Structural properties and vibrational spectra of 2-formyloxazole and its 2-thioformyl and 2-selenoformyl derivatives

The molecular structure, internal rotation and vibrational spectra of 2-formyloxazole and its chalcogen analogs (namely sulfur and selenium) have been investigated using B3LYP density functional and ab initio second- and fourth-order Møller-Plesset perturbation methods by utilizing the 6-311++G(d,p) basis set. Calculations showed that the three molecules maintain the planar syn and anti structures as stable forms as a result of the mesomeric effect, with the syn being about 2.5 kcal/mol more stable than the anti form. The isomerization barrier between the two rotamers in the gas phase was predicted to increase as the carbonyl's oxygen is replaced with sulfur and then selenium. The study was extended to the solution phase, and it was concluded that the syn conformer of the three molecules is more stable compared to the anti conformer in solutions too. The effect of substituting the oxygen atom with sulfur and selenium on the isomerization barrier in terms of activation energy and rate constant was also studied.