Compensation for pulse imperfections in rotational-echo double-resonance NMR by composite pulses and EXORCYCLE

We describe a simple method to compensate for pulse-angle errors in rotational-echo double resonance (REDOR) experiments for determining heteronuclear distances in solids. By using composite 180 degrees pulses on the unobserved dephasing spin and EXORCYCLE for the single pi pulse on the observed channel, the REDOR curve becomes much less sensitive to pulse-angle errors. Both improvements are demonstrated by experiments on the model compound, (15)N, (13)Calpha -labeled N-t-BOC-glycine, and are confirmed by numerical simulations. The advantage of EXORCYCLE is also shown analytically using the product operator formalism. The proposed simple schemes compensate for unavoidable pulse-angle errors that arise, for example, from radiofrequency field inhomogeneity. They also make REDOR experiments more accurate and robust for low-sensitivity samples where direct pulse-length calibration is difficult.