VIBRATIONAL OVERTONE STUDY OF 5-MEMBERED AROMATIC HETEROCYCLES - LOCAL MODE INTERPRETATIONS

The vibrational overtone spectra of some simple five-membered aromatic heterocycles are measured in the liquid and gas phases. Overall, the aryl CH stretching overtone regions are described very well by the local mode model. Distinct aryl CH stretching overtone progressions can be identified for each structurally nonequivalent aryl oscillator. The assignments of the progressions are based on a comparison between the overtone spectra of the parent and substituted molecules. The assignments are substantiated further by geometry-optimized ab initio calculations at the SCF level and with the aid of ab initio anharmonic force fields. Although qualitatively the overtone spectra can be understood in terms of the bond length-frequency correlation equations that were originally developed to account for the spectra of substituted benzenes, a difference in behavior is noted for the furans.