Aromaticity as a quantitative concept part V: A comparison of semi-empirical methods for the calculation of molecular geometries of heteroaromatic compounds and application of the AM1 and MNDO methods to the calculation of Bird's aromaticity indices

The MINDO/3, MNDO, and AM1 geometries for five and six membered heteroaromatics have been compared with available experimental data and with some ab initio geometries. Geometry optimizations using the AM1 and MNDO methods gave the best results of the semi-empirical methods examined and yielded molecular geometries in good agreement with available experimental bond angles of all types and for C-C, C-N, C-O and C-S bond distances. The AM1 and MNDO calculated that N-N, N-O and C=S bond distances are significantly shorter than experimental values due to a systematic error. In general, AM1 ring geometries provide a reliable estimate for the majority of heteroaromatic compounds.

The Bird I₆ and I₅ aromaticity indices calculated from semiempirical and ab initio geometries are compared with those calculated from experimental bond lengths. None of these semiempirical theoretical methods are successful for rings when the number of heteroatoms exceed the number of carbon atoms. For other heterocycles, AM1 and ab initio 3–21G basis set give the best results, followed by MNDO and then by MINDO/3. Rings containing carbonyl groups are an exception in that MINDO/3 provides the best 16 estimates.