NMR contact shifts and local conformation in poly(4-vinylpyridine N-oxide)-Ni(II) complexes

The ¹H and ¹³C NMR contact shifts induced by Ni²⁺ ions in aqueous solutions were used to elucidate the conformation of poly[1-(1-oxo-4-pyridyl)ethylene] ( 1 ) and of the model compound 1,3-bis(1-oxo-4-pyridyl)propane ( 2 ). The equilibrium position of the pyridine N-oxide ring relative to the adjacent methine or methylene group was deduced from the angular dependence of the hyperfine coupling of β-protons and -carbons, assuming a twofold potential energy for the rotation about the C⁴? N axis. In the case of 1 it could be shown that the dihedral angle between the plane H^β? C^α? C⁴ and the plane of the ring is 15±5°.