AB-INITIO AND PM3 CALCULATED MOLECULAR-STRUCTURES AND ENERGIES OF ACETOIN OXIME - THE LIX63 PROGENITOR

The conformational potential energy surface of both nonionized and ionized acetoin (E)- and (Z)-oximes was studied by applying the ab initio HF/6-31G** and the semi-empirical PM3 method. The hydrogen bond parameters of selected stationary structures of several conformers are reported. (E)- and (Z)-acetoin oximes are almost iso-energetic, but show an (E)-isomer preference. At the HF/6-31G** level of theory the (E)-cyclic conformer is preferred by 1.09 kcal mol(-1), whereas in the PM3 approximation the preferences for the cyclic (E)-form decreases to 0.92 kcal mol(-1), demonstrating the intramolecular hydrogen bond. The theoretical results suggest that open and cyclic structures are thermodynamically equivalent or similar in the (E)- and (Z)-set of conformers. The calculated geometries of the (Z)-stereoisomers agree well with the theoretical prediction of a prochiral character of both oxygen atoms: epimeric pairs of conformers are found. Calculations for the corresponding tautomers are also discussed. Comparison of the results for the nonionized and for the ionized tautomeric pairs provides evidence for the nonionic nature of this class of compounds at least in the gas-phase.