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 Deltai 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 r0. Here, we report analogous calculations for 6 Deltai 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 Fe12CN and Fe13CN 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, omega1=2179 cm-1, omega2=173 cm-1, omega3=420 cm-1, dipole moment = 4.59 D, spin-orbit coupling constant A SO=-83 cm-1, \langle r(Fe-C)\rangle0 = 2.082 Å, and \langle r(C-N)\rangle0 = 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) \rangle0 = 170(5)o (where the number in parentheses is the quantum-mechanical uncertainty). We compare the 6 Deltai FeCN results with those obtained for 6 Deltai 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.