* 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; MICHIKO AMANO, YUKARI MITSUI, SACHIKO S. ITONO, Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan; PER JENSEN, Theoretische Chemie, Bergische Universität, D-42097 Wuppertal, Germany.

We have previously reported a computational molecular spectroscopic study of ^{6}
_{i} FeNC,\footnoteT. Hirano, R. Okuda, U. Nagashima, V.~Spirko, and P. Jensen, *J. Mol. Spectrosc.* , \textbf236, 234-247 (2006). where we showed that the experimentally derived, too-short C-N bond length (r_{ e}(C-N) = 1.03(8) Å) can be ascribed to an inadequate treatment of the large amplitude bending motion in the experimental determination of r_{0}. Here, we report analogous calculations for ^{6}
_{i} FeCN.

Based on the three-dimensional potential energy surface calculated at the MR-SDCI+Q+E_{ rel}/[Roos~ANO (Fe), aug-cc-pVQZ (C, N)] level of theory, the standard spectroscopic parameters of Fe^{12}CN and Fe^{13}CN are derived by perturbation methods, and ro-vibrationally averaged bond lengths \langle r \rangle have been predicted as expectation values obtained with ro-vibrational wavefunctions from the MORBID program. Some of the spectroscopic constants thus determined are: r_{ e}(Fe-C) = 2.048 Å and r_{ e}(C-N) = 1.168 Å,
_{1}=2179 cm^{-1},
_{2}=173 cm^{-1},
_{3}=420 cm^{-1}, dipole moment = 4.59 D, spin-orbit coupling constant A_{ SO}=-83 cm^{-1}, \langle r(Fe-C)\rangle_{0} = 2.082 Å, and \langle r(C-N)\rangle_{0} = 1.172 Å. In variational MORBID calculations, rovibronic energy levels are determined, and some vibrational bands are simulated. The bending potential is shallow, and the MORBID calculations show that the zero-point averaged structure is bent with the expectation value \langle \angle(Fe-C-N) \rangle_{0} = 170(5)^{o} (where the number in parentheses is the quantum-mechanical uncertainty). We compare the ^{6}
_{i} FeCN results with those obtained for ^{6}
_{i} FeNC. Since there are no experimental spectroscopic data available for FeCN, we hope that the predictions made here may be useful in the experimental investigation of this molecule.