Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p- Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o -, m -, 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-d6 with O-, while the m- and p-benzyne ions were prepared by the gas-phase reaction between the corresponding 3- and 4-(trimethylsilyl)phenyl anions and molecular fluorine, F2. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and triplet states of the biradicals. The electron affinities of o- and p-benzyne are found to be 0.564 ± 0.007 and 1.265 ± 0.008 eV, respectively, 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 because the origin peak could not be assigned unequivocally. For o- and p-benzyne, the singlet−triplet energy splittin...