Resolving Two Closely Overlapping −CN Vibrations and Structure in the Langmuir Monolayer of the Long-Chain Nonadecanenitrile by Polarization Sum Frequency Generation Vibrational Spectroscopy

Abstract

Polarization sum frequency generation vibrational spectra (SFG-VS) reveals that there are two distinctively different but closely overlapping −CN vibrations at 2244.5 and 2251.1 cm–1, respectively, in the Langmuir monolayer of the long-chain nonadecanenitrile (CH3(CH2)17CN, or C18CN) at the air/water interface. The blue-shifted −CN group at the 2251.1 cm–1 peak is about 1.8 times broader than that of the 2244.5 cm–1. Both the spectral shift and spectral width are consistent with the picture that this blue-shifted peak corresponds to the solvated −CN group, while the 2244.5 cm–1 peak is the signature of the less solvated −CN group. Polarization dependence of these two peaks further suggests that the −CN group corresponding to the 2251.1 cm–1 peak is tilted with an average angle of 50° from interface normal, while that corresponding to the 2244.5 cm–1 peak is tilted with an angle around 67°. The relative population for the −CN groups corresponding to the 2251.1 cm–1 peak is about three times that of the 224...