Broadband dipolar recoupling in the nuclear magnetic resonance of rotating solids:: A compensated C7 pulse sequence

We introduce an improved variant of the C7 pulse-sequence for efficient recoupling of spin-1/2 pair dipolar interactions in magic-angle spinning solid-state NMR spectroscopy.:The tolerance of C7 toward isotropic as well as anisotropic chemical shift offsets and rf inhomogeneity is improved considerably by replacing the original basic element C(phi)(<4(4)over bar>) = (2 pi)(phi)(2 pi)(phi+pi) With the cyclically permuted element C-phi(1(4) over bar 3) = (pi/2)(phi)(2 pi)(phi+pi)(3 pi 1/2)(phi). The improved performance of this permutationally offset stabilized variant of C7 is analyzed by average Hamiltonian theory to fifth order, numerical simulations, and demonstrated by experiments on powder samples of doubly C-13-labeled barium oxalate hemihydrate and diammonium fumarate. (C) 1998 American Institute of Physics.