Hydration and protonation of O-vinylacetoxime in the gas phase: an ab initio study

Ab initio HF/6-31G* calculations of O-vinylacetoxime monohydrates and catio ns were performed. Each conformer forms two stable H-complexes with partici pation of N and O atoms. The former have planar heavy-atom skeletons, where as the water molecule in the latter is located above the plane of the proto n-acceptor complex. The complexes stabilized by N...HO and O...HO bonds hav e different dipole moments and frequencies of the OH stretching vibrations. The most energetically favorable cation is formed by adding a proton to th e C-beta atom of the vinyl group of O-vinylacetoxime. The ap,ap-conformer ( ap is antiperiplanar) of this cation is 6.5 and 34.9 kcal mol(-1) more stab le than the onium cations with the NH+ and OH+ fragments, respectively, and is characterized by polarization and appreciable lengthening of the N-O an d C=C bonds.