Computational molecular spectroscopy for X∼2Δ NiCN: Large-amplitude bending motion

Three-dimensional potential energy surfaces for the 2Δ electronic ground state of NiCN have been calculated at the MR-SDCI+Q+Erel/[Roos ANO (Ni), aug-cc-pVQZ (C, N)] level of theory together with electric dipole moments and transition moments. From the ab initio data, standard molecular constants have been determined by the second-order perturbation method, rovibronic term values, rotational constants, and dipole moment expectation values have been determined by the variational RENNER program, and averaged structures have been computed with the variational MORBID method. The perturbation method failed because of a severe Fermi resonance between 2ν2 and ν3. The ab initio calculations yield a linear equilibrium structure with re(Ni-C)=1.8141Å and re(C-N)=1.1665Å. The zero-point averaged structure, determined as expectation values in terms of MORBID wavefunctions, is bent with 〈ρ¯〉0≡180°-〈∠(Ni-C-N)〉0=9(5)°,〈r(Ni-C)〉0=1.8159Å, and 〈r(C-N)〉0=1.1705Å. The bending potential is shallow and gives rise to large-amplitude bending motion. Kingston et al. [C.T. Kingston, A.J. Merer, T.D. Varberg, J. Mol. Spectrosc. 215 (2002) 106-127] have obtained r0,Ω=5/2(C-N)=1.1591(29)Å in LIF experiments, and Sheridan and Ziurys [P.M. Sheridan, L.M. Ziurys, J. Chem. Phys. 118 (2003) 6370-6379] obtained the value 1.1590(2) Å from microwave spectra. These experimentally derived r0,Ω=5/2(C-N)-values are 'too short' when compared with 〈r(C-N)〉0 but not so seriously short as in the cases of FeNC and CoCN.