Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan.
J Chem Phys. 2011 Jun 21;134(23):234508. doi: 10.1063/1.3602159.
Vibrational Raman spectra of C=C stretching modes of ethylene derivates (cis-C(2)H(2)Cl(2), cis-stilbene, and trans-stilbene) were measured in supercritical fluids along an isotherm as functions of their densities. The substitution effect of the Raman shift is so significant that a difference among three solutes can be 20 times and is observed similarly in dipolar (CHF(3)) and non-dipolar (CO(2)) fluids. In particular, the shifts of trans-stilbene were enormously large among all systems for studies of vibrational spectroscopies of supercritical fluids and were equivalent to those of typical hydrogen-bonded fluids. Such large shifts arising from the significant attractive energy between solute and solvent molecules were attributed to a site-selective solvation around a phenyl group, which was driven by a dispersion force in the absence of steric hindrance. We found that the absence of steric hindrance causes the significant local density augmentation. To the best of our knowledge, Raman experiments and their theoretical analysis are the first ones quantifying how the difference of steric hindrance produces solvation structures in solution as well as supercritical solutions.
顺式-1,2-二氯乙烯、顺式-二苯乙烯和反式-二苯乙烯的 C=C 伸缩振动模式的振动拉曼光谱沿等压线在超临界流体中作为其密度的函数进行了测量。拉曼位移的取代效应非常显著，三种溶质之间的差异可达 20 倍，并且在偶极（CHF3）和非偶极（CO2）流体中也可以观察到类似的差异。特别是，对于超临界流体振动光谱研究的所有体系，反式-二苯乙烯的位移都非常大，与典型的氢键流体相当。这种由于溶质和溶剂分子之间存在显著吸引力而产生的较大位移归因于苯环周围的选择性溶剂化，这种溶剂化是由没有空间位阻的分散力驱动的。我们发现，没有空间位阻会导致局部密度显著增加。据我们所知，拉曼实验及其理论分析首次定量说明了空间位阻的差异如何在溶液以及超临界溶液中产生溶剂化结构。
