Isotopic shifts versus potential energy distribution?

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-13C2O2F2, cis-13C2O2F2, trans-14C2O2F2 and cis-14C2O2F2 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-12C2O2F2 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 (ag/a1) depend dramatically on which of these is employed in making the determination. The validity of the latter approach is clearly demonstrated.