BIRADICAL THERMOCHEMISTRY FROM COLLISION-INDUCED DISSOCIATION THRESHOLD ENERGY MEASUREMENTS .2. EXPERIMENTAL AND THEORETICAL-STUDIES OF THEMECHANISM AND THERMOCHEMISTRY OF FORMATION OF ALPHA,N-DEHYDROTOLUENE BIRADICALS FROM GAS-PHASE HALIDE ELIMINATION-REACTIONS

Absolute heats of formation for alpha,2-, alpha,3-, and alpha,4-dehydr otoluene biradicals have been determined from the measured threshold e nergies for dissociation of chloride, bromide, and iodide ion from the corresponding o-, m-, and p-halobenzyl anions in the gas phase. The a pparent heats of formation derived for the alpha,2- and alpha,4-dehydr otoluene biradicals exhibit a dependence upon the particular halide io n used for the threshold energy measurement (decreasing with increasin g halide atomic number), while the final heat of formation obtained fo r the alpha,3-dehydrotoluene biradical is invariant with changes in th e halide. The 298 K heats of formation derived from the iodobenzyl ani on results for alpha,2-, alpha,3-, and alpha,4-dehydrotoluene are all found to be 103 +/- 3 kcal/mol. This value is in fair agreement with t he predicted heats of formation for the ground state of each biradical obtained from MCSCF calculations (105-106 kcal/mol) and significantly lower than the value of 107.6 +/- 1.7 kcal/mol predicted by a simple bond energy additivity calculation. The MCSCF calculations indicate al pha,2- and alpha,4-dehydrotoluene to be ground-state triplet biradical s with open-shell singlets lying 7.4 and 8.1 kcal/mol higher in energy , respectively, while alpha,3-dehydrotoluene is found to be a ground-s tate singlet with the triplet lying 3.0 kcal/mol higher in energy. The halide ion dependence of the apparent heats of formation for the alph a,2- and alpha,4-dehydrotoluene biradicals is attributed to the spin-f orbidden nature of the dissociation reactions that produce them. The i ntersystem crossing required to form ground-state triplet products fro m the halobenzyl anion precursors is associated with a reverse activat ion energy and/or a kinetic shift in the reaction onset due to slow un imolecular decomposition kinetics. Both effects would be expected to d iminish with the heavier halides. In contrast, dissociation of a m-hal obenzyl anion to produce alpha,3-dehydrotoluene is spin-allowed, so th e reaction occurs at the true thermochemical limit.