Analysis of long-range NMR spin-spin coupling in polyenes and the pi-mechanism.

The J-OC-PSP (decomposition of J into Orbital Contributions using Orbital Currents and Partial Spin Polarization) method is applied to analyze NMR spin-spin coupling constants in polyenes, which were calculated using coupled perturbed density functional theory in connection with the B3LYP hybrid functional and a [7s,6p,2d/4s,2p] basis set. The analysis revealed that the pi-mechanism for Fermi contact (FC) spin coupling is based on passive pi-orbital contributions. The pi-orbitals transfer spin information between sigma orbitals (spin-transport mechanism) or increase the spin information of a sigma orbital by an echo effect. The calculated FC(pi) values are rather constant for small polyenes ranging between 3.5-5.5 Hz for a double bond. They decay more slowly with the distance between perturbing and responding nucleus than the sigma contributions to the FC term. The sign of the passive FC(pi) contribution can be assessed from a Dirac vector model. The limits for long-range coupling in a polyene were determined and their practical implications discussed.