Kavitha G, Narayana Chandrabhas
Chemistry and Physics of Material Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India.
J Phys Chem B. 2006 May 4;110(17):8777-81. doi: 10.1021/jp056176a.
High-pressure Raman scattering studies at ambient temperature are performed on n-heptane. We observe a liquid-solid transition around 1.5 GPa from the changes in the Raman spectra. This has been reported in earlier works. With increasing pressure, we observe large changes in the Raman modes and the spectra show a distinct change around 7.5 GPa. This marks the solid-solid transition at 7.5 GPa observed in n-heptane for the first time. As predicted in theoretical work, we observe dampening of methyl rotation in n-heptane below 7.5 GPa. With increase in pressure above 7.5 GPa we observe a definitive conversion of gauche to trans conformation in the solid phase. Upon release of pressure we do not observe any hysteresis, which suggests that the solid-solid transition takes place with no volume change or is a second-order transition. In this paper we propose this transition to be an orientational order-disorder transition driven by the dampening of the rotation of the methyl group.
在室温下对正庚烷进行了高压拉曼散射研究。通过拉曼光谱的变化，我们观察到在1.5吉帕左右发生了液-固转变。这在早期的研究中已有报道。随着压力增加，我们观察到拉曼模式发生了很大变化，并且光谱在7.5吉帕左右出现了明显变化。这首次表明在正庚烷中7.5吉帕处发生了固-固转变。正如理论研究中所预测的，我们观察到在7.5吉帕以下正庚烷中甲基旋转受到抑制。当压力增加到7.5吉帕以上时，我们观察到在固相中正庚烷从gauche构象到反式构象的明确转变。压力释放后，我们未观察到任何滞后现象，这表明固-固转变发生时没有体积变化，或者是一个二级转变。在本文中，我们提出这种转变是由甲基旋转受到抑制驱动的取向有序-无序转变。
