Rapid Identification of Halogen Bonds in Co-Crystalline Powders via 127 I Nuclear Quadrupole Resonance Spectroscopy.

127 I nuclear quadrupole resonance (NQR) spectroscopy is established as a rapid and robust method to indicate the formation of iodine-nitrogen halogen bonds in co-crystalline powders. Once the relevant spectral frequency range has been established, diagnostic 127 I NQR spectra can be acquired in seconds. The method is demonstrated for a series of co-crystals of 1,4-diiodobenzene. Changes in the 127 I quadrupolar coupling constant (CQ ) by up to 74.4 MHz correlate with the length of the C-I donor covalent bond and inversely with the I⋅⋅⋅N halogen-bond length. The predictive power of this technique is validated on two previously unknown co-crystalline powders prepared mechanochemically. Single-crystal growth via co-sublimation and structure determination by single-crystal X-ray diffraction cross-validates the findings. Natural localized molecular-orbital analyses provide insight into the origins of the quadrupolar coupling constants.