Probing alkylsilane molecular structure on amorphous silica surfaces by sum frequency generation vibrational...

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_l is set to 1.1. To highlight the origin of the SFG signatures, two chemical models were used, one that includes explicitly the SiO₂ 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^- CH₃ 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 CH₃ 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°.