Processing of CP MAS kinetics: Towards NMR crystallography for complex solids

摘要

Variable temperature and high data point density measurements of H-1-P-31 cross-polarization kinetics in the powdered ammonium dihydrogen phosphate (ADP) have been carried out in the range of -40 degrees C to +90 degrees C upon 7 and 10 kHz MAS. The advanced route of processing CP MAS kinetic data has been developed. It is based on reducing the incoherent far range order spin couplings and extracting the CP oscillatory term with the sequent mathematical treatment. The proper replica has been found, which allowed to reduce the Fourier-Bessel (Hankel) transform calculating the angularly averaged and purely distance-depending spin distribution profile to the routine Fourier transform. The shortest P-31-H-1 distances determined by CP MAS kinetics get between the values obtained by neutron and X-ray diffraction, whereas those for more remote protons are slightly larger. The changes in P center dot center dot center dot H distances are hardly noticeable, though a certain trend to increase upon the heating can be deduced. The clearly pronounced effect was the increase of the spin-diffusion rate constant upon heating. It allows to state that the communication between interacting spins is the process extremely easy to activate. Published by AIP Publishing.