Phase-modulated LA-REDOR: a robust, accurate and efficient solid-state NMR technique for distance measurements...

Distances between a spin-1/2 and a spin>1/2 can be efficiently measured by a variety of magic-angle spinning solid state NMR methods such as Rotational Echo Adiabatic Passage Double Resonance (REAPDOR), Low-Alpha/Low-Amplitude REDOR (LA-REDOR) and Rotational-Echo Saturation-Pulse Double-Resonance (R/S-RESPDOR). In this manuscript we show that the incorporation of a phase modulation into a long quadrupolar recoupling pulse, lasting 10 rotor periods that are sandwiched between rotor-synchronized pairs of dipolar recoupling π pulses, extends significantly the range of the values of the quadrupole moments that can be accessed by the experiment. We show by a combination of simulations and experiments that the new method, phase-modulated LA-REDOR, is very weakly dependent on the actual value of the radio-frequency field, and is highly robust with respect to off-resonance irradiation. The experimental results can be fitted by numerical simulations or using a universal formula corresponding to an equal-transition-probability model. Phase-modulated LA-REDOR (13)C{(11)B} and (15)N{(51)V} dipolar recoupling experiments confirm the accuracy and applicability of this new method.