Ultraviolet photoelectron spectroscopy of o-, m-, and p-halobenzyl anions

The 351 nm photoelectron spectra of the ortho, meta, and para isomers of th e 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-fluoro benzyl 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-c hlorobenzyl 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-bromoben zyl-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 halobe nzyl spectra, corresponding to the ring-deformation mode and either a ring stretching mode or the CH2 bending mode. The measured electron affinities a re 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, a nd 87.6 +/- 2.1 kcal/mol, respectively; DH298(C-H) for o-, m-, and p-chloro toluenes are 89.0 +/- 3.1, 89.8 +/- 2.1, and 87.5 +/- 2.1 kcal/mol, respect ively, 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.