Flowing afterglow studies of the reactivity and energetics of biradicals and biradical negative ions

A method for measuring heats of formation for biradicals is described. The procedure involves the measurement of the collision-induced dissociation (CID) threshold energy for a halocarbanion in a flowing afterglow-triple quadrupole instrument. Heats of formation for a variety of organic biradicals, including dehydrobenzenes (benzynes), dehydrotoluenes, and dehydrophenols, are determined. The heats of formation for o-, m-, and p-benzyne were found to be 106.6 $\pm$ 3.0, 121.9 $\pm$ 3.0, and 137.8 $\pm$ 2.9 kcal/mol, respectively. The heats of formation obtained for o- and p-benzyne are in excellent agreement with independently measured values reported recently. The heats of formation for the dehydrotoluenes are all found to be 103 $\pm$ 3 kcal/mol form the CID threshold energies for dissociation of iodobenzyl anions. The heat of formation of $\alpha$,4-dehydrotoluene was confirmed using p-chloromethylphenyl anions. The heat of formation for $\alpha$,3-dehydrotoluene was found to be 103.3 $\pm$ 3.8 kcal/mol using CID threshold energy measurements for dissociations halotoluene and halobenzyl cations. The halide ion dependence of the apparent heats of formation for $\alpha$,2- and $\alpha$,4-dehydrotoluene obtained using halobenzyl anions is proposed to be due to a curve-crossing required for adiabatic dissociation of these ions. The absence of such behavior for $\alpha$,3-dehydrotoluene is taken to be evidence that it has a singlet ground state. Collision-induced dissociation threshold energies have been obtained for o-, m-, and p-halophenoxide ions. The o- and p-halophenoxides probably dissociate to form bicyclic isomers, while dissociation of the m-halophenoxides proceeds to give either the singlet biradical or 1,3-hexadien-1-one-5-yne. Finally, the reactions and energetics of a number of biradical negative ions are investigated. Among the biradical negative ions studied are the m- and p-benzyne radical anions and the trimethylenemethane radical anion. These ions show reactivity that is consistent with that expected for ions containing a radical site