Isotopic shifts versus potential energy distribution?

Abstract

Optimizations of geometries and calculations of vibrational wavenumbers and potential energy distributions for trans/ cis oxalyl fluoride were performed at the MP2(FC)/aug-cc-pVQZ theoretical level. The isotopic shifts were calculated for trans- 13C 2O 2F 2, cis- 13C 2O 2F 2, trans- 14C 2O 2F 2 and cis- 14C 2O 2F 2 from the unscaled force fields. The mutual co-assignments of all the calculated wavenumbers were obtained using matrices analogous to the Duschinsky matrix. The assignments of the calculated wavenumbers of trans- and cis- 12C 2O 2F 2 to a specific vibration based on the potential energy distribution and the isotopic shifts were compared, and it is shown that the assignments of the C–F and C–C stretching vibrations ( a g / a 1) depend dramatically on which of these is employed in making the determination. The validity of the latter approach is clearly demonstrated.