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ELECTRONIC GROUND STATE.
TSUNEO HIRANO, REI OKUDA AND UMPEI NAGASHIMA, Research Institute for Computational Sciences, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan; PER JENSEN, Theoretische Chemie, Bergische Universität, D-42097 Wuppertal, Germany.
The three-dimensional potential energy surface of X 2
i NiCN has been calculated ab initio at the MR-SDCI+Q+E rel/[Roos~ANO (Ni), aug-cc-pVQZ (C, N)] level of theory. The equilibrium geometry derived from this surface is linear with r\mathrm e(Ni-C) = 1.814 [1.8292(28), 1.8293(1)] Å and r\mathrm e(C-N) = 1.167 [1.1591(29), 1.1590(2)] Å, where the values in brackets are r0 values for the ground
=5/2 spin-substate determined experimentally by Kingston et al . and Sheridan et al ., respectively. From the electronic structure given in terms of natural orbitals, and the Mulliken population of +0.83 on Ni, we conclude that the Ni-C bond is basically ionic but less ionic than those of FeNC and CoCN. The electron from Ni goes into the Ni-mediated CN
* orbital, giving the electron distribution Ni+0.8(CN)-0.8. The 3d-
* backbonding is not observed. Molecular constants determined from the ab initio potential energy surface by perturbation methods and in variational calculations will be reported: For example,
1 = 2198 cm-1,
2 = 254 cm-1, and
3 = 511 cm-1. A severe Fermi resonance between 2
2 and
3 is expected. A spin-orbit interaction scheme including the ab initio predicted spin-orbit coupling constant A_\mathrmSO = -613 cm-1\footnote cf .~the unperturbed A_\mathrmSO-value of -594.2(5) cm-1 for X 2
NiH; J.~A. Gray, M. Li, T. Nelis, and R.~W. Field, J. Chem. Phys ., \textbf95, 7164 (1991). will be presented.