HIGH-PRESSURE SOLID-PHASES OF BENZENE .2. CALCULATIONS OF THE VIBRATION FREQUENCIES AND EVOLUTION OF THE BONDS IN C6H6 AND C6D6 UP TO 20 GPA

The complete vibrational spectra of crystalline C6H6 and C6D6 have been calculated for the different pressure-induced solid phases recently determined at 293 K up to 25 GPa, and compared to Raman scattering data. The normal coordinate analysis has been carried out by using intermolecular Buckingham-type atom-atom interactions and the intramolecular force field of the free molecule. Results of such frequency calculations are compared to experimental values at ambient pressure. The variation of the relevant crystalline parameters is discussed to construct a model and calculate the vibrational frequencies under pressure. The quantitative fit of the frequency shift of the Raman active modes under pressure demonstrates the necessity of including different C-C and C-H (C-D) bond compressibilities within the benzene molecule. Such intramolecular distance variations which allow to estimate the frequency corrections for the totally symmetric (a1g) breathing modes, have been determined from the observed pressure-frequency dependence of these internal modes. The behavior of other nonsymmetric (e2g) internal modes which become comparatively weak under pressure, suggests a charge delocalization within-and possibly out of-the benzene ring, eventually leading to irreversible opening of the hexagonal cycle. This can be directly related to the irreversible transformation of benzene to a polymer which is observed after pressurization above 20 GPa.