Danten Y, Tassaing T, Besnard M
Laboratoire de Physico-Chimie Moléculaire, Unite Mixte de Recherche (UMR) Centre National de la Recherche (CNRS) 5803, Université Bordeaux I, 351 Cours de la Libération, 33405 Talence, France.
J Chem Phys. 2005 Aug 15;123(7):074505. doi: 10.1063/1.1953561.
Far-infrared (FIR) and mid-infrared (MIR) profiles of D2O infinitely dilute in supercritical CO2 have been studied using molecular-dynamics simulations. For this purpose, we have proposed an intermolecular potential model taking implicitly into account electron donor-acceptor (EDA) interactions between water and CO2 evaluated from ab initio calculations of the intermolecular potential-energy surface (IPS). Interaction-induced dipole mechanisms have been also taken into account in addition to the water permanent dipole to evaluate the simulated FIR profiles of water and CO2 polarizable molecules. They were found to play a minor role in the genesis of the FIR profiles of water/CO2 under supercritical conditions. The analysis of the reorientational dynamics of D2O shows that the rotational dynamics of water is weakly anisotropic due to the EDA interactions which affect more specifically the reorientational motions of the C2 symmetry axis of solute. These results have been used to assess the contribution of the vibrational relaxation in the experimental mid-infrared profiles associated with the nu1 symmetric and nu3 antisymmetric stretching and nu2 bending modes of D2O. It was found that the rotational dynamics mainly contribute to the broadening of the infrared (IR) profiles. Nevertheless, the vibrational processes play a role in the frequency shifts of the band centers and the relative intensity enhancements of the nu1 and nu3 modes of D2O. In particular, the EDA interactions between water and CO2 lead to the appearance of a well-defined IR band of the nu1 mode of D2O. Finally, a comparison with another model taking only into account dipole-quadrupole electrostatic interactions between water and CO2 molecules clearly reveals that EDA interactions have to be considered to reproduce both MIR and FIR measurements. From this point of view CO2 can be classified on a hydrophilic solvent scale based upon the solubility criterion as an intermediate solvent between "inert" xenon and carbon tetrachloride.
利用分子动力学模拟研究了超临界二氧化碳中无限稀释的重水（D₂O）的远红外（FIR）和中红外（MIR）光谱。为此，我们提出了一种分子间势模型，该模型隐含地考虑了从分子间势能面（IPS）的从头算计算中评估得到的水与二氧化碳之间的电子供体-受体（EDA）相互作用。除了水的永久偶极矩外，还考虑了相互作用诱导的偶极机制，以评估水和二氧化碳可极化分子的模拟FIR光谱。发现在超临界条件下，它们在水/二氧化碳的FIR光谱形成中起次要作用。对D₂O重取向动力学的分析表明，由于EDA相互作用，水的旋转动力学呈弱各向异性，这更具体地影响了溶质C₂对称轴的重取向运动。这些结果已被用于评估与D₂O的ν₁对称和ν₃反对称伸缩以及ν₂弯曲模式相关的实验中红外光谱中振动弛豫的贡献。发现旋转动力学主要导致红外（IR）光谱的展宽。然而，振动过程在D₂O的ν₁和ν₃模式的带中心频移和相对强度增强中起作用。特别是，水与二氧化碳之间的EDA相互作用导致了D₂O的ν₁模式出现明确的红外带。最后，与另一个仅考虑水和二氧化碳分子之间偶极-四极静电相互作用的模型进行比较，清楚地表明必须考虑EDA相互作用才能重现MIR和FIR测量结果。从这个角度来看，根据溶解度标准，二氧化碳在亲水性溶剂尺度上可被归类为介于“惰性”氙和四氯化碳之间的中间溶剂。
