title: Empirical determination of the harmonic force constants in benzene. 4. The Fermi resonances.
abstract: In this article, we present a continuation of our work on the refinement of the harmonic force constants Fi,k in benzene (in symmetrized Whiffen's coordinates) and on a growing number of higher order (anharmonic) force constants, Fi,j,k and Fi,j,k,l, that are of importance for the benzene isotopomer invariant potential energy surface. The refined set of harmonic and anharmonic force constants improves the agreement between the experimental levels and those calculated theoretically. The emphasis of the present work is on the analysis of the two notable Fermi resonances in benzene (nu8 +nnu1 <=> (n +1)nu1 + nu6, where n = 0, 1, ... m, and nu20 <=> nu8 + nu19 <=> nu1 + nu6 + nu19). For this purpose, we have further extended our fully dimensional, fully symmetrized, and nonperturbative vibrational procedure to the vibrational structure of the benzene isotopic species with D6h symmetry.
authors.affiliation: Institute of Solid State Physics, Bulgarian Academy of Sciences, Tsarigradsko chaussee 72, 1784 Sofia, Bulgaria. rashev@issp.bas.bg
authors.forename: Svetoslav
authors.initials: S
authors.lastname: Rashev
authors.forename: David C
authors.initials: DC
authors.lastname: Moule
authors.forename: Svetlana T
authors.initials: ST
authors.lastname: Djambova