Halogen Bond Strength in Solids Quantified via Zeeman-Perturbed Nuclear Quadrupole Resonance Spectroscopy

Proton NMR is a ubiquitous and valuable probe of hydrogen bonds. Conversely, ¹²⁷I NMR of strong halogen bond (XB) donors is hopeless due to quadrupolar coupling constants (<i>C</i>_Q) on the order of GHz. We report here an innovative implementation of Zeeman-perturbed nuclear quadrupole resonance (Zp-NQR) spectroscopy, employing adjustable magnetic fields on the order of mT, which renders possible the acquisition and analysis of spectra of ¹²⁷I and ⁷⁹Br nuclei subject to quadrupolar couplings of up to 2.3 GHz in solid powders. This approach is demonstrated on three series of halogen-bonded cocrystals based on so-called "iconic" strong XB donors <i>p</i>-diiodotetrafluorobenzene, <i>sym</i>-trifluorotriiodobenzene, and <i>p</i>-dibromotetrafluorobenzene (27 compounds). Analysis of the spectra using a diagonalization of the Zeeman-quadrupolar Hamiltonian provides <i>C</i>_Q values and quadrupolar asymmetry parameters, thereby overcoming various limitations encountered in pure NMR and pure NQR. Inspection of the data reveals strong correlations with geometrical and structural features of the halogen bond, including its length. Dispersion-corrected zeroth-order regular approximation relativistic DFT computations of the interaction energies of the XB donor are strongly correlated with experimental and computed values of <i>C</i>_Q(¹²⁷I) and <i>C</i>_Q(⁷⁹Br). It is concluded that the electric field gradient at the XB donor site is a useful metric for quantifying XB strength in solids. The XB interaction energies range from ∼5 to 10 kcal mol^-1 for the systems studied herein. The Zp-NQR approach is amenable to widespread application to diverse problems in the chemical and materials sciences related to energy materials, crystal engineering, and many systems comprising strongly quadrupolar isotopes.