An AM1 Study of an α-Nucleophile: Geometries and Interconversion Modes of Oximate Anion Stereomers

Abstract

The geometries, the relative stabilities and the interconversions of the four isomers of glyoxal monoximate anion ( 2 ) were studied at the AM1 level. In the ground state all four geometries were found to be planar, with the stability order S 1 > W > S > U . There are three factors that tend to stabilize U : (a) non-bonded interactions; b) n N −σsu* CC interaction; and (c) electrostatic repulsion between n N and the partial negative charge on the carbonyl oxygen. In contrast, in the all-carbon system 3 , W is more stable than S . The reversal in stability in the case of 2 probably results from an n N −σ * CC interaction which favours S 1 over W . The barrier to rotation of the carbonyl about the CC bond is ca. 12 kcal/mol, reflecting the partial double bond character in the latter. Rotation and in-plane-inversion are the two possible paths for tilting the oximate oxygen from one side to the other. In the case of formaldoximate anion, the activation energies for these two processes are nearly the same. In 2 , however, rotation is strongly assisted by the presence of the carbonyl group, which lowers its TS energy to ca. 23 kcal/mol as compared to 51 kcal/mol for in-plane-inversion. The AM1 results were corroborated, with one minor exception, by MNDO calculations.