Multinuclear Solid-State Magnetic Resonance as a Sensitive Probe of Structural Changes upon the Occurrence of Halogen Bonding in Co-crystals

Although the understanding of intermolecular interactions, such as hydrogen bonding, is relatively well-de- veloped, many additional weak interac- tions work both in tandem and compet- itively to stabilize a given crystal struc- ture. Due to a wide array of potential applications, a substantial effort has been invested in understanding the hal- ogen bond. Here, we explore the utility of multinuclear ( 13 C, 14/15 N, 19 F, and 127 I) solid-state magnetic resonance experi- ments in characterizing the electronic and structural changes which take place upon the formation of five halo- gen-bonded co-crystalline product ma- terials. Single-crystal X-ray diffraction (XRD) structures of three novel co- crystals which exhibit a 1:1 stoichiome- try between decamethonium diiodide (i.e., ((CH3)3N + A2)10N + A3)3)A I � )) and different para-dihalogen-substitut- ed benzene moieties (i.e., p-C6X2Y4, X = Br, I; Y = H, F) are presented. 13 C and 15 N NMR experiments carried out on these and related systems validate sample purity, but also serve as indirect probes of the formation of a halogen bond in the co-crystal complexes in the solid state. Long-range changes in the electronic environment, which manifest through changes in the electric field gradient (EFG) tensor, are quantita- tively measured using 14 N NMR spec- troscopy, with a systematic decrease in the 14 N quadrupolar coupling constant (CQ) observed upon halogen bond for- mation. Attempts at 127 I solid-state NMR spectroscopy experiments are presented and variable-temperature 19 F NMR experiments are used to dis- tinguish between dynamic and static disorder in selected product materials, which could not be conclusively estab- lished using solely XRD. Quantum chemical calculations using the gauge- including projector augmented-wave (GIPAW) or relativistic zeroth-order regular approximation (ZORA) densi- ty functional theory (DFT) approaches complement the experimental NMR measurements and provide theoretical corroboration for the changes in NMR parameters observed upon the forma- tion of a halogen bond.