A theoretical study of FeNC in the 6Δ electronic ground state

We report an ab initio calculation, at the MR-SDCI + Q + E-rel/[Roos ANO (Fe), aug-cc-pVQZ (C, N)] level of theory, of the potential energy surface for (6)Delta(i) FeNC. From the ab initio results, we have computed values for the standard spectroscopic parameters of (FeNC)-C-12 and (FeNC)-C-13. Analytical representations of the potential energy surfaces have been fitted through the ab initio points, and the resulting functions have been used for directly solving the rotation-vibration Schrodinger equation by means of the MORBID program and by means of an adiabatic-separation method. For (6)Delta(i) FeNC, our ab initio calculations show that the equilibrium structure is linear with r(e)(FeN) = 1.9354 angstrom and r(e)(N-C) = 1. 1823 angstrom. We find that the bending potential is very shallow, and the MORBID calculations show that the zero-point averaged structure is bent with the expectation values (r(Fe-N)) = 1.9672 angstrom, (r(N-C)) = 1.1866 angstrom, and ((rho) over bar) = 180 degrees - (angle(Fe-N-C)) = 13 degrees. The experimentally derived bond length r(0)(N-C) = 1.03(8) angstrom reported for (6)Delta(i) FeNC by Lie and Dagdigian [J. Chem. Phys. 114 (2001) 2137-2143] is much shorter than the corresponding ab initio r(e)-value and the averaged value from MORBID. Our calculations suggest that this discrepancy is caused by the inadequate treatment of the large-amplitude bending motion of (6)Delta(i) FeNC. It would appear that for floppy triatomic molecules such as FeNC, r(0)-values have little physical meaning, at least when they are determined with the effects of the large-amplitude bending motion being ignored, i.e., under the assumption that the r(0) structure is linear. (C) 2006 Elsevier Inc. All rights reserved.