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

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