Resolving two closely overlapping -CN vibrations and structure in the langmuir monolayer of the long-chain nonadecanenitrile by polarization sum frequency generation vibrational spectroscopy

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 (CH ₃(CH ₂) 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 2244.5 cm ^-1 peak. These results suggest that the -CN head groups in the C18CN Langmuir monolayer are not aligned uniformly at slightly different depth, in order to avoid the strong repulsive forces between the strong -CN dipoles. The SFG-VS spectra of the O-H stretches at C18CN Langmuir monolayer are similar to those of the 4″-n-pentyl-4-cyano-p-terphenyl (5CT) monolayer, indicating complete exclusion of the water molecules from the C18CN Langmuir monolayer, but significantly different from those of the 4″-n-octyl-4-p- cyanobiphenyl (8CB) monolayer, as well as those of the air/acetonitrile aqueous solution interface. Different from previously held understandings, these results suggest that the structure of the insoluble long-chain C18CN Langmuir monolayer is significantly different from that of the Gibbs adsorption layer of the short-chain soluble acetonitrile or propanenitrile aqueous solution surfaces. These observations not only shed new light on understanding the detailed structure and interactions in the molecular monolayer and films but also suggest the importance of the polarization and spectral resolution in the SFG studies.