HIGH OVERTONES OF C-H STRETCHING VIBRATIONS IN ISOXAZOLE, THIAZOLE, AND RELATED METHYL AND DIMETHYL DERIVATIVES

The high overtone spectra of liquid isoxazole and thiazole, two five-m embered heterocyclic molecules with different aromaticity characters, have been investigated in the C-H stretching region by visible absorpt ion spectroscopy. The local-mode model describes satisfactorily the C- H stretching vibrations at the fifth and sixth quanta of excitation, w here the localized character is increased. Each overtone band generall y displays a number of peaks corresponding to the different types of C -H oscillators. Comparison between the overtone spectra of the parent molecules and those of the methyl derivatives provided sufficient info rmation for a conclusive assignment of the progressions. Deconvolution of the fifth overtone bands gave further evidence regarding the shape and the width of the distinct peaks. Anharmonicity values of the over tone vibrations, C-H bond lengths, and force constants were obtained f rom the local-mode analysis and discussed in comparison with the resul ts obtained from the normal-mode approximation. A correlation between overtone frequencies and chemical shifts in proton magnetic resonance, generally observed in aromatic compounds, has been found only for thi azole, whose aromaticity is responsible for a significant electron cur rent in the ring.