O-BENZYNE, M-BENZYNE, 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 ph ase in a flowing afterglow-triple quadrupole instrument from-the react ions of molecular fluorine (F-2) with m- and p-(trimethylsilyl)phenyl anions. The mechanism of the F-2 reaction involves electron transfer f rom 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 clas sical derivatization scheme wherein the ions are first converted to th e corresponding o-, m-, and p-nitrobenzoates by stepwise addition of C O2 and NO2. These derivatives are then identified by forming proton-bo und dimers of each ion with CHF2CO2H, and comparing the O(2)NC(6)H4(C) O(2)(-)/CHF2CO2- yield ratios obtained by collision-induced dissociati on (CID) with those obtained from analogous experiments with the authe ntic nitrobenzoate ions. Estimates of the electron affinities of m- an d p-benzyne have been determined by the kinetic method. Adducts of SO2 with phenide ion and each of the isomeric benzyne anions are formed t hat produce measurable yields of SO2.- and the corresponding C6Hn- ion upon CID. A calibration relation is derived between the CID yield rat ios obtained for the C6H5SO2- and o-C6H4SO2.- adducts and the known el ectron-affinities of phenyl radical (25.3 +/- 0.1 kcal/mol) and o-benz yne (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 kc al/mol. These data are used to derive the following gas-phase aciditie s for the different ring positions of phenyl radical and the correspon ding C-H bond strengths for the phenide ion (Delta H-acid(C6H5), DH298 [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.