Bent valence excited states of CO2

Abstract

The conical intersection regions on the potential energy functions of the valence excited ^1,3Σ_u⁻, ^1,3Δ_u, ³Σ_u⁺, and ^1,3Π_g states of CO₂ have been investigated by ab initio calculations. Using large scale multireference configuration interaction the ordering of the lowest valence excited states of CO₂ has been calculated to be ³B₂, ³A₂, ¹A₂ followed by ¹B₂. All these states have bent equilibrium structures and lie energetically below their dissociation asymptotes. The near equilibrium parts of the potential energy functions have been mapped in three dimensions by multiconfiguration self‐consistent field calculations. The ^1,3B₂ and ^1,3A₂ states differ in their equilibrium angles (118° and 127°, respectively), and have much longer equilibrium distances (around 1.26 Å) than the electronic ground state. Anomalously low values of ca. 800 cm⁻¹ have been calculated for the wave number of the antisymmetric stretching vibrations of the ¹A₂, ³B₂, and ¹B₂ states. The crossings between the bent valence excited states in the geometry region 90°≤α_OCO≤150° and 1.1 Å≤R_CO≤1.4 Å have been located.