Principal Component Analysis Of The Cis- And Trans-difluoroethylene Polar Tensors

The isotopic invariance criterion, molecular orbital calculations at the Hartree-Fock and Moller-Plesset levels, and principal component analysis have been used to determine the in-plane atomic polar tensor (APT) elements of cis-C2H2F2 and cis-C2D2F2 and the out-of-plane elements of their trans isomers. The in-plane APT elements of the cis-difluoroethylenes are shown to be transferable to their trans isomers. On the other hand, the out-of-plane cis elements transferred to the trans compound lead to calculated A6 and A7 intensities of 46.9 and 3.7 km mol-1 for trans-C2H2F2, significantly lower than the measured 56.7 and 12.7 km mol-1 values. The differences in these intensity values stem primarily from the different pyy(F) APT out-of-plane elements of these isomers, -0.193e and -0.096e for the trans and cis molecules, respectively, suggesting different electronic densities for the fluorine atoms. Recent high-resolution infrared spectra indicate geometry differences for these isomers that chemical valency arguments can attribute to different fluorine atomic charges in these isomers. The transferable cis and trans in-plane APT elements appear to be predominantly determined by charge flux contributions to the APT elements, as suggested previously. © 1993 American Chemical Society.97236161616