Deriving the complete molecular conformation of self-assembled alkanethiol molecules from sum-frequency generation vibrational spectra

Abstract

The sum frequency generation (SFG) spectrum of ${\text{CH}}_{2}$ and ${\text{CH}}_{3}$ modes in the self-assembled monolayer of octadecanethiol is modeled in order to understand the reasons of the very low SFG intensity of ${\text{CH}}_{2}$ modes and to investigate quantitatively the two types of L-shaped conformations qualitatively suggested in the literature. ${\text{CH}}_{2}$ modes are assumed local in order to calculate easily the spectrum of any conformation, and ab initio molecular hyperpolarizabilities are used. It is found that the absence of ${\text{CH}}_{2}$ bands does not imply an all-trans conformation and vice versa. Several conformations are compatible with SFG, but only one of them can be arranged on Au(111) and completely agrees with x-ray diffraction. All conformational details of the L-shape molecules are obtained.