Bonds or not bonds? Pancake bonding in 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl radical dimers and their derivatives

Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to pi-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (pi*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.