Complete dipolar decoupling of 13C and its use in two-dimensional double-quantum solid-state NMR for determining polymer conformations

A multiple-pulse technique for complete dipolar decoupling of directly bonded C-13-labeled sites is described. It achieves significant spectral simplifications in a recently introduced two-dimensional double-quantum solid-state NMR experiment for determining torsion angles. Both homonuclear and heteronuclear dipolar couplings are removed by combining a C-13 multiple-pulse sequence with continuous-wave irradiation on the protons. The C-13 sequence has a fundamental 10-pulse cycle which is a significantly modified magic-sandwich-echo sequence. The crucial heteronuclear decoupling is achieved by breaking the 360 degrees "inner" pulses in the magic sandwich into 90 degrees pulses and spacing them by H-1 360 degrees pulse lengths. Spectral artifacts typical of multiple-pulse sequences are eliminated by phase shifts between cycles. In contrast to many other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar coupling, which makes the sequence practical for direct detection even with long pulse ring-down times. A modification of the sequence to scale the chemical shift and increase the effective spectral width is also presented. The 1D and double-quantum 2D experiments are demonstrated on polyethylene with 4% C-13 - C-13 spin pairs. The potential of this approach for distinguishing segmental conformations is illustrated by spectral simulations of the two-dimensional ridge patterns that correlate double-quantum and single-quantum chemical-shift anisotropies. (C) 1998 Academic Press.