Threshold C 7H +7 formation from the benzyl halides by photoionization mass spectrometry

Abstract

Threshold photoionization mass spectrometry has been used to measure appearance energies for the C 7H + 7 fragment ions produced from benzyl chloride, benzyl bromide and benzyl iodide. Based on the stationary electron convention, the derived 298 K cationic heats of formation are 865 ± 8 kJ mol −1, 863 ± kJ mol −1 and 868 ± kJ mol −1, respectively. These are consistent with a threshold tropylium structure, rather than the benzyl structure proposed from low ionizing electron energy collisional activation mass spectra. High level ab initio molecular orbital calculations for both the benzyl cation and tropylium cation support a value of 865 ± 3 kJ mol −1 for Δ H o f298 (tropylium +).