Only very recently have chemically defined, highly ordered self-assembled monolayers (SAMs) of HS(CH2CH2O)nCH3, n = 5, 6, become accessible for systematic infrared spectroscopy analysis. Here we report the results of the first ab initio modeling of reflection-absorption (RA) spectra of specifically oriented non-interacting molecules which represent these SAM constituents. The RA spectra of HS(CH2CH2O)5,6CH3, are obtained by DFT methods with gradient corrections and using a variety of basis sets, including 6-311++G**. Positioning and relative intensities of all model spectra are unambiguously identified in all but one intense band in the CH2-stretching region of experimental SAM spectra. Arguments which show that the observed band (which earlier was attributed to symmetric CH3-stretching vibrations) has a distinctively different character are presented.