Pushing the limits of NMR crystallography: a study on three forms of glycylglycine

NMR crystallography (NMRX), which integrates solid-state NMR (SSNMR) spectroscopy, X-ray diffraction (XRD), and density functional theory (DFT) calculations, provides deeper insight into crystal structures than any of these methods individually. With the goal of pushing the limits of NMRX, we have characterized three solid forms of the dipeptide glycylglycine, including a free base, a hydrochloride salt, and an unusual hemihydrochloride salt-the latter form is synthesized and characterized by single-crystal XRD here for the first time. SSNMR data include 2D ¹H-¹H DQ-SQ BABA spectra that are used to measure ¹H chemical shifts and provide assignments through homonuclear dipolar correlations, slow-spinning ¹H-¹³C and ¹H-¹⁵N CP/MAS spectra to determine the ¹³C and ¹⁵N chemical shift tensors, ultra-wideline ¹⁴N WURST-CPMG spectra to measure the ¹⁴N electric field gradient (EFG) tensors, 1D ³⁵Cl spectra to determine the ³⁵Cl EFG and chemical shift tensors, and 2D ³⁵Cl → ¹H D-RINEPT spectra to determine heteronuclear dipolar correlations between the chloride ions and nearby protons. Together with state-of-the-art DFT calculations, these parameters are related directly to molecular- and crystal-level structure and intermolecular hydrogen bonding. Modern DFT approaches using double-hybrid functionals are critical for predicting subtle differences in ¹³C and ¹⁵N chemical shift tensors between the three solid forms. By highlighting the current capabilities of NMRX, it is hoped that these results will open new avenues for the exploration of a plethora of materials and biosolids.