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 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 h 4 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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The absolute heats of formation of 1,2-, 1,3-, and 1,4-dehydrobenzene (ortho-, meta-, and para-benzyne) have been determined from measurements of the threshold energies for collision-induced dissociation (CID) of ortho-, meta-, and para-chlorophenyl anions in a flowing afterglow-triple quadrupole apparatus. The 298 K heats of formation for ortho-, meta-, and para-benzyne derived in this manner are 106.6 ± 3.0, 122.0 ± 3.1, and 137.3 ± 3.3 kcal/mol, respectively. The values for meta-and para-benzyne are higher than those reported previously (Wenthold, P. G.;

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The isomeric m- and p-benzyne anions have been generated in the gas phase in a flowing afterglow-triple quadrupole instrument from the reactions of molecular fluorine (F2) with m- and p-(trimethylsilyl)phenyl anions. The mechanism of the F2 reaction involves electron transfer from the (trimethylsilyl)phenyl anion to F2, followed by nucleophilic attack on the resulting (trimethylsilyl)phenyl radical by the nascent F- formed within the intermediate ion/molecule complex. The structures of o-, m-, and p-benzyne anion are unambiguously identified by a classical derivatization scheme wherein the ions are first converted to the corresponding o-, m-, and p-nitrobenzoates by stepwise addition of CO2 and NO2. These derivatives are then identified by forming proton-bound dimers of each ion with CHF2CO2H, and comparing the O2NC6H4CO2 -/CHF2CO2 - yield ratios obtained by collision-induced dissociation (CID) with those obtained from analogous experiments with the authentic nitrobenzoate ions. Estimates of the electron affinities of m- and p-benzyne have been determined by the kinetic method. Adducts of SO2 with phenide ion and each of the isomeric benzyne anions are formed that produce measurable yields of SO2 •- and the corresponding C6H n - ion upon CID. A calibration relation is derived between the CID yield ratios obtained for the C6H5SO2 - and o-C6H4SO2 •- adducts and the known electron affinities of phenyl radical (25.3 ± 0.1 kcal/mol) and o-benzyne (12.9 ± 0.2 kcal/mol). The measured CID ratios for the m- and p-benzyne SO2 adducts are then combined with this relation to obtain EA(m-benzyne) = 19.5 ± 0.3 kcal/mol and EA(p-benzyne) = 28.8 ± 0.5 kcal/mol. These data are used to derive the following gas-phase acidities for the different ring positions of phenyl radical and the corresponding C−H bond strengths for the phenide ion (ΔH acid(C6H5), DH 298 [C6H4 -−H], kcal/mol):  ortho 378.2 ± 3.1, 93.7 ± 3.1; meta 386.9 ± 3.1, 102.4 ± 3.1; para 393.5 ± 2.9, 109.0 ± 2.9.