Symmetry breaking in the NO2 σ radical: Construction of the 2A1 and 2B2 states with C s symmetry complete active space self-consistent-field wave functions

Abstract

Jackels and Davidson’s interpretation of the symmetry-breaking problem in the NO2 radical is examined using complete active space self-consistent-field (CASSCF) wave functions. Jackels and Davidson provided a Lewis-structure analysis of the necessary forms of C2v -restricted wave functions for 2B2 NO2. In particular, C2v and single-configuration restrictions force the molecule to ‘‘choose’’ between the beneficial effects of resonance and the adverse effects of inclusion of charged components in the wave function. This choice, according to Jackels and Davidson, results in a distortion to Cs symmetry in the region of the 2B2 minimum. The Cs symmetry complete active space treatment enables NO2 to achieve wave functions of proper symmetry (for both 2A1 and 2B2 states, when C2v nuclear symmetry is present) while simultaneously yielding significantly localized orbitals, thus offering a means to verify the earlier analysis. Although the Cs complete active space results contradict one important Jackels–Davidson result by predicting a very shallow C2v 2B2 energy minimum for NO2, the authors find the earlier molecular orbital picture of NO2 to be a useful tool for qualitatively understanding its electronic structure.