Vibrational transition moment directions of a terminally p-nitrobenzyl substituted long-chain alkanethiol by polarized infrared spectra and DFT calculations

Transition moment directions of the vibrational states of nitro and carbonyl groups of p-nitrobenzyl-16-mercaptohexadecanoate are evaluated by infrared linear dichroism (IR LD) to be further exploited as film orientation markers in self-assembled monolayers (SAMs) that the respective compound forms on metal surfaces. DFT calculations followed by a complete normal coordinate analysis were employed to assist in the vibrational bands assignments. The analysis of the experimental IR LD spectra in conjunction with the step-wise reduction procedure of Thulstrup–Eggers indicated that the transition moment directions of the antisymmetric NO₂ stretching and the carbonyl stretching modes are collinear, and confirmed previous results that those of the symmetric and the antisymmetric NO₂ stretching vibrations are not exactly mutually perpendicular.