On the chain length dependence of CH 3 vibrational mode relative intensities in sum frequency generation spectra of self assembled alkanethiols

Abstract

A global modelling of Sum Frequency Generation (SFG) spectra of the self assembly of alkanethiols is presented. All CH modes are modelled together. CH 2 modes are assumed local to allow easy calculation of the spectrum as a function of molecular conformation. Ab initio molecular hyperpolarizabilities are used. A phenomenological approach is used to include the Fermi resonance in the model: the calculated intensity of the CH 3 symmetric stretch is redistributed according to intensity borrowing by the Fermi resonance. The model is applied to octadecanethiol (ODT) and pentadecanethiol (PDT). The even/odd intensity alternation of SFG CH 3 vibrational bands as a function of chainlength is used to test the model. It is found that the relative magnitudes of the hyperpolarizability tensors of the CH 3 asymmetric and symmetric modes need to be adjusted. The model provides tilt and twist angles of the alkyl chain in agreement with the literature. It shows that the all trans conformation does not imply the absence of CH 2 bands in SFG spectra. In addition, new information is obtained. In particular, if two types of molecular conformations are assumed, the C planes of the two molecules must be perpendicular to each other.