Some ab initio valence-bond calculations for NO2

Abstract

The results of some STO-6G valence-bond calculations, with single-zeta and double-zeta basis sets, are reported for the ground state of NO₂. 18 Lewis structures that differ in the locations of the four π electrons and the odd electron are included in the main calculations, subsequent to the determination of the approximate structure of the σ-electron core via energy optimization with a smaller number of structures. Regardless of the nature of the orbital basis set in the 18 structure calculations, the spin-paired diradical structure A, [graphic omitted] as well as structures of types B and C, [graphic omitted] is calculated to be a primary contributor to the resonance scheme. Therefore, NO₂ has considerable spin-paired diradical as well as free-radical character.

The results of some single-zeta VB calculations are also reported for CO^–₂, and consideration is given to the increased-valence and non-paired spatial orbital representations for the π electrons of NO₂. One formulation of the wavefunction for resonance between the increased-valence structures is equivalent to the best linear combination of the wavefunctions for the 18 Lewis structures.