Effects of pressure on conformer equilibria in liquid n-hexane

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Abstract
From a study of the dependence of the polarized Raman spectra of liquid n‐hexane (30 °C) on pressure (0.26–7.38 kbar), we have confirmed the findings of Schoen et al. [Nature 270, 412 (1977)] that pressure is effective in perturbing the distribution of conformers. However, our results on the behavior of the gauche forms differ from these authors. We find that increased pressure decreases the concentration of the ttt conformer and increases the concentrations of both the single‐ and double‐gauche forms. Surprisingly, the pressure dependence of the single‐ and double‐gauche conformers is nearly the same. Thus while the volume change for ttt→gtt is found to be about −1.1±0.0 cm3/mol, that for the single‐gauche to double‐gauche conversion is about 0.0±0.0 cm3/mol. These results appear to rule out a simple additivity model for the n‐alkanes in which CC bonds respond independently to pressure. Our measurements also suggest that the concentration of the all‐trans conformer in liquid n‐hexane is significantly higher than the value obtained from the rotational‐isomeric‐state model with E g and E gg′ equal to the widely used values of 500 and 3000 cal/mol.