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 depe ndence. Experimental data using model compounds are shown that demonst rate this; as proton RF decoupling powers are increased, the REDOR dif ference curves change in shape with the maximum REDOR difference appro aching unity. This is an important consideration when fitting data wit h simulations in an attempt to determine internuclear distances from d ipole-dipole couplings. Often, the desired decoupling powers that achi eve convergence of REDOR difference curves require proton RF powers ou tside the capabilities of conventional MAS solid-state probes. To alle viate this problem, the two-pulse phase-modulated (TPPM) pulse train i s applied during the mixing period of the REDOR pulse sequence. The re sults show improved efficiency of proton decoupling when TPPM is emplo yed, as compared to the continuous wave case at the same decoupling RF strength. Simulations of a four spin-1/2 system qualitatively agree w ith the experiments. (C) 1998 Elsevier Science B.V. All rights reserve d.