Inter- and intramolecular temperature-dependent vibrational perturbations of alkanethiol self-assembled monolayers

The infrared spectra of octanethiol, dodecanethiol, and hexadecanethiol (C-8, C-12, and C-16, respectively) monolayers adsorbed on Au(111) textured surfaces have been explored in the 25-300 K regime. The C-H stretching modes typically shift by several wavenumbers to lower frequencies and the intensities increase by as much as 75% as the temperature is decreased, providing evidence of, among other effects, the coupling of these stretching modes with lower energy vibrational modes. In contrast, for all temperatures below 300 K, the positions of the C-H bands of fully hydrogenated C-16 shift by several wavenumbers to higher frequencies as the hydrogenated adsorbates are increasingly diluted in a matrix of fully deuterated C-16, showing that all bands are subject to intermolecular couplings. The analysis of the behavior of the C-H stretching bands suggests that the temperature dependence of the vibrational frequencies associated with the methylene stretching modes is principally due to intermolecular couplings, whereas the temperature dependence of the vibrations associated with the methyl terminations is largely due to intramolecular couplings. It is suggested that the highly constrained geometry of the isotopically diluted monolayer may provide an environment that is less sensitive to intramolecular couplings with low-frequency modes than that of urea-clathrate-isolated species.