~(31)P MAS Refocused INADEQUATE Spin-Echo (REINE) NMR Spectroscopy: Revealing J Coupling and Chemical Shift Two-Dimensional Correlations in Disordered Solids

摘要

Two-dimensional (2D) variations in ~2J_(p1,p1), ~2J_(p1,p2). and ~2J_(p1,p2) are obtained-using the REINE (REfocused INADEQUATE spin-Echo) pulse sequence presented by Cadars et al. (Phys. Chem. Chem. Phys. 2007, 9, 92-103)-from pixel-by-pixel fittings of the spin-echo modulation for the 2D correlation peaks due to linked phosphate tetrahedra (P_1-P_1, P_1-P_2, P_2-P_1, and P_2-P_2) in a ~(31)P refocused INADEQUATE solid-state MAS NMR spectrum of a cadmium phosphate glass, 0.575CdO-0.425P_2O_5. In particular, separate variations for each 2D ~(31)P REINE peak are obtained which reveal correlations between the J couplings and the ~(31)P chemical shifts of the coupled nuclei that are much clearer than those evident in previously presented 2D z-filtered ~(31)P spin-echo spectra. Notably, such correlations between the J couplings and the ~(31)P chemical shifts are observed even though the conditional probability distributions extracted using the protocol of Cadars et al. (J. Am. Chem. Soc. 2005, 127, 4466-4476) indicate that there is no marked correlation between the ~(31)P chemical shifts of neighboring phosphate tetrahedra. For 2D peaks at the P_2 ~(31)P chemical shift in the direct dimension, there can be contributions from chains of three units (P1-P2-P1), chains of four units (P1-P2-P_2-P_1), or longer chains or rings (-P_2-P_2-P_2-): for the representative glass considered here, best fits are obtained assuming a glass comprised predominantly of chains of four units. The following variations are found: ~2J_(p1,p1) = 13.4 ± 0.3 to 14.8 ± 0.5 Hz, ~2J_(p1,p1) = 15.0 ± 0.3 to 18.2 ± 0.3 Hz, and ~2J_(p1,p1) = 5.9 ± 0.6 to 9.1 ± 0.9 Hz from the fits to the P1-P1, P1-P2, and P2-P2 peaks, respectively. The correlation of a particular J coupling with the ~(31)P chemical shifts of the considered nucleus and the coupled nucleus is quantified by the coefficients C_(F2) and C_(F1_), that correspond to the average pixel-by-pixel change in the J coupling with respect to the chemical shift of the observed (F_2) and neighboring (F_1) ~(31)P nuclei, respectively.