Ethylene glycol: Infrared spectra,AB initio calculations,vibrational analysis and conformations of 5 matrix isolated isotopic modifications

An extension of earlier ab initio calculations is reported which consists in a more detailed structure optimization of conformations investigated earlier, using the previously described as well as a more flexible basis set. The measurement and analysis of infrared spectra of five isotopic modifications, isolated in Ar and Xe matrices, are described. The analysis of the spectra, carried out within the framework of two H-bonded conformations r_e1 and r_e2 predicted by ab initio calculations to be the most stable ones, is complicated by the occurrence of unusually large matrix splittings and irreversible changes of the spectra under irradiation by infrared light. The spectra are interpreted in terms of essentially two slightly different conformers of type r_e1, deformed by the matrix field. The ¹⁸O isotope shifts prove to be important for correct assignment and the least squares fit. A valence force field is given, whose dependence on the conformation of the OH groups and especially on H-bonding is discussed. According to normal coordinate analysis, the effect of the internal H-bond is most pronounced in the COH bend and OH torsion region. The differences of the potential constants of free and bonded OH groups are responsible for a zero point energy differences of about 20 cm^?1 of two isomers of the modification CH₂ODCH₂OH. From experimental evidence for at least a partial equilibration of these two isomers at liquid helium temperature, internal rotation of the OH groups in an Ar matrix may be deduced. Thermodynamic functions of the r_e1 conformation of glycol in the rigid rotor-harmonic oscillator approximation are reported.