SEMIEMPIRICAL TREATMENT OF HYDROGEN-BONDING - THE ACETOIN OXIME CASE

The study presents a comparison of MNDO/M, MNDO/H, MNDO/R, PM3 and HF/ 6-31G* calculations applied to the acetoin (E)- and (Z)-oxime stereoi somers. The obtained results suggest that none of semiempirical method s accurately model both geometries and energies of the various conform ers studied. The MNDO/M approach predicts structural characteristics t hat are similar to ab initio data with respect to the intramolecular O -H ... O interaction, but yields significantly poorer values for the O -H ... N moiety. The MNDO modifications (except PM3) often lead to an overestimation of the intramolecular hydrogen bonding energies and, th us, reduce the ratio of open (non-hydrogen bonded) structures in the c onformers. Some stereoelectronic effects predicted by the PM3 method a re also discussed.