Theoretical calculation of the potential surfaces and vibrational frequencies of the A ²Σ⁺ and X ²Π electronic states of the NCO radical

Abstract

The multireference single- and double-excitation configuration interaction method (MRD-CI) is applied for the computation of the potential energy surfaces of each of the component electronic states in the A ²Σ⁺-X ²Π transition of the NCO radical. The ability of this theoretical approach to deal with more than one state of the same symmetry is advantageous in this study because both the A ²Σ⁺ and the X ²Π states have ²A′ components for bent molecular geometries. The calculations are found to be quite accurate (±0·01a ₀) in predicting equilibrium bond length changes which occur between the (linear) A ²Σ⁺ and X ²Π states. Similarly good experience is noted for the corresponding changes in vibrational frequencies. The T ₀ value for the transition is computed to be 23 131 cm^-1, in good agreement with the observed result of 22 754 cm^-1. The A ²Σ⁺ lifetime is computed to be 267 ns, whereas observed values for several of its lowest vibronic levels range from 322 to 361 ns. Rotational coupling between the two electronic states is kept to a minimum by virtue of the fact that the corresponding potential surfaces do not approach each other more closely at non-equilibrium geometries than the T ₀ value indicated above.