Ab initio molecular orbital study of ground and low-lying electronic states of NiCN

摘要

The electronic ground state and some low-lying excited states of NiCN have been studied by ab initio multi-reference single and double excitation configuration interaction calculations (MR-SDCI) with Davidson's correction Q and Cowan-Griffin's relativistic corrections Eel. The electronic ground state of NiCN is (2)Delta(i) and the equilibrium geometry is linear with bond lengths of r(e)(Ni-C) = 1.8141 angstrom and r(e)(C-N) = 1.1665 angstrom. According to multi-reference averaged quadratic coupled-cluster theory (MR-AQCC) with E-rel, the first excited state is (2)Sigma(+), separated from the ground state by 446 cm(-1), the second and third excited states are (2)Pi(i) (at 1594 cm(-1)) and (4)Delta(i) (at 8930 cm(-1)), respectively, and other quartet states follow. The relative stabilities and the molecular properties of the low- and high-spin states having the same electronic total angular momentum are shown to be determined by dynamical electron correlation effects. A spin-orbit interaction scheme for the lowest doublet manifold is proposed. The predicted term values (in cm(-1)) are 0 ((X) over tilde (2) Delta(5/2)), 793 ((2)Sigma(+)(1/2))1891 ((2)Delta(3/2)), 2321 ((2)Pi(3/2)), and 2773 ((2)Pi(1/2)). Thus, an unassigned state reported by Kingston et al. [C.T. Kingston, A.J. Merer, T.D. Varberg, J. Mol. Spectrosc. 215 (2002) 106] at 755 cm(-1) could be the 1 (2)Sigma(+)(1/2) state. (C) 2008 Elsevier B.V. All rights reserved.