Monitoring the refinement of crystal structures with 15N solid-state NMR shift tensor data

The N-15 chemical shift tensor is shown to be extremely sensitive to lattice structure and a powerful metric for monitoring density functional theory refinements of crystal structures. These refinements include lattice effects and are applied here to five crystal structures. All structures improve based on a better agreement between experimental and calculated N-15 tensors, with an average improvement of 47.0 ppm. Structural improvement is further indicated by a decrease in forces on the atoms by 2-3 orders of magnitude and a greater similarity in atom positions to neutron diffraction structures. These refinements change bond lengths by more than the diffraction errors including adjustments to X-Y and X-H bonds (X, Y = C, N, and O) of 0.028 +/- 0.002 angstrom and 0.144 +/- 0.036 angstrom, respectively. The acquisition of N-15 tensors at natural abundance is challenging and this limitation is overcome by improved H-1 decoupling in the FIREMAT method. This decoupling dramatically narrows linewidths, improves signal-to-noise by up to 317%, and significantly improves the accuracy of measured tensors. A total of 39 tensors are measured with shifts distributed over a range of more than 400 ppm. Overall, experimental N-15 tensors are at least 5 times more sensitive to crystal structure than C-13 tensors due to nitrogen's greater polarizability and larger range of chemical shifts. (C) 2015 AIP Publishing LLC.