BIRADICAL THERMOCHEMISTRY FROM COLLISION-INDUCED DISSOCIATION THRESHOLD ENERGY MEASUREMENTS - ABSOLUTE HEATS OF FORMATION OF ORTHO-BENZYNE, META-BENZYNE, AND PARA-BENZYNE

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.;Paulino, J. A.;Squires, R. R., J. Am. Chem. Sec. 1991, 113, 7414) but are in excellent agreement with recently reported MCSCF and CI calculations. Several control experiments are described which demonstrate that the earlier results for meta- and para-benzyne suffered from an acid-catalyzed isomerization of the reactant chlorophenyl anions in the flowing afterglow prior to CID threshold analysis. It is shown that CID threshold measurements with chlorophenyl-d(4) anions are necessary in order to obtain the correct dissociation energies for the isomerically pure species. The gas-phase acidities of the meta and para positions of chlorobenzene were determined at 298 K using the silane cleavage method to be 391.1 +/- 2.0 and 393.9 +/- 2.0 kcal/mol, respectively. The heats of formation for the benzynes determined in this study are used to derive the gas-phase acidity of the 2-position of meta-benzyne (365 +/- 3 kcal/mol) and the ortho, meta, and para C-H bond strengths in phenyl radical (79.9 +/- 3.1, 95.3 +/- 3.2, and 110.6 +/- 3.4 kcal/mol, respectively). The cycloaromatization of cis-3-hexene-1,5-diyne to para-benzyne (the Bergman cyclization) is calculated to be endothermic by 13 kcal/mol.