Effect of Electronegative Substituents and Angular Dependence on the Heteronuclear Spin-Spin Coupling Constant 3JC-H: An Empirical Prediction Equation Derived by Density Functional Theory Calculations

A general carbon-proton vicinal Coupling constant ((3)J(C-H)) prediction equation has been empirically derived by a coupling constant database of 2157 (3)J(C-H) calculations (at the hybrid DFT MPW IPW91/6-31G(d,p) level). The equation includes the electronegativity effect of the substituents attached to the C-13-C-C-H-1 fragment and the dihedral (Phi) dependence of the heteronuclear spin-coupling. A set of butane and pentane models were built, systematically varying both the Phi torsion angle in 30 degrees steps and the substitution pattern with several electronegative substituents (Br, NH2, F, Cl, SH, OH) in order to obtain the coupling constant database. The here reported (3)J(C-H) equation is a quantitative prediction tool, particularly useful as a support in the analysis of NMR data for the structural elucidation of organic compounds characterized by specific substitution patterns. To confirm the accuracy of our equation in the prediction of the experimental (3)J(C-H) couplings, we tested the equation, comparing 114 experimental (3)J(C-H) values obtained from 29 polysubstituded benchmark organic compounds with the predicted data. In addition, a set of (3)J(C-H) coupling bidimensional Karplus-type curves correlating the calculated (3)J(C-H) values to the specific dihedral angle for every substitution pattern considered were built in order to evaluate the magnitude of the electronegativity effect.