An NMR crystallographic approach for the determination of the hydrogen substructure of nitrogen bonded protons

We present an approach for determining the positions of the hydrogen atoms in NHx groups of crystalline materials. It is based on a combination of quantum-chemical DFT calculations and quantitative solid-state NMR measurements of N–H and H–H distances. The former provide the alignment of the NHx groups within the crystal structure whereas the latter define their internal geometry. For the model system melem (C6N7(NH2)3) the N–H and H–H distances were determined to 1.055(7) Å using a Lee–Goldburg CP experiment and to 1.79(2) Å based on homonuclear double-quantum excitation with a R1462 sequence, respectively. The thus-obtained positions of the hydrogen atoms were verified by analysing 1H–13C solid-state NMR cross-polarization build-up curves. The calculated polarization transfer rates depend on both the hetero- and the homonuclear second moments MHC2 and MHH2. Thus this experiment is highly sensitive to the positions of the hydrogen atoms within a given crystal structure. The agreement between calculated and experimentally observed transfer rate constants turned out to be poor if the calculations were based on single crystal diffraction data only. While the use of quantum chemical relaxed structure models improve the situation significantly, a satisfactory agreement could only be reached with the incorporation of the NMR distances into the optimized structure. Our results prove that the combination of DFT structure optimizations with quantitative solid-state NMR experiments is a powerful and very accurate tool for the determination of the hydrogen substructure for a known structure model of heavy atoms only. Since the localization of the hydrogen atoms is often not possible based on X-ray diffraction data, the presented approach appears to be very promising for future applications.