Model studies of hydrogen atom addition and abstraction processes involving ortho-, meta-, and para-benzynes

H-atom addition and abstraction processes involving ortho-, meta-, and ara- benzyne have been investigated by multiconfigurational self-consistent fiel d methods. The H-A + H-B. . .H-C reaction (where r(BC) is adjusted to mimic the appropriate singlet-triplet energy ga) is shown to-effectively model H -atom addition to benzyne. The doublet multiconfiguration wave functions ar e shown to mix the "singlet" and "triplet" valence bond structures of H-B. . .H-C along the reaction coordinate; however, the extent of mixing is depe ndent on the singlet-triplet energy gap (DeltaE(ST)) of the H-B. . .H-C dir adical. Early in the reaction, the ground-state wave function is essentiall y the "singlet" VB function, yet it gains significant "triplet" VB characte r along the reaction coordinate that allows H-A-H-B bond formation. Convers ely, the wave function of the first excited state is' predominantly the "tr iplet" VB configuration early in the reaction coordinate, but gains "single t" VB character when the H-atom is close to a radical center. As a result, the potential energy surface-(PES) for H-atom. addition to triplet H-B. . . H-C diradical is repulsive! The H-3 model predicts, in agreement with the a ctual calculations on benzyne, that the singlet diradical electrons are not coupled strongly enough to give rise to an activation barrier associated w ith C-H bond formation. Moreover, this model predicts' that the PES for H-a tom addition to triplet benzyne will be characterized by a repulsive curve early in the reaction coordinate, followed by a potential avoided crossing with the (pi)(1)(sigma*)(1) state of the phenyl radical. In contrast to H-a tom addition, large activation barriers characterize the abstraction proces s in both the singlet ground state and first triplet state. In the ground s tate, this barrier results from the weakly avoided, crossing of the dominan t VB configurations, in the ground-state singlet (S-0) and first excited si nglet (S-1) because of the large energy gap between S-0 and S-1 early in th e reaction coordinate. Because the S-1 state is best described as the combi nation of the triplet X-H bond and the triplet H-B. . .H-C spin couplings, the activation barrier along the So abstraction PES will have much less dep endence on the DeltaE(ST) of H-B. . .H-C than previously speculated. For si milar reasons, the T-1 potential surface is quite comparable to the S-0 PES .