Vibrational fundamentals and natural bond orbitals analysis of some tri-fluorinated benzonitriles in ground state

The theoretical IR and Raman spectra of the 2,3,4-, 2,3,6-, 2,4,5- and 3,4,5-tri-fluorobenzonitrile molecules have been calculated by using the density functional method in the ground state. The rigorous normal coordinate analyses based upon both an empirical force field and quantum chemical calculations have been performed and the detailed vibrational assignment has been made on the basis of the calculated potential energy distributions (PEDs). A comparison of molecular geometries, atomic charges and vibrational fundamentals of these molecules has been reported. The effects of fluorination upon the geometries, atomic charges and vibrational frequencies of benzonitrile have been discussed. Several ambiguities and contradictions in the previously reported vibrational assignments have been clarified. In addition, the variation of Raman intensity with excitation frequency and with temperature has also been studied.