Knowledge Bank

The three-dimensional potential energy surface of $X~2\Delta i$ NiCN has been calculated {\it ab initio} at the MR-SDCI+Q+$Erel$/[Roos ANO (Ni), aug-cc-pVQZ (C, N)] level of theory. The equilibrium geometry derived from this surface is linear with $re$(Ni-C) $=$ 1.814 [1.8292(28), 1.8293(1)] \AA \ and $re$(C-N) $=$ 1.167 [1.1591(29), 1.1590(2)] \AA, where the values in brackets are $r0$ values for the ground $\Omega =5/2$ spin-substate determined experimentally by Kingston \textit{et al}.}., \textbf{215}, 106 (2002).} and Sheridan \textit{et al}.,}., \textbf{118}, 6370 (2003).} 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 $\sigma$* orbital, giving the electron distribution Ni$\mathrm{<mrow\; data-mjx-texclass="ORD">+0.8</mrow>}$(CN)$\mathrm{<mrow\; data-mjx-texclass="ORD">-0.8</mrow>}$. The $3d$-$\pi$* backbonding is not observed. Molecular constants determined from the \textit{ab initio} potential energy surface by perturbation methods and in variational calculations will be reported: For example, $\omega 1$ = 2198 \wn, $\omega 2$ $=$ 254 \wn, and $\omega 3$ $=$ 511 \wn. A severe Fermi resonance between $2\nu 2$ and $\nu 3$ is expected. A spin-orbit interaction scheme including the \textit{ab initio} predicted spin-orbit coupling constant $A\mathrm{SO}$ $=$ $-$613 \wn}. the unperturbed $A\mathrm{SO}$-value of $-$594.2(5) \wn\ for $X2\Delta$ NiH; J. A. Gray, M. Li, T. Nelis, and R. W. Field, \textit{J. Chem. Phys}., \textbf{95}, 7164 (1991).} will be presented.