Systematic strategy for decoding the NMR spin-spin coupling mechanism: the J-OC-PSP method.

Criteria for analyzing the NMR spin-spin coupling mechanism were derived. Advantages and disadvantages of parallel- and post-processing analytical methods are discussed. An orbital decomposition into just one-orbital contributions provides less information than a decomposition into one-, two- and m-orbital effects. If just the last orbital in an orbital path leading from the perturbing to the responding nucleus is considered, the physics of the transport of spin information cannot be described. The theory of the J-OC-PSP (decomposition of J into Orbital Contributions using Orbital Currents and Partial Spin Polarization) is described to demonstrate the role of orbital contributions, orbital spin densities and orbital currents for the coupling mechanism. J-OC-PSP1 provides a decomposition into one- and two-orbital contributions with distinct physical reference (Ramsey perturbation of orbitals, steric exchange interactions, etc.) whereas J-OC-PSP2 introduces distinct orbital paths from perturbing to responding nucleus, clarifies the difference between active, passive and frozen orbitals and makes it possible to separate through-space from through-bond spin-spin coupling mechanism. Fermi contact coupling in hydrocarbons over more than two bonds is found to occur preferentially through space by tail interactions of the orbitals, as was anticipated in the early work of Barfield.