Ab initio calculations of equilibrium geometries and vibrational excitations of helical ethylene-glycol oligomers: application to modeling of monolayer infrared spectra

The density functional theory methods are used to calculate the equilibrium molecular structures and vibrational spectra of helical H(CH2CH2O)nH (OEG) oligomers (n=4–7) at a level of precision that has not been accomplished before. The largest deviation between experimentally observed frequencies, obtained from infrared reflection–absorption spectra of OEG-monolayers on gold, and calculated, single molecule frequencies (unscaled), is slightly above 2%. Moreover, the most intense peak in the CH2-stretching region at about 2890 cm−1, commonly regarded as a trademark of the OEG helical conformation, is reassigned in this study to the asymmetric CH2-stretching mode.