Spin–orbit coupling in the X2Π state of NCS

The three-dimensional spin–orbit function coupling the A′ and A″ components of the X2Π state of NCS was generated by ab initio calculations. New electronic potential energy functions have also been determined using the cc-pVQZ basis set and the MRCI internally contracted approach. It is shown that the variation of the spin–orbit coupling strongly influences the calculated rovibronic Renner–Teller levels. It is found that particularly the CN stretching frequency is sensitive to this effect. The empirical value of the Renner–Teller parameter ϵ, evaluated from experiment using an effective spin–orbit constant, is not directly comparable with the theoretical values since it is deduced from a limited perturbational approach.