Investigation of chalcogen bonding in ebselen·DMAP co-crystals using synchrotron far-infrared spectroscopy

Chalcogen bonding is a non-covalent interaction gaining increasing interest in various fields including materials science and biochemistry. Although chalcogen bonding in the solid-state is commonly characterised using NMR and X-ray crystallography, complementary techniques are needed to fully understand the solid-state behaviour of this interaction in order to establish robust structure-property relationships. This work presents the synchrotron far-infrared spectra of five chalcogen bonding co-crystal systems comprising ebselen derived chalcogen bond donors with the Lewis base, DMAP. The experimental results are in good agreement with calculated periodic DFT vibrational modes at the B3LYP-D3/def2-SVP level. Crystalline powder samples show strong signals between 60 and 115 cm^-1, which upon visualisation of the calculated eigenvectors, were confidently assigned to chalcogen bonding vibrations. The Se⋯N chalcogen bond vibrational frequency and infrared band profile is system specific, however, and there is no correlation observed between the frequency and chalcogen bond strength. Ultimately, this work represents, to our knowledge, one of the first uses of far-infrared spectroscopy to identify chalcogen bonding interactions in solid-state crystalline systems.