DFT and TDDFT study of carbon trioxide

Carbon trioxide has been studied using DFT and TDDFT methods with different combinations of GGA functionals and correlation consistent basis sets. The C2v isomer is calculated to be 2.5 kcal mol−1 and 12.5 kcal mol−1 lower in energy than D3h with a conversion energy barrier of 5.0 kcal mol−1 and 12.9 kcal mol−1 by PW91PW91 and B3LYP, respectively. All of the above include zero-point energy corrections. The geometry changes almost linearly along the reaction coordinate and the TS structure is approximately an average of D3h and C2v with an imaginary O-C-O bending mode of 421i cm−1. However, D3h is more favored in low energy excited singlet states. These states also promote the C2v → D3h conversion.