Distribution effects on 1H double-quantum MAS NMR spectra

The effect of a distribution in the H-1-H-1 dipolar coupling on H-1 double-quantum (DQ) magic angle spinning (MAS) nuclear magnetic resonance (NMR) spinning sideband patterns is considered. In disordered or amorphous materials a distribution in the magnitude of the H-1-(1) H dipolar coupling is a realistic possibility. Simulations of the H-1 DQ MAS NMR spinning sideband spectra were performed with the two-spin approximation. These simulations reveal that a dipolar coupling distribution can greatly affect the DQ spectral shape and behavior of the DQ build-up. The spectral line shapes are quantified by measurement of the relative intensities of the DQ sidebands. These variations in the H-1 DQ NMR spectra are evaluated as a function of the width of the dipolar coupling distribution. As an example, the experimental DQ spinning sideband spectrum for a hydrated polyoxoniobate containing 15 H2O molecules per hexaniobate cluster, are better simulated with a distribution of dipolar couplings opposed to a single coupling constant. (C) 2004 Elsevier Inc. All rights reserved.