High pressure solid phases of benzene. II. Calculations of the vibration frequencies and evolution of the bonds in C6H6 and C6D6 up to 20 GPa

Abstract

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.

High Pressure Photoinduced Ring Opening of Benzene

“…This change of the molecular geometry was ascribed to an efficient mixing of this excited state with the S 0 ground state. A similar hypothesis, but suggesting a S 0 -T 1 mixing, was already formulated by Thiery et al [22]. The benzene ring destabilization is sensitive to photochemical effects.…”

High‐pressure polymerization of phenylacetylene and of the benzene and acetylene moieties

“…The same observation, even though limited to lower pressure, has been reported for the Raman internal active modes. 19,34 For all the modes exhibiting a high pressure soft behavior, a remarkable wavenumber decrease in the S 1 state was also found. 35,36 This correspondence was interpreted on the basis of a progressive distortion of the ring with increasing pressure resembling the geometry of the molecule in the first electronic excited state, thus suggesting that the high-pressure reaction can possibly be achieved at lower pressure by inducing the ring deformation through optical excitation.…”

Intermolecular interactions in solid benzene

“…The aim of the present work is to understand the intermolecular interactions at a microscopic level that lead to molecular packing, lattice dynamics, phase behavior, and ultimately the possibility of new bulk properties in aromatic systems. Over the years there have been a number of studies that are pertinent to this work: crystal structure, 1 phases, 2,3 and vibrational spectra, [4][5][6][7][8][9] from both the experimental and theoretical standpoints.…”