The evolution of the structural, vibrational and electronic properties of the cyclic ethers – on ring size. An ab initio study

• The COC bond angles and the CH bond lengths are found to increase steadily with increasing ring size.
• The wavenumbers of the CH2 wagging and twisting modes follow the same trend.
• The CH2 stretching and bending wavenumbers exhibit the reverse behaviour.
• The charges of the oxygen, carbon and hydrogen atoms do not show any obvious dependence on ring size.

The molecular structures, vibrational spectra and atomic charges of the alicyclic ethers containing from two to five carbon atoms have been determined by means of ab initio calculations, at the level of second order Møller–Plesset perturbation theory and using Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. Two isomers of the oxetane, tetrahydrofuran and tetrahydropyran molecules have been identified and their relative energies determined. Structural properties, such as the COC bond angles and the CH bond lengths, are found to increase steadily with increasing ring size and with decreasing ionization energy. The mean CH2 stretching and bending wavenumbers exhibit the reverse behaviour, while the mean wavenumbers of the CH2 wagging and twisting modes follow the same trend as the structural features. The ring mode wavenumbers vary in a less regular way. The charges of the oxygen, α-carbon and axial and equatorial α- and β-hydrogen atoms also do not show systematic dependences on ring size or ionization energy. The trends in the values of these properties have been rationalized.