THEORETICAL-STUDIES OF O-, M-, AND P-BENZYNE NEGATIVE-IONS

A general theoretical description of ''distonic'' radical anions is pr esented, along with an account of an extensive series of ab initio mol ecular orbital and density functional theory calculations on the negat ive ions of o-, m- and p-benzyne. The performance of several different computational methods is evaluated with respect to artifactual symmet ry-breaking. QCISD(T), CCSD(T), CASPT2N, and DFT(B3LYP) calculations e mploying double-zeta quality basis sets predict that o-, m-, and p-ben zyne anions all have high-symmetry, delocalized radical anion ground e lectronic states: B-2(2) (C-2 nu), B-2(2) (C-2 nu), and (2)A(g) (D-2h) , respectively. The meta and pam isomers also exhibit low-lying, local ized radical anion forms with distorted geometries that arise by pseud o Jahn-Teller interactions (vibronic coupling): (2)A' (C-s) for m-benz yne anion and (2)A(1) (C-2 nu) for p-benzyne anion. Broken-symmetry wa ve functions are obtained at symmetrical geometries from MCSCF and CIS D calculations for p-benzyne anion, but not for o- and m-benzyne anion s. The calculated potential energy surfaces for in-plane distortion of m- and p-benzyne anions are found to be quite flat. An isodesmic reac tion approach is used to calculate the electron, proton, and hydrogen atom-binding energies for each of the minimum energy states. Good agre ement is achieved between the experimental estimates for these quantit ies and the calculated values for the lowest-energy anion states. The implications of the theoretical findings for negative ion photoelectro n spectroscopic measurements with the m- and p-benzyne anions are disc ussed.