Conformational and vibrational analyses of 2-methoxyethanol and 2-(methylthio)ethanol by density functional theory

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Conformational and vibrational analyses were performed on 2-methoxyethanol (ME) and 2-(methylthio)ethanol (MTE) by density functional theory (DFT). The energies, molecular geometries and vibrational wavenumbers were calculated for the TGg′, GGg′, TTt and TGt conformers of ME and the GGg′, G′Gg′, TGg′ and GGt conformers of MTE (T or t: trans; G or g: gauche) by the BLYP, B3LYP and B3PW91 methods using the 6-31G∗ basis set. The calculations by the HF and MP2 methods were also carried out on the same conformers. The calculated energies are consistent with the experimental findings that the TGg′ conformer of ME and the GGg′ and G′Gg′ conformers of MTE are present in low-temperature matrix. The DFT calculations give the stabilization energy by the OH⋯S hydrogen bonding substantially the same as the energy by the OH⋯O hydrogen bonding. The vibrational wavenumbers for the TGg′ conformer of ME and the GGg′ conformer of MTE are successfully predicted by the MP2, B3LYP and B3PW91 methods using the uniform scale factors for the respective methods. The experimental large wavenumber difference between ME and MTE of the intramolecular hydrogen bonded O-H stretching vibrations is reproduced much better with the DFT calculations than with the ab initio MO calculations. The present study has shown that Becke's three-parameter exchange functional methods overall give the most accurate results.