Ultraviolet Photoelectron Spectroscopy of o-, m-, and p-Halobenzyl Anions

The 351 nm photoelectron spectra of the ortho, meta, and para isomers of the fluorobenzyl, chlorobenzyl, and bromobenzyl anions all exhibit resolved, analyzable vibrational structure. For a given isomer, the electron affinity increases with increasing halide atomic number, while for a given halogen substituent, the meta isomer has the largest electron affinity and the para isomer has the lowest. The electron affinities of the o-, m-, and p-fluorobenzyl radicals are found to be 1.091 ± 0.008, 1.173 ± 0.008, and 0.937 ± 0.008 eV, respectively, the electron affinities of the o-, m-, and p-chlorobenzyl radicals are 1.257 ± 0.008, 1.272 ± 0.008, and 1.174 ± 0.008 eV, respectively, and the electron affinities of o-, m-, and p-bromobenzyl radicals are 1.308 ± 0.008, 1.307 ± 0.008, and 1.229 ± 0.008 eV, respectively. Two vibrational progressions are present in all of the halobenzyl spectra, corresponding to the ring-deformation mode and either a ring stretching mode or the CH2 bending mode. The measured electron affinities are used together with previously reported gas-phase acidities to derive the 298 K, methyl C−H bond energies for the corresponding halotoluenes. DH298(C−H) for o-, m-, and p-fluorotoluenes are found to be 90.6, 88.8 ± 2.1, and 87.6 ± 2.1 kcal/mol, respectively, DH298(C−H) for o-, m-, and p-chlorotoluenes are 89.0 ± 3.1, 89.8 ± 2.1, and 87.5 ± 2.1 kcal/mol, respectively, and DH298(C−H) for o-, m-, and p-bromotoluenes are 90.2 ± 3.1, 90.1 ± 3.1, and 88.5 ± 3.1 kcal/mol, respectively.