Electronic and structural effects on the 13C contact shifts of σ-bonded molecules

Abstract

Carbon-13 n.m.r. contact shifts induced by nickel acetylacetonate have been observed for various nitrogen heterocyclic compounds in order to elucidate the manner of stereospecific electron spin transmission through the σ-bonded skeleton. It is revealed that the substituent at the α-carbon atom has a sensitive effect on the mode of the carbon-13 contact shifts. The experimental trend is discussed in relation to changes in the electronic structure of ligands or in metal–ligand bonding caused by the introduction of substituents. It is also found that hybridization of the nitrogen non-bonded orbital in ligand molecules plays an important role in intramolecular electron spin transmission. These results are discussed with the aid of molecular orbital calculations of electron spin densities.