Principles of spin-echo modulation by J-couplings in magic-angle-spinning solid-state NMR. |
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Authors: | Luminita Duma Wai Cheu Lai Marina Carravetta Lyndon Emsley Steven P Brown Malcolm H Levitt |
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Affiliation: | Chemistry Department, Southampton University, SO17 1BJ, UK. |
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Abstract: | In magic-angle-spinning solid-state NMR, the homonuclear J-couplings between pairs of spin-1/2 nuclei may be determined by studying the modulation of the spin echo induced by a pi-pulse, as a function of the echo duration. We present the theory of J-induced spin-echo modulation in magic-angle-spinning solids, and derive a set of modulation regimes which apply under different experimental conditions. In most cases, the dominant spin-echo modulation frequency is exactly equal to the J-coupling. Somewhat surprisingly, the chemical shift anisotropies and dipole-dipole couplings tend to stabilise--rather than abscure--the J-modulation. The theoretical conclusions are supported by numerical simulations and experimental results obtained for three representative samples containing 13C spin pairs. |
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