Abstract
Although the electron-mediated spin-spin or J coupling is conventionally viewed as transmitted via covalent bonds, examples of J couplings between atoms that are not formally bonded but are in close proximity (termed "through-space" J couplings) have been reported. In this work, we investigate the observation of homonuclear 31P J couplings in organochalcogen heterocycles, which occur between 31P in two separate molecules, confirming without doubt their through-space nature. The presence of this interaction is even more surprising for one compound, where it occurs between crystallographically equivalent species. Although crystallographically equivalent species need not be magnetically equivalent in the solid state, owing to the presence of anisotropic interactions, we demonstrate that it is not the shielding anisotropy that lifts magnetic equivalence, in this case, but the presence of heteronuclear couplings to 77Se. We support our experimental observations with periodic scalar-relativistic density functional theory calculations and coupling density deformation plots to visualize the mechanism of these interesting interactions.
Highlights
Indirect spin−spin (J) couplings are invaluable for the determination of chemical structure in solution-state NMR spectroscopy.[1]
In one compound the coupling can be observed directly in the 31P spectrum, while in a second it is not directly resolved, but can be measured using J-resolved spectroscopy. This latter interaction is of particular note as it occurs between two crystallographically equivalent species, and we show that magnetic equivalence is lifted only by the presence of heteronuclear J couplings to 77Se, allowing experimental measurement
We have demonstrated the observation of unusual intermolecular homonuclear through-space J couplings between P species in different molecules in the solid state
Summary
Indirect spin−spin (J) couplings are invaluable for the determination of chemical structure in solution-state NMR spectroscopy.[1]. As there is only one P per molecule, the second coupling has, by definition, to be to 31P in an adjacent molecule, confirming the lack of any conventional bonding pathway between the spins This observation was supported by periodic DFT calculations, which predicted two 31P−77Se J couplings, one through space and one through bond, with very similar magnitudes. Unlike the previously observed heteronuclear interactions, it is not possible to remove homonuclear J couplings using decoupling, and so we support our conclusions using periodic relativistic density functional theory (DFT) calculations and exploit coupling density deformation (CDD)[20] plots to visualize the mechanism by which these interesting interactions take place These observations add to the rich diversity of weak interactions that contribute to crystal packing and are of particular importance for chalcogen-rich molecular materials, which contain large and polarizable atoms.
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