Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p-Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o-, m-, and p-Benzyne

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Abstract

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 splittings can be obtained directly from the photoelectron spectrum, with values of 37.5 ± 0.3 and 3.8 ± 0.5 kcal/mol, respectively, obtained for the h4 species and 37.6 ± 0.3 and 3.9 ± 0.5 kcal/mol, respectively, obtained for the fully deuterated molecules. Using a previously reported value for the electron affinity of m-benzyne, the singlet−triplet splitting for this molecule is found to be 21.0 ± 0.3 kcal/mol. Vibrational frequencies are reported for the deuterated and nondeuterated forms of all three biradicals 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 para 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 found to be 89.3 ± 3.3, 98.7 ± 3.1, and 105.0 ± 2.9 kcal/mol, respectively. The heats of formation of the singlet and triplet states of the benzynes are found to be in excellent agreement with the predictions derived from simple valence promotion energy models.

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