Contact Chemical Shifts for the Carbon Atoms of Nickel Complexes of the 4-Alkylanilines. The Factors Governing the EPR Hyperfine Constants of Carbon Atoms1

The proton and carbon contact chemical shifts of the nickel acetylacetonate complexes of 20 aniline derivatives have been measured. The signs and magnitudes of the contact shifts for the meta and para carbon atoms of these anilines are consistent with the dominant π delocalization of spin density. The contact chemical shifts for the α carbon atoms of the substituents in 4-alkylaniline derivatives depend on the hybridization of the bonding orbital of the α-carbon atom and on the degree of substitution of the α atom. The contact chemical shifts for the β-carbon atoms in 4-alkylaniline derivatives exhibit an angular dependence. The results for molecules in which the dihedral anle is defined by structural constraints are well described by aβ= pcπ(~1.1 + 23(cos2 θ)). In general, the contact chemical shifts both for the β-hydrogen and the β-carbon atoms of 4-alkyl groups conform to relationships based on (cos2 θ). The shifts for the 4-cyclopropylanilines deviate from this relationship. These data indicate that more spin density is delocalized to cyclopropyl groups in the bisected conformation than in the perpendicular conformation. The concept of carbon-carbon hyperconjugation provides a basis for the interpretation of the results.