Studies on the NMR contact shifts induced by hydrogen bonding with nitroxide radical. A correlation between 13C contact shifts induced by 13C-H...nitroxide hydrogen bond and 13C-H nuclear spin coupling constants

Abstract

1H and 13C NMR contact shifts have been observed for chloroform, methylene chloride and phenylacetylene in the presence of di-tertbutyl nitroxide radical (DBNO). Upfield and downfield contact shifts were observed for 1H and 13C NMR resonances, respectively. The relative values of 13C contact shifts with respect to 1H contact shifts are linearly correlated with directly bonded 13C-H spin coupling constants. This correlation is interpreted in terms of finite perturbation theory of nuclear spin coupling constants in which the 13C-H coupling constant is related to the electron spin density on the 13C nucleus induced when spin density is placed finitely on the proton. The potential utility of this relation in the prediction of sign and magnitude of long-range 13C-H coupling constants is also stated.