First-principle DFT and MP2 modeling of infrared reflection - Absorption spectra of oriented helical ethylene glycol oligomers

First-principle modeling is used to obtain a comprehensive understanding of infrared reflection absorption (RA) spectra of helical oligo(ethylene glycol) (OEG) containing self-assembled monolayers (SAMs). Highly ordered SAMs of methyl-terminated 1-thiaoligo(ethylene glycols) [HS(CH2CH2O)(n)CH3, n = 5, 6] on gold recently became accessible for systematic infrared analyses [Vanderah et al., Langmuir, 2003, 19, 3752]. We utilized the quoted experimental data to validate the first-principle modeling of infrared RA spectra of HS(CH2CH2O)(5,6)CH3 obtained by (i) DFT methods with gradient corrections (using different basis sets, including 6-311++G**) and (ii) HF method followed by a Moller-Plesset (MP2) correlation energy correction. In focus are fundamental modes in the fingerprint and CH-stretching regions. The frequencies and relative intensities in the calculated spectra for a single molecule are unambiguously identified with the bands observed in the experimental RA spectra of the corresponding SAMs. In addition to confirming our earlier assignment of the dominating peak in the CH-stretching region to CH2 asymmetric stretching vibrations, all other spectral features observed in that region have received an interpretation consistent (but not in all cases coinciding) with previous investigations. The obtained results provide an improved understanding of the orientation and conformation of the molecular building blocks within OEG-containing assemblies, which, in our opinion, is crucial for being able to predict the folding and phase characteristics and interaction of OEG-SAMs with water and proteins.