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Distribution effects on 1H double-quantum MAS NMR spectra
G P Holland1, B R Cherry, T M Alam
1Department of Organic Materials, Sandia National Laboratories, Albuquerque, NM 87185, USA.
Abstract:
The effect of a distribution in the (1)H-(1)H dipolar coupling on (1)H 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 (1)H-(1)H dipolar coupling is a realistic possibility. Simulations of the (1)H 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 (1)H 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 H(2)O molecules per hexaniobate cluster, are better simulated with a distribution of dipolar couplings opposed to a single coupling constant.
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