Comparison of theoretical and experimental studies of infrared and microwave spectral data for 5- and 6-membered ring heterocycles: The rotation constants, centrifugal distortion and vibration rotation constants

A systematic study of the rotation constants, vibration-rotation constants and quartic centrifugal distortion constants has been performed for furan, pyrrole, thiophene, both oxazoles and thiazoles, 1,2,4- and 1,2,5-oxa- and -thiadiazoles, 1,2,5-selenadiazole, and several 6-membered heterocycles (azines). The main study used cc-pVTZ basis sets. There is a very close correlation between the observed constants above and those calculated, especially using the cc-pVTZ + B3LYP procedures; some trends in these appear on substitution of HC by N in the azoles. This strong correlation between experimental values and this theoretical procedure is also apparent with vibrational differences in rotation constants, all over large numbers of measured values. However, a small number of calculated values do not correlate well with experiment; this may be a result of some of the experimental values being subject to resonance perturbations not included in the calculations. In general, a TZ2P basis set with B3LYP methodology, and cc-pVTZ results with MP2 methodology, lead to correlations of similar quality, but the latter require markedly more computing power. The present study draws attention to a tetrad in the IR spectrum of 1,2,5-oxadiazole, where further analysis is necessary.