Raman intensity measurements for determining conformer populations as a function of pressure

Raman spectroscopy of fluids under pressure has been increasingly studied1,2 and applied3,4 over the past few years. Essential to the extension of these investigations is the reliable interpretation of the pressure-induced spectral changes of model systems in terms of variations in conformation and molecular dynamics. Conformational investigations have been made on systems that were suspected of undergoing conformational change at high pressures, for example, n-alkanes5–9, haloalkanes10–13 and substituted cyclo-hexanes14–19. Raman investigations reported here reveal significant pressure-induced spectral changes in both conformationally sensitive and insensitive systems. We suggest that these arise from a breakdown in the tenet of the pressure invariance of a direct proportionality between Raman scattering intensity and concentration. This is most apparent from the differential behaviour of symmetrical and antisymmetrical vibrations. We propose a re-evaluation of some previous interpretations of the pressure-induced spectral changes.