Abstract
The high overtone spectra of liquid isoxazole and thiazole, two five-membered heterocyclic molecules with different aromaticity characters, have been investigated in the C−H stretching region by visible absorption spectroscopy. The local-mode model describes satisfactorily the C−H stretching vibrations at the fifth and sixth quanta of excitation, where the localized character is increased. Each overtone band generally 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 information 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 overtone vibrations, C−H bond lengths, and force constants were obtained from the local-mode analysis and discussed in comparison with the results 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 thiazole, whose aromaticity is responsible for a significant electron current in the ring.
Published Version
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