Resonance assisted hydrogen bonds between oxime and carboxyl group. The comparison of tetrameric structures of 4-methyl-2-oxopentanoic acid oxime and levulinic acid oxime

Abstract

Crystal structures of oximes of two aliphatic keto-carboxylic acids, 4-methyl-2-oxopentanoic acid oxime [(PEN) 4-methyl-2-(hydroxylimino)pentanoic acid] and levulinic acid oxime [(LEV) 4-(hydroxylimino)pentanoic acid], were determined by X-ray structural methods. The molecular structures of monomers interacting via hydrogen-bond networks were analysed with the use of theoretical ab initio self-consistent field (SCF) calculations on model systems. The results of present crystallographic and theoretical studies show that a different structure of PEN and LEV crystals originates mostly from electron charge distribution, depending on the relative position of the oxime and carboxyl moieties. The observed N--O and N--C bond lengths and OmH • • .N and O--H. • .O hydrogen-bond contacts in PEN crystals are affected by the rr-electron resonance of the oxime and carboxyl moieties in a way similar to that found in /~diketones.