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Rienstra, C.M., Hatcher, M.E., Mueller, L.J., Sun, B.Q., Herzfeld, J., and Griffin, R.G.. 1998. Efficient Multispin Homonuclear Double Quantum Recoupling for Magic-Angle Spinning NMR: 13C-13C Correlation Spectroscopy of u-13C-Erythromycin A. J. Amer. Chem. Soc. 120: 10602-10612. Full Article

We introduce a radio frequency (rf) pulse sequence for efficient homonuclear double-quantum dipolar recoupling under magic-angle spinning NMR. The sequence is optimized for two-dimensional double-quantum 13C-13C chemical shift correlation spectroscopy in multiple spin systems, such as the U-13C-labeled antibiotic erythromycin A. Spin systems such as this display a wide range of isotropic and anisotropic chemical shifts and, therefore, require a broadband dipolar recoupling sequence that minimizes the errors arising from the interaction of chemical shifts and rf inhomogeneity. The sequence should also preserve the theoretical efficiency over the powder average (~73%) provided by the C7 experiment of Levitt and co-workers (Lee, Y. K.; Kurur, N. D.; Helmle, M.; Johannessen, O. G.; Nielsen, N. C.; Levitt, M. H. Chem. Phys. Lett. 1995, 242, 304-309). We satisfy these criteria by combining the standard C7 (2-2+180) elements with -pulse permuted elements (-2+180-, in analogy to the MLEV decoupling scheme) to remove error terms over a 10% range of rf amplitude. The new sequence, which we refer to as CMR7 (combined MLEV refocusing and C7), yields for two-spin systems broadband double-quantum filtering efficiencies greater than 70%. For multispin systems, the improved polarization transfer efficiency results in greater cross-peak intensities, facilitating assignment of U-13C-labeled molecules in the solid state.

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