Direct Investigation of Halogen Bonds by Solid-State Multinuclear Magnetic Resonance Spectroscopy and Molecular Orbital Analysis

Noncovalent interactions play a ubiquitous role in the structure, stability, and reactivity of a wide range of molecular and ionic cocrystals, pharmaceuticals, materials, and biomolecules. The halogen bond continues to be the focus of much attention, due in part to its strength and unique directionality. Here, we report a multifaceted experimental and computational study of halogen bonds in the solid state. A series of cocrystals of three different diiodobenzene molecules and various onium halide (Cl- or Br-) salts, designed to exhibit moderately strong halogen bonds (C-I center dot center dot center dot X-) in the absence of competing hydrogen bonds, has been prepared and characterized by single-crystal X-ray diffraction. Interestingly, a wide range of geometries about the halide anion are observed. Cl-35/37 and Br-79/81 solid-state NMR spectroscopy is applied to characterize the nuclear quadrupolar coupling constants (C-Q) and asymmetry parameters (eta(Q)) for the halogen-bonded anions at the center of bonding environments ranging from approximately linear to distorted square planar to octahedral. The relationship between the halogen bond environment and the quadrupolar parameters is elucidated through a natural localized molecular orbital (NLMO) analysis in the framework of density functional theory (DFT). These calculations reveal that the lone pair type orbitals on the halogen-bonded anion govern the magnitude and orientation of the quadrupolar tensor as the geometry about the anion is systematically altered. In -C-I center dot center dot center dot X--I-C environments, the value of eta(Q) is well-correlated to the I center dot center dot center dot X-center dot center dot center dot I angle. C-13 NMR and DFT calculations show a correlation between chemical shifts and halogen bond strength (through the C I distance) in o-diiodotetrafluorobenzene cocrystals. Overall, this work provides a chemically intuitive understanding of the connection between the geometry and electronic structure of halogen bonds and various NMR parameters with the aid of NLMO analysis.