Anti-Parallel Dimer Structure of p-Cyanophenol Adsorbed at the Aqueous Solution Surface Probed by Sub-1 cm-1 High Resolution

Sub-1 cm-1 High Resolution Broadband Sum-Frequency Generation Vibrational Spectra (HR-BB-SFG-VS) in different polarization combinations and experimental configurations was used to measure the spectral lineshape in the -CN stretching vibration region of p-cyanophenol (PCP) molecules at the air/water interface. The HR-BB-SFG-VS with a spectral resolution of ~ 0.6 cm-1 provides accurate and nearly intrinsic SFG spectral lineshape with excellent signal-to-noise ratio (SNR). From analyzing the high-resolution spectra in the 2160-2300 cm-1 range, we found that there are two closely overlapping peaks with opposite phases, implying the existence of two -CN groups with opposite orientations at the interface. This finding is unexpected from the conventional perception that PCP molecules at the air/liquid interface forms single Gibbs monolayer at the air/PCP solution interface. In addition, we also found that the -CN spectral intensity dependence on the bulk PCP concentration does not follow the typical Gibbs adsorption layer behavior, that is to follow the Langmuir isotherm, nor follow the anti-parallel double layer adsorption behavior, that is to follow the double-layer adsorption model (DAM) isotherm. By comparing with the -CN signal intensity and lineshape of the 4-(Octyloxy) benzonitrile (C8-PCP) monolayer, as well as orientation analysis of the two opposite peaks with polarization data, we further validated the anti-parallel dimer structure regarding the interfacial configuration of the PCP molecules. These findings not only provided detailed information about the adsorbed organic molecular layers, such as PCP, at the liquid interfaces, but also demonstrated the power of HR-BB-SFG-VS as a uniquely powerful interfacial selective spectroscopy in resolving and discovering detailed and unexpected molecular structure at interfaces.