Comparison of computational methods applied to oxazole, thiazole, and other heterocyclic compounds

A variety of computational methods, including the semiempirical techniques AM1, PM3, and MNDO, and the thermochemical basis sets of Benson and Stine, was used to calculate and compare heats of formation (ΔHf°) data for optimized geometries of a variety of aromatic and nonaromatic heterocycles. Detailed analyses, including 6‐31G* and MP2/6‐31G* ab initio calculations, were performed for the oxazole and thiazole heterocycles. The results indicate a scatter among the methods sensitive to the nature of the heterocycle. This was in particular evident in the oxazole molecule, where AM1 gave a singularly high value of ΔHf° consistent with longer calculated bond lengths, particularly about the oxygen atom. Aromatic stabilization energy appears to be addressed differently among the employed methods. Implications of this contrast applied to calculation of macromolecular systems containing heterocyclic units are discussed.