Orientational Time Correlation Functions for Vibrational Sum-Frequency Generation. 2. Propionitrile

Molecular dynamics (MD) simulations of propionitrile have been performed to assess the influence of reorientation on vibrational sum-frequency-generation (VSFG) spectra at the liquid/vapor (LV) and liquid/silica (LS) interfaces. Orientational time–correlation functions (TCFs) are derived for the VSFG spectroscopy of the symmetric and asymmetric stretches of functional groups such as methylene groups and rotationally hindered methyl groups. The MD simulations are used to compute VSFG orientational TCFs for the methyl, methylene, and cyanide groups of propionitrile at the LV and LS interfaces. Although propionitrile exhibits relatively fast reorientation in the bulk liquid, we find that for symmetric stretching modes at these interfaces, reorientation only plays a significant role in VSFG spectra under SPS polarization conditions. For asymmetric stretches, reorientation affects the VSFG spectra significantly under all polarization conditions. Azimuthal dynamics tend to dominate the orientational TCFs