Improved assignment of vibrational modes in sum-frequency spectra in the CH stretch region for surface-bound C18 alkylsilanes

Previous assignments of the CH stretching modes for surface-bound alkylsilanes in vibrationally resonant sum-frequency generation (VR-SFG) spectra have differed, leading to uncertainty in how to interpret vibrational spectra of these systems. In particular, the assignment of Fermi resonances, including which modes are coupled, has been unclear. To aid in these assignments, partially deuterated alkylsilanes were synthesized and characterized by FT-IR and VR-SFG. Density functional theory (DFT) calculations complement the spectroscopic investigation. Based on these results, we identify multiple contributions to the VR-SFG feature at ∼2950 cm−1; this is primarily a Fermi resonance between the symmetric methyl stretch and symmetric methyl bending modes at 2945 cm−1 with a high-frequency shoulder at ∼2960 cm−1, assigned to the methyl antisymmetric stretch, and a small contribution from the antisymmetric stretch of the ω CH2 group at ∼2930 cm−1. The feature at 2880 cm−1 is assigned as the symmetric methyl stretch. Improved mode assignments will aid the interpretation of vibrational spectra with an aim toward developing a better understanding of the molecular basis of retention in liquid chromatography.