Empirical Determination of the Harmonic Force Constants in Benzene. 3. The Harmonic Frequencies

Abstract

The aim of this work is to present a full set of 34 empirically determined harmonic force constants F i , k for benzene (in symmetrized Whiffen's coordinates), as well as the corresponding set of 20 harmonic normal mode (NM) frequencies for all four D 6 h isotopomers: C 6 H 6 , C 6 D 6 , 1 3 C 6 H 6 , 1 3 C 6 D 6 . The reliability of the obtained harmonic force constant values is reinforced by their ability to reproduce satisfactorily the experimentally measured fundamental vibrational frequencies of the four D 6 h benzene isotopomers. A specific combined LM (local mode)/SM (symmetrized modes) complex symmetrized nonperturbative vibrational model, developed in our previous work, has been employed for calculations on the vibrational energy levels in benzene, using the harmonic force constants F i , k and a few diagonal anharmonic parameters, as input data. A set of local (valence) harmonic force constants has also been derived from the empirically determined symmetrized force constants F i , k , and their physical meaning was discussed. The set of 20 harmonic NM frequencies ω k for all four D 6 h benzene isotopomers, calculated in the present work using the empirically determined set of F i , k values, have been analyzed and compared to previous empirical and ab initio determinations by other authors.