o -, m -, and p -Benzyne Negative Ions in the Gas Phase:  Synthesis, Authentication, and Thermochemistry

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 (F 2 ) with m - and p -(trimethylsilyl)phenyl anions. The mechanism of the F 2 reaction involves electron transfer from the (trimethylsilyl)phenyl anion to F 2, 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 CO 2 and NO 2 . These derivatives are then identified by forming proton-bound dimers of each ion with CHF 2 CO 2 H, and comparing the O 2 NC 6 H 4 CO 2 - /CHF 2 CO 2 - 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 SO 2 with phenide ion and each of the isomeric benzyne anions are formed that produce measurable yields of SO 2 • - and the corresponding C 6 H n - ion upon CID. A calibration relation is derived between the CID yield ratios obtained for the C 6 H 5 SO 2 - and o -C 6 H 4 SO 2 • - 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 SO 2 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 (Δ H acid (C 6 H 5 ), DH 298 [C 6 H 4 - −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.