Department of Chemistry and Research Institute for Basic Sciences, Korea University, Seoul 136-713, South Korea.
J Chem Phys. 2012 Sep 21;137(11):114307. doi: 10.1063/1.4751477.
Small infrared probe molecules have been widely used to study local electrostatic environment in solutions and proteins. Using a variety of time- and frequency-resolved vibrational spectroscopic methods, one can accurately measure the solvation-induced vibrational frequency shifts and the timescales and amplitudes of frequency fluctuations of such IR probes. Since the corresponding frequency shifts are directly related to the local electric field and its spatial derivatives of the surrounding solvent molecules or amino acids in proteins, one can extract information on local electric field around an IR probe directly from the vibrational spectroscopic results. Here, we show that, carrying out a multipole analysis of the solvatochromic frequency shift, the solvatochromic dipole contribution to the frequency shift is not always the dominant factor. In the cases of the nitrile-, thiocyanato-, and azido-derivatized molecules, the solvatochromic quadrupole contributions to the corresponding stretch mode frequency shifts are particularly large and often comparable to the solvatochromic dipole contributions. Noting that the higher multipole moment-solvent electric field interactions are short range effects in comparison to the dipole interaction, the H-bonding interaction-induced vibrational frequency shift can be caused by such short-range multipole-field interaction effects. We anticipate that the present multipole analysis method specifically developed to describe the solvatochromic vibrational frequency shifts will be useful to understand the intermolecular interaction-induced vibrational property changes and to find out a relationship between vibrational solvatochromism and electrochromism of IR probes in condensed phases.
小的红外探针分子已被广泛用于研究溶液和蛋白质中的局部静电环境。利用各种时间和频率分辨的振动光谱方法，可以准确测量溶剂化诱导的振动频率位移以及这些 IR 探针的频率波动的时间尺度和幅度。由于相应的频率位移与周围溶剂分子或蛋白质中氨基酸的局部电场及其空间导数直接相关，因此可以直接从振动光谱结果中提取有关 IR 探针周围局部电场的信息。在这里，我们表明，通过对溶剂化变色频率位移进行多极分析，溶剂化变色偶极子对频率位移的贡献并不总是主要因素。在腈基、硫氰酸根和叠氮基衍生分子的情况下，相应伸缩模式频率位移的溶剂化变色四极子贡献特别大，并且通常与溶剂化变色偶极子贡献相当。值得注意的是，与偶极子相互作用相比，更高的多极矩-溶剂电场相互作用是短程效应，氢键相互作用诱导的振动频率位移可能是由这种短程多极场相互作用效应引起的。我们预计，专门开发用于描述溶剂化变色振动频率位移的这种多极分析方法将有助于理解分子间相互作用诱导的振动特性变化，并找出凝聚相中的 IR 探针振动溶剂化变色和电致变色之间的关系。
