Dissociation dynamics of halotoluene ions, production of tolyl, benzyl and tropylium ([C₇H₇]⁺)ions

Abstract

The dissociation rates and energetics of the loss of halogen atoms from energy-selected halotoluene ions were investigated by photoelectron photoion coincidence (PEPICO) and collisional activation (CA) mass spectrometric experiments. Dissociation onsets, determined from the dissociation rates measured as a function of the internal energy of the parent ion, revealed the formation of three [C7H,]+ isomers, which were identified, on the basis of the CA data, as the tolyl, benzyl and tropylium ions. All of the ions investigated produced a mixture of isomeric ions. Only iodotoluene ions produced any tolyl product ions by a direct bond cleavage. The bromo-and chlorotoluene ions produced mixtures of benzyl and tropyl ions. The observed two-component decay rates of the iodotoluene ions revealed the participation of a lower energy [C,H,Il+' isomer in the dissociation process. The identity of this isomer is not known but it probably does not have the cycloheptatriene ion structure because considerable kinetic energy was released in this dissociation.

Energy- and Time-Dependent Branching to Competing Paths in Coupled Unimolecular Dissociations of Chlorotoluene Radical Cations

“…At the SCF level of theory, the transition state for [1,3] α-H migration (1-x ↔ 3-x) is lower in energy than that for [1,2] α-H migration (1-x ↔ 2-x). Thus, the benzylium and tropylium ions are split directly from 1-x and the 1-x ↔ 2-x process proceeds far slower than the 1-x ↔ 3-x process.…”

“…Theoretical rate constants are in good agreement with the photoelectron photoion coincidence (PEPICO) experiments. 3 In the internal energy range 2.0-3.6 eV, the o-isomer dissociates approximately five times faster than both the m-and p-isomers. The m-isomer is the slowest one in the low energy region but it starts dissociating faster than the p-isomer in the high energy region above 2.9 eV.…”

“…Both experiments [1][2][3][4][5][6][7][8] and theory [9][10][11][12][13] have shown that the benzylium ion is kinetically favored over the thermodynamically more stable tropylium ion.…”

“…No coupling between isomers was considered because the two product ions were thought to be originated from the two separate paths involving [1,3] and [1,2] α-H migrations from the reactant. However, the recent study with the density functional theory (DFT) have indicated that the [1,2] α-H migration is the first step of the complex rearrangement processes and the direct [1,3] α-H migration is not necessary for the formation of the benzylium ion.…”

“…However, the recent study with the density functional theory (DFT) have indicated that the [1,2] α-H migration is the first step of the complex rearrangement processes and the direct [1,3] α-H migration is not necessary for the formation of the benzylium ion. 13 In theoretical studies of the kinetics of coupled unimolecular dissociations of the bromotoluene radical cations, we have found that the product branching ratio is time-dependent because of the coupling and the decay of reactant is not single exponential.…”