AB-INITIO AND RESONANCE RAMAN STUDIES OF HEXAFLUORO-1,3-BUTADIENE

Fluorine substitution can have a major effect on the electronic spectr a and sometimes on the ground electronic state conformations of organi c compounds. In this work we investigate the effect of perfluorination on the resonance Raman spectra of the simple diene hexafluoro-1,3-but adiene (HFBD) where the preferred ground state geometry is believed to be skew s-cis. Ab initio calculations at the 6-31+G level (which inc ludes diffuse functions (+) and polarization functions () on both C a nd F atoms) together with MP2, BLYP-DFT, and hybrid B3LYP-DFT/HF treat ments of electron correlation were performed. The most stable conforme r of HFBD is confirmed to be the skew s-cis form (phi approximate to 5 8 degrees) with a fundamental vibrational C-C torsional mode at 44 cm( -1). Ab initio calculations of the gradient of the potential energy su rface of the resonant excited singlet state provide a prediction of th e relative resonance Raman intensities. A detailed comparison of theor y with experiment is possible, and excellent agreement is observed. Ac tivity in the fundamental transition of the torsional mode confirms th e low frequency of this mode and the nonplanar nature of the ground st ate.