Improved heteronuclear decoupling in REDOR with the use of TPPM

Rotational-echo double-resonance (REDOR) solid-state NMR spectroscopy exhibits an explicit proton radio frequency (RF) decoupling power dependence. Experimental data using model compounds are shown that demonstrate this; as proton RF decoupling powers are increased, the REDOR difference curves change in shape with the maximum REDOR difference approaching unity. This is an important consideration when fitting data with simulations in an attempt to determine internuclear distances from dipole–dipole couplings. Often, the desired decoupling powers that achieve convergence of REDOR difference curves require proton RF powers outside the capabilities of conventional MAS solid-state probes. To alleviate this problem, the two-pulse phase-modulated (TPPM) pulse train is applied during the mixing period of the REDOR pulse sequence. The results show improved efficiency of proton decoupling when TPPM is employed, as compared to the continuous wave case at the same decoupling RF strength. Simulations of a four spin–1/2 system qualitatively agree with the experiments.