77Se Solid-State NMR Investigation of Selenium Chemical Shift Tensors of Chalcogen Bonds in Selenadiazole Cocrystals

This study focuses on 3,4-dicyano-1,2,5-selenadiazole and substituted 2,1,3-benzoselenadiazole-based cocrystals synthesized via mechanochemical methods and characterized by a combination of X-ray diffraction and solid-state NMR spectroscopy. Eight new single-crystal structures are reported, revealing a variety of chalcogen bond (ChB) geometries and binding motifs that are found to promote low-dimensional molecular architectures. We find that ⁷⁷Se isotropic chemical shifts follow exponential decay or growth trends along with the ChB length, while also depending on the electrostatic contribution of the ChB donor. These trends are shown to be governed by changes to the intermediate selenium chemical shift tensor component, δ₂₂. Such behavior is further exploited to estimate ChB lengths in compounds unsuitable for single-crystal structure determination. This methodology highlights the utility of solid-state NMR as a powerful alternative for probing ChB interactions, particularly in systems where traditional crystallographic techniques are not applicable. The results offer critical physical insights into the origins of the selenium chemical shift tensors of ChB-based materials.