Tetsassi Feugmo Conrard Giresse, Liégeois Vincent, Champagne Benoît
Laboratoire de Chimie Théorique (LCT), Unité de Chimie-Physique Théorique et Structurale (UCPTS), University of Namur, Rue de Bruxelles, 61, B-5000 Namur, Belgium.
Phys Chem Chem Phys. 2017 Nov 15;19(44):29822-29832. doi: 10.1039/c7cp03509h.
The first vibrational sum frequency generation (SFG) spectra based on molecular properties calculated at the coupled cluster singles and doubles (CCSD) level of approximation have been simulated for interfacial model alkyl chains, providing benchmark data for comparisons with approximate methods, including density functional theory (DFT). The approach proceeds in three steps. In the first two steps, the molecular spectral properties are determined: the vibrational normal modes and frequencies and then the derivatives of the dipole moment and of the polarizability with respect to the normal coordinates. These derivatives are evaluated with a numerical differentiation approach, of which the accuracy was monitored using Romberg's procedure. Then, in the last step, a three-layer model is employed to evaluate the macroscopic second-order nonlinear optical responses and thereby the simulated SFG spectra of the alkyl interface. Results emphasize the following facts: (i) the dipole and polarizability derivatives calculated at the DFT level with the B3LYP exchange-correlation functional can differ, with respect to CCSD, by as much as ±10 to 20% and ±20 to 50% for the CH and CH vibrations, respectively; (ii) these differences are enhanced when considering the SFG intensities as well as their variations as a function of the experimental configuration (ppp versus ssp) and as a function of the tilt and rotation angles, defining the orientation of the alkyl chain at the interface; (iii) these differences originate from both the vibrational normal coordinates and the Cartesian derivatives of the dipole moment and polarizability; (iv) freezing the successive fragments of the alkyl chain strongly modifies the SFG spectrum and enables highlighting the delocalization effects between the terminal CH group and its neighboring CH units; and finally (v) going from the free chain to the free methyl model, and further to C constraints on leads to large variations of two ratios that are frequently used to probe the molecular orientation at the interface, the (r + r)/r ratio for both antisymmetric and symmetric CH vibrations and the I/I ratio.
基于耦合簇单双激发（CCSD）近似水平计算的分子性质，首次对界面模型烷基链的振动和频光谱（SFG）进行了模拟，为与包括密度泛函理论（DFT）在内的近似方法进行比较提供了基准数据。该方法分三步进行。在前两步中，确定分子光谱性质：振动简正模式和频率，然后是偶极矩和极化率相对于简正坐标的导数。这些导数采用数值微分方法进行评估，并使用龙贝格算法监测其精度。然后，在最后一步中，采用三层模型评估宏观二阶非线性光学响应，从而得到烷基界面的模拟SFG光谱。结果强调了以下事实：（i）使用B3LYP交换相关泛函在DFT水平计算的偶极矩和极化率导数，相对于CCSD，对于CH和CH振动，分别相差高达±10%至20%和±20%至50%；（ii）当考虑SFG强度及其随实验配置（ppp与ssp）以及随倾斜和旋转角度（定义烷基链在界面处的取向）的变化时，这些差异会增大；（iii）这些差异源于振动简正坐标以及偶极矩和极化率的笛卡尔导数；（iv）冻结烷基链的连续片段会强烈改变SFG光谱，并能够突出末端CH基团与其相邻CH单元之间的离域效应；最后（v）从自由链到自由甲基模型，再到对C的约束，会导致两个常用于探测界面分子取向的比率发生很大变化，即反对称和对称CH振动的（r + r）/r比率以及I/I比率。
