THE OXIMES OF CYCLOPROPENONE, CYCLOPROPANONE AND ACETONE AND THEIR O-PROTONATED AND N-PROTONATED FORMS - AN AB-INITIO AND SEMIEMPIRICAL QUANTUM-CHEMICAL STUDY

Applying ab initio and semiempirical quantum chemical methods to study cyclopropenone 2, cyclopropanone 3, and acetone oximes 4 and also bot h their O- and N-protonated forms, a strong dependence of the energy d ifference between O- and N-protonation on the structure of oximes was obtained. Geometries were fully optimized using the 3-21G basis set, a nd the best energy calculations were carried out at the 6-31G level. The results were compared with those for formaldoxime 1. All predicted ab initio structures are in adequate agreement with available experim ental data. At the 6-31G//3-21 level the following differences for O- and N-protonation were obtained: 1, 22.9; 2, 29.6; 3, 20.5, 4, 27.7 k cal mol-1. Although AM1 is in good agreement with the ab initio calcul ations, MNDO is not sensitive enough to changes of structure. Although AM shows very good agreement with 6-31G//3-21G for 3 and its protona ted forms in calculating strain energies, MNDO gives only satisfactory results for the three corresponding molecules 2 and its two protonate d forms. The results are discussed and some explanations are given.