Institute of Chemistry, Faculty of Science, Ss. Cyril and Methodius University, P.O. Box 162, 1001 Skopje, Republic of Macedonia.
J Chem Phys. 2013 Aug 7;139(5):054504. doi: 10.1063/1.4816282.
A hybrid, sequential statistical physics-quantum mechanical electronic-quantum mechanical nuclei approach has been applied to study the C-H stretching frequencies of bare fluoroform dissolved in liquid krypton under cryogenic conditions (at 130 K), as well as upon blue shifting hydrogen bonding interactions with dimethylether in the same solvent. The structure of the liquid at 130 K was generated by Monte Carlo simulations of cryogenic Kr solutions containing either fluoroform or fluoroform and dimethylether molecules. Statistically uncorrelated configurations were appropriately chosen from the equilibrated MC runs and supermolecular clusters containing solute and solvent molecules (either standalone or embedded in the "bulk" part of the solvent treated as a polarizable continuum) were subjected to quantum mechanical electronic (QMel) and subsequent quantum mechanical nuclei (QMnuc) calculations. QMel calculations were implemented to generate the in-liquid 1D intramolecular C-H stretching vibrational potential of the fluoroform moiety and subsequently in the QMnuc phase the corresponding anharmonic C-H stretching frequency was computed by diagonalization techniques. Finally, the constructed vibrational density of states histograms were compared to the experimental Raman bands. The calculated anharmonic vibrational frequency shifts of the fluoroform C-H stretching mode upon interaction with dimethylether in liquid Kr are in very good agreement with the experimental data (20.3 at MP2 level vs. 16.6 cm(-1) experimentally). Most of this relatively large frequency blue shift is governed by configurations characterized by a direct C-H···O contact between monomers. The second population detected during MC simulations, characterized by reversed orientation of the monomers, has a minor contribution to the spectral appearance. The experimentally observed trend in the corresponding bandwidths is also correctly reproduced by our theoretical approach. Solvation of the fluoroform monomer, according to experiment, results in small C-H stretching frequency red shift (-2 cm(-1)), while our approach predicts a blue shift of about 10 cm(-1). By a detailed analysis of the anharmonic C-H stretching frequency dependence on the position of the nearest solvent krypton atom and also by analyzing the vibrational Stark effect induced by the local fluctuating field component parallel to the C-H axis, we have derived several conclusions related to these observations. The frequency vs. C···Kr distance dependence shows appreciable fluctuations and even changes in sign at R values close to the maximum of the C···Kr radial distribution function, so that most of the first-shell Kr atoms are located at positions at which the CH frequency shifts acquire either small negative or small positive values. It so happens, therefore, that even the actual sign of the frequency shift is strongly dependent on the correct description of the first solvation shell around CF3H by the Monte Carlo method, much more than the other in-liquid properties calculated by similar approaches.
一种混合的、顺序的统计物理-量子力学电子-量子力学核方法已被应用于研究在低温条件下（约 130 K）裸氟仿在液氖中的 C-H 伸缩频率，以及与同溶剂中的二甲醚发生蓝移氢键相互作用后的情况。在 130 K 下的液体结构通过含有氟仿或氟仿和二甲醚分子的低温 Kr 溶液的蒙特卡罗模拟生成。从平衡的 MC 运行中适当选择统计上不相关的构型，并对包含溶质和溶剂分子的超分子簇（独立存在或嵌入作为可极化连续体处理的“主体”部分）进行量子力学电子（QMel）和随后的量子力学核（QMnuc）计算。实施 QMel 计算以生成氟仿部分的液相 1D 分子内 C-H 伸缩振动势能，随后在 QMnuc 相中通过对角化技术计算相应的非谐 C-H 伸缩振动频率。最后，将构建的振动态密度直方图与实验拉曼带进行比较。在与液体 Kr 中的二甲醚相互作用时，氟仿 C-H 伸缩模式的计算非谐振动频率位移与实验数据非常吻合（在 MP2 水平为 20.3，实验值为 16.6 cm(-1)）。这种相对较大的频率蓝移主要由单体之间直接 C-H···O 接触的构型控制。在 MC 模拟中检测到的第二种构型，其单体的取向相反，对光谱外观的贡献较小。我们的理论方法还正确再现了对应带宽的实验观察趋势。根据实验，氟仿单体的溶剂化导致 C-H 伸缩频率轻微红移（约-2 cm(-1)），而我们的方法预测约 10 cm(-1)的蓝移。通过分析非谐 C-H 伸缩频率对最近溶剂氪原子位置的依赖性，以及通过分析平行于 C-H 轴的局部涨落场分量引起的振动斯塔克效应，我们得出了与这些观察结果相关的几个结论。频率与 C···Kr 距离的依赖性显示出明显的波动，甚至在接近 C···Kr 径向分布函数最大值的 R 值处发生符号变化，因此，大部分第一层 Kr 原子位于 CH 频率获得小负或小正值的位置。因此，即使是频率位移的实际符号也强烈依赖于 MC 方法对 CF3H 第一层溶剂化壳的正确描述，比通过类似方法计算的其他液相性质更依赖于此。
