Laboratoire de Chimie Théorique (LCT), Namur Institute of Structured Matter (NISM), UNamur, Rue de Bruxelles, 61, B-5000 Namur, Belgium.
Unité de Recherche Lasers et Spectroscopies, Namur Institute of Structured Matter (NISM), UNamur, Rue de Bruxelles, 61, B-5000 Namur, Belgium.
J Chem Phys. 2019 Feb 21;150(7):074703. doi: 10.1063/1.5080007.
The sum frequency generation (SFG) signatures of octadecyl-trichlorosilane (OTS) and dodecyl-dimethyl-chlorosilane (DDCS) monolayers on silica were simulated in the C-H stretching region for three polarization combinations (ppp, sps, and ssp), showing the impact of the additional Si-linked methyl groups of DDCS on its SFG signatures. These simulations are based on a two-step procedure where (i) the molecular properties (vibrational frequencies, IR and Raman intensities) are evaluated using first principles methods and (ii) the three-layer model is employed to calculate the macroscopic responses using these molecular responses, the geometry of the experimental setup, and the optical properties of the layers. These first principles calculations adopt the own N-layered integrated orbital molecular mechanics (ONIOM) approach, which divides the system and enables different levels of approximation to be applied to its different parts. Here, the same ωB97X-D exchange-correlation functional is used for all parts, while the underlying silica layers are described with a smaller atomic basis set (STO-3G, 3-21G, or 6-31G) than the alkylsilane and the top silica layer (6-311G*). Calculations show that for describing the lower layer the minimal STO-3G basis set already provides reliable spectral profiles. For OTS, the results are compared to the experiment, demonstrating a good agreement for ppp and sps configurations, provided the refractive index of the layer n is set to 1.1. To highlight the origin of the SFG signatures, two chemical models were used, one that includes explicitly the SiO surface in the first principles calculations (adsorbed-model) and the other that only considers the silane chain (isolated-model). Simulations show that OTS and DDCS display similar spectral patterns where, for ppp and sps configurations, the r CH stretching vibrations are dominant in comparison to the r stretching ones. Still, in the case of DDCS, the r peak presents a shoulder, which is assigned to the vibrations of the Si-linked methyl groups. This shoulder vanishes when these CH groups are frozen. Then, using the isolated-model, the rotation angle (ξ) is gradually changed, showing that in the ppp SFG spectrum the r/r intensity ratio decreases from 73.4 at 0° to 1.7 at 180°.
十八烷基三氯硅烷（OTS）和十二烷基二甲基氯硅烷（DDCS）在二氧化硅上单分子层的和频信号（SFG）在 C-H 伸缩区域的三种偏振组合（ppp、sps 和 ssp）下进行了模拟，显示了 DDCS 中额外的 Si 键合甲基基团对其 SFG 特征的影响。这些模拟基于两步程序，其中（i）使用第一性原理方法评估分子性质（振动频率、IR 和拉曼强度），（ii）采用三层模型使用这些分子响应、实验装置的几何形状和各层的光学性质来计算宏观响应。这些第一性原理计算采用了自有 N 层集成轨道分子力学（ONIOM）方法，该方法对体系进行了划分，并允许对其不同部分应用不同的近似水平。在这里，相同的 ωB97X-D 交换相关泛函用于所有部分，而烷基硅烷和顶层二氧化硅层（6-311G*）则使用更小的原子基组（STO-3G、3-21G 或 6-31G）来描述底层二氧化硅层。计算表明，对于描述较低层，最小的 STO-3G 基组已经提供了可靠的光谱轮廓。对于 OTS，将结果与实验进行了比较，对于 ppp 和 sps 构型，提供了良好的一致性，前提是层 n 的折射率 n 设置为 1.1。为了突出 SFG 特征的起源，使用了两种化学模型，一种在第一性原理计算中明确包含 SiO 表面（吸附模型），另一种仅考虑硅烷链（孤立模型）。模拟表明，OTS 和 DDCS 显示出相似的光谱模式，对于 ppp 和 sps 构型，r CH 伸缩振动比 r 伸缩振动更为突出。然而，在 DDCS 的情况下，r 峰呈现出一个肩峰，该肩峰归因于 Si 键合甲基基团的振动。当这些 CH 基团被冻结时，这个肩峰就会消失。然后，使用孤立模型，逐渐改变旋转角（ξ），表明在 ppp SFG 光谱中，r/r 强度比从 0°时的 73.4 降低到 180°时的 1.7。
