Theoretical Simulation of Vibrational Sum-Frequency Generation Spectra from Density Functional Theory: Application to p-Nitrothiophenol and 2,4-Dinitroaniline

• Published in: Chemphyschem 2009-08, Vol.10 (12), p.2132-2142
• Main Authors: Guthmuller, Julien, Cecchet, Francesca, Lis, Dan, Caudano, Yves, Mani, Alaa Addin, Thiry, Paul A., Peremans, André, Champagne, Benoît
• Format: Article
• Language: English
• Subjects: Chemistry; density functional calculations; Exact sciences and technology; General and physical chemistry; IR spectroscopy; Raman spectroscopy; self-assembled monolayers; Surface physical chemistry; vibrational sum-frequency generation
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200900184

The molecular orientation of adsorbed molecules forming self‐assembled monolayers can be determined by combining vibrational sum‐frequency generation (SFG) measurements with quantum chemical calculations. Herein, we present a theoretical methodology used to simulate the SFG spectra for different combinations of polarizations. These simulations are based on calculations of the IR vectors and Raman tensors, which are obtained from density functional theory computations. The dependency of the SFG vibrational signature with respect to the molecular orientation is presented for the molecules p‐nitrothiophenol and 2,4‐dinitroaniline. It is found that a suitable choice of basis set as well as of exchange‐correlation (XC) functional is mandatory to correctly simulate the SFG intensities and consequently provide an accurate estimation of the adsorbed molecule orientation. Comparison with experimental data shows that calculations performed at the B3LYP/6‐311++G(d,p) level of approximation provide good agreement with experimental frequencies, and with IR and Raman intensities. In particular, it is demonstrated that polarization and diffuse functions are compulsory for reproducing the IR and Raman spectra, and consequently vibrational SFG spectra, of systems such as p‐nitrothiophenol. Moreover, the investigated XC functionals reveal their influence on the relative intensities, which show rather systematic variations with the amount of Hartree–Fock exchange. Finally, further aspects of the modeling are revealed by considering the frequency dependence of the Raman tensors. Summing up: A theoretical methodology simulates vibrational sum‐frequency generation (SFG) spectra for different combinations of polarizations and is applied to p‐nitrothiophenol and 2,4‐dinitroaniline. The IR vectors and Raman tensors are calculated with DFT. Comparison of such simulations with experiment allows determination of the molecular orientation (see picture).