Comparison of theoretical and experimental studies of infrared spectral data for the 5-membered ring heterocycles

A systematic study of the anharmonic vibration frequencies has been performed for furan, pyrrole, thiophene, both oxazoles and thiazoles, and both oxa- and thiadiazoles. The calculations were carried out under standard conditions with both the cc-pVTZ and a TZ2P basis set, and used both the B3LYP density functional (both bases) and MP2 ab initio techniques (cc-pVTZ only). The correlations of the MP2 results with experiment were much less favorable, with mean absolute differences (∣d∣) being nearly twice as large as those with B3LYP. The TZ2P + B3LYP produced results very similar to the cc-pVTZ + B3LYP; the mean absolute frequency difference over 199 values between the two basis sets was ∣d∣ 2.96 cm−1. In correlation with experimental frequencies, TZ2P + B3LYP over all experimental bands gave ∣d∣ 10.13 cm−1; cc-pVTZ + B3LYP gave ∣d∣ 11.56 cm−1. However, in some cases, such as oxazole, where band origins have been more accurately determined and Coriolis effects included, the correlation between theory and experiment is much closer, ∣d∣ 7.2 cm−1.Since TZ2P processes much more rapidly than cc-pVTZ, it is more practicable for larger molecules, and hence is preferred.