Analysis of multipath transmission of spin–spin coupling constants in cyclic compounds with the help of partially spin-polarized orbital contributions

Abstract

The multipath spin–spin coupling mechanism in cyclic compounds leads to NMR spin–spin coupling constants (SSCCs) strongly deviating from normal SSCCs. This is shown for ⁿJ(CC), ⁿJ(CH), and ⁿJ(HH) coupling constants of cyclic and bicyclic compounds using the recently developed decomposition of of J into orbital contributions using orbital currents and partial spin polarization (J-OC-PSP). A typical multipath SSCC depends on a through-space part, two or more through-bond parts, and the path-path interaction part where the latter results from steric exchange repulsion between the bond paths. The sign and magnitude of the through-space contribution as well as the through-bond contributions can be predicted by analysis of the Fermi contact spin density distribution calculated from the localized molecular orbitals involved in the coupling mechanism. In this way, unusual SSCCs of cyclopropane and bicyclo[1.1.1]pentane are explained for the first time. Also, measured and calculated ²J(CC) coupling constants of cyclopentane and tetrahydrofuran are shown to be averages over the pseudorotational motion of these ring molecules where each individual SSCC of a conformation passed in the pseudorotation is the sum of different path contributions. The three-bond path contributions dominate the ²J(CC) coupling constants of the pseudorotating molecules and introduce its dependence on the pseudorotational phase angle.