Gas-Phase Heats of Formation of C7H7+ Isomers: m-Tolyl, p-Tolyl, and Benzyl Ions

Article abstract of DOI:10.1021/ja00225a008

The dissotiation onsets of C7H7⁺ formation from the ionic precursors of benzyl bromide and m- and p-nitrotoluene have been investigated by photoelectron photoion coincidence in order to determine accurate heats of formation of the resulting C7H7⁺ ion structures.These parent ions are known to produce 100 percent benzyl and tolyl ions, respectively.Because the benzyl bromide ion dissociation was found to be rapid, the dissociation onset was determined from the crossover energy in the breakdown diagram and appropriately corrected for the precursor thermal energy.The nitrobenzene ions dissociate slowly near the dissotiacion onset, so that the thermochemical onset was found by extrapolating the measured decay rates with the RRKM/QET statistical theory.The rate data indicate that p- and m-nitrotoluene ions do not isomerize to a common structure prior to dissociation.This suggest that the p- and m-tolyl ions also exist as unique structures.However, no information about the o-tolyl ion was obtained.The derived ionic heats of formation were ΔH_f⁰₂₉₈(m-tolyl) = 1054 +/- 10 kJ/mol, ΔH_f⁰₂₉₈(p-tolyl) = 1074 +/ 10 kJ/mol, and ΔH_f⁰₂₉₈(benzyl) = 897 +/- 5 kJ/mol.An upper limit for the heat of formation of C6H4CH3O⁺, the NO loss product from the p-nitrotoluene ion, was determined to be 890 kJ/mol.These heats of formation allow the methyl and ring C-H bond energies in the toluene ion to be calculated.They are 2.18 and 3.9, respectively.