal/mol, while in other systems (e.g., 3-methylcyclopentene, trans-1,3,5-hexatriene) the conjugate base of the nonbenzenium isomer has a proton affinity of 205 kcal/mol. When the C6H7+ ions are formed by fragmentation of a C6H8+ ion (in the cyclohexadienes or trans-1,3,5-hexatriene) which has a well-defined internal energy, it is seen that the fraction of C6H7+ ions exhibiting the benzenium structure decreases with increasing energy. The fraction of ions with the benzenium structure also decreases with increasing internal energy of the (C3H4++C3H4) reaction complex from 0.72 in allene (energy level of separated reactants: 315 kcal/mol) to 0.62 in propyne (energy level of separated reactants: 327 kcal/mol). The energetics of the fragmentation process leading to C6H7+ formation have been examined in a photoelectron-photoion coincidence spectrometer for trans-1,3,5,-hexatriene and 1,3- and 1,4-cyclohexadiene. It is found that the transition state for the fragmentation process (C6H8+→C6H7++H) is effectively the same in all three systems but lies at an energy level higher than (benzenium ion +H) products. Rate constants for reactions of benzenium ions with a variety of organic and inorganic compounds have been determined.