ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY OF THE O-BENZYNE, M-BENZYNE, AND P-BENZYNE NEGATIVE-IONS - ELECTRON-AFFINITIES AND SINGLET-TRIPLET SPLITTINGS FOR O-BENZYNE, M-BENZYNE, AND P-BENZYNE

The 351 nm photoelectron spectra of the negative ions of o-, m-, and p -benzyne (1,2-, 1,3-, and 1,4-dehydrobenzene, respectively) and their perdeuterated isotopomers have been obtained. The o-benzyne ions were generated by the reaction of benzene and benzene-d(6) with O-, while t he m-and p-benzyne ions were prepared by the gas-phase reaction betwee n the corresponding 3- and 4-(trimethylsilyl)phenyl anions and molecul ar fluorine, Fz. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and tr iplet states of the biradicals. The electron affinities of o- and p-be nzyne are found to be 0.564 +/- 0.007 and 1.265 +/- 0.008 eV, respecti vely, while the electron affinities of deuterated o- and p-benzyne are found to be 8 and 5 meV lower, respectively. The electron affinity of m-benzyne could not be determined from the photoelectron spectrum bec ause the origin peak could not be assigned unequivocally. For o- and p -benzyne, the singlet-tripiet energy splittings can be obtained direct ly from the photoelectron spectrum, with values of 37.5 +/- 0.3 and 3. 8 +/- 0.5 kcal/mol, respectively, obtained for the h(4) species and 37 .6 +/- 0.3 and 3.9 +/- 0.5 kcal/mol, respectively, obtained for the fu lly deuterated molecules. Using a previously reported value for the el ectron affinity of m-benzyne, the singlet-triplet splitting for this m olecule is found to be 21.0 +/- 0.3 kcal/mol. Vibrational frequencies are reported for the deuterated and nondeuterated forms of all three b iradicals and for the corresponding negative ions. Using the measured electron affinities and previously reported heats of formation of o-, m-, and p-benzyne, the gas-phase acidities of the ortho, meta, and par a positions of phenyl radical are calculated to be 377.4 +/- 3.4, 386. 8 +/- 3.2, and 393.1 +/- 3.0 kcal/mol, respectively, and the C-H bond energies at the ortho, meta, and para positions of phenyl anion are fo und to be 89.3 +/- 3.3, 98.7 +/- 3.1,and 105.0 +/- 2.9 kcal/mol, respe ctively. The heats of formation of the singlet and triplet states of t he benzynes are found to be in excellent agreement with the prediction s derived from simple valence promotion energy models.