Raman Spectroscopic Investigations of Pressure-Induced Phase Transitions in n-Hexane

We report high-pressure Raman studies on n-hexane up to 16 GPa. n-Hexane undergoes solid−solid transition around 9.1 GPa along with an already reported liquid−solid transition around 1.4 GPa. The intensity ratio of the Raman modes relating the all-trans conformation (1147 and 2872 cm^-1) to that of the gauche conformation (1074 and 2923 cm^-1) shows a sudden change across 9.1 GPa, suggesting an increase in the all-trans population conformers above 9.1 GPa. The disappearance of the torsional modes suggests a steric hindrance to the methyl end group, similar to the n-heptane case, suggesting that the high-pressure phase (above 9.1 GPa) is an orientationally disordered phase. In general, the transition pressure for the solid−solid transition is inversely proportional to the length of the carbon backbone in the medium chain length n-alkanes.