Theoretical Calculations and Experimental Measurements of the Vibrational Response of p-NTP SAMs: An Orientational Analysis

The vibrational response of p-nitrothiophenol (p-NTP) self-assembled monolayers (SAMs), on Pt and oil Au, has been investigated by combining theoretical methods with vibrational spectroscopies. Experimentally, the vibrational signatures have been measured using infrared spectroscopy (IR), polarization-modulation reflection absorption infrared spectroscopy (RAIRS), and with sum frequency generation spectroscopy (SFG). Theoretically, density functional theory calculations (DFT) have predicted the molecular vibrations and have estimated their IR vectors and Raman tensors, necessary to simulate the spectra and therefore to interpret the vibrational signatures as well as to retrieve the molecular orientation. So, a tilt angle (theta) of 60 degrees for the molecular 1,4-axis of p-NTP has been directly estirnated for both Pt and Au SAMS by the polarization-dependent SFG data. Then, combining these results with RAIRS measurements has allowed to determine rotation angles (xi) of 56 degrees and 66 degrees for p-NTP SAMS on Pt and Au respectively.