Ab initio investigation of the ... πg2 (X3Σg-, 1 1Δg, 1 1Σg+) electronic states of NCN.: Study of the Renner-Teller effect in the 1 1Δg state

Large-scale CI calculations are carried out to obtain accurate potential energy surfaces for the ...pi (2)(g) manifold (X(3)Sigma (-)(g), 1 (1)Delta (g), 1(1)Sigma (+)(g)) of electronic states of NCN. Separation of the low-lying singlet states from the triplet ground state is computed in close agreement with the results of a recent photoelectron study by T. R. Taylor, R. T. Bise, K. R. Asmis, and D. M. Neumark [Chem. Phys. Lett. 301, 413-416 (1999)]. Vibronic coupling (Renner-Teller effect) in the 1(1)Delta (g) stare is studied by means of a perturbative and a variational approach. Results of the present ab initio study confirm explicitly the rho (4) dependence postulated for the splitting of bending potential curves in Delta electronic states at small deviations from Linearity. (C) 2000 Academic Press.