Investigation of the π Character of a C−C Bond with the Help of the Diamagnetic and Paramagnetic Spin−Orbit Term of the NMR Spin−Spin Coupling Constant

Abstract

The NMR spin-spin coupling constant (SSCC) 1 J(C-C) is a sensitive antenna for investigating the electronic structure in hyper- and π-conjugated hydrocarbons. The magnetic dipole field of the perturbed nucleus generates orbital currents, which are experienced by the responding nucleus, thus leading to the paramagnetic spin-orbit (PSO) coupling mechanism. The strength of the orbital currents depends on the availability of high-lying occupied and low-lying virtual orbitals with distinct p π or p σ character, the nodal properties of these orbitals, and the overlap between zeroth- and first-order orbitals. For C-C bonds in unsaturated hydrocarbons, the value of 1 PSO(C-C) depends on opposing effects such as the p σ and the p π character of the C-C bond (i.e., the degree of hybridization at the C atoms and the degree of π delocalization in the conjugated system). This dependence leads to the overall cubic behavior of the PSO term when given as a function of the C-C bond character and the strong scattering of PSO values for a given bond type. Under certain conditions, the degree of hybridization can be determined with the help of the PSO term. Also, the PSO term helps to distinguish hyperconjugation and π-conjugation effects. For a given C-C bond type with fixed or similar hybridization at the C atoms, the PSO term can be given as a function of the C-C bond order, thus representing a sensitive descriptor for the degree of π delocalization.