Analysis of the Transmission Mechanism of NMR Spin-Spin Coupling Constants Using Fermi Contact Spin Density Distribution, Partial Spin Polarization, and Orbital Currents: XH_n Molecules

摘要

Trends in calculated and measured one-bond reduced spin-spin coupling constants (SSCSs) ~1K(XH) for twelve XH_n hydrides (X = C, Si, Ge, N, P, As, O, S, Se, F, Cl, Br) are explained using orbital contributions obtained with the J-OC-PSP (decomposition of J into Orbital Contributions using the Orbital Currents and partial Spin Polarization) approach. The sign and magnitudes of the orbital contributions can be rationalized with the help of the Fermi contact spin density distribution, the s-density of an orbital at the nucleus, the electronegativity, and the polarizability of the central atom X. Partitioning of Fermi contact, the paramagnetic spin-orbit, the diamagnetic spin-orbit, and the spin dipole terms as well as the total SSCC ~nK into one-orbital contributions ~nK~k and orbital interaction contributions ~nK~(k,l) (n, type of SSCC; k and l, indices of occupied orbitals) reveals that each of the four Ramsey terms adds to the spin-spin coupling mechanism; however, many of the orbital contributions cancel each other so that, for example, DSO and SD terms make only negligible contributions to ~1K(XH). The two types of orbital contributions are associated with two different transmission mechanisms via the exchange antisymmetry property of the wave function. ~nK~k is the result of an orbital relaxation mechanism whereas ~nK~(k,l) is closely related to the concept of stearic exchange antisymmetry. Trends in measured ~K(XH) SSCCs can be explained by an interplay of bond and lone pair contributions. Sign and magnitude of ~1K(XH) are rationalized by utilizing the nodal behavior of zeroth- and first-order orbitals. Results are converted into simple Dirac models.