Analysis of large-anisotropy-spin recoupling pulses for distance measurement under magic-angle spinning NMR shows the superiority and robustness of a phase modulated saturation pulse.

The distance between a spin one-half and an attached spin possessing a large anisotropy can be obtained using different dipolar recoupling sequences that are based on the rotational-echo double resonance technique under magic-angle spinning solid-state NMR. The general difference between these sequences with respect to the coupled spin is the set of pulses applied in order to drive this spin out of equilibrium, thereby recoupling the dipolar interaction. Since complete inversion is practically not possible due to the coupled-spin anisotropy, using one or another pulse depends on the experimental and spin conditions: the spinning speed, the strength of the radio frequency field, the size of the anisotropic interaction (quadrupolar or chemical shiftanisotropy couplings), the offset, and the accuracy of setting the magic angle. Here we present a detailed description of the behavior of the anisotropic spin magnetization, including the macroscopic level transition probabilities, the degree of inversion, and the microscopic and macroscopic magnetizations during the applications of these pulses under different experimental conditions. As simulations show, a complete randomization of spin populations under a wide range of experimental conditions occurs under a specific phase modulation of the recoupling pulse while for all other cases dependence on experimental conditions is large and the achievable bandwidth is limited. A result of this detailed analysis is that the extension of the phase modulated pulse extends even further its robustness. The saturation capability is demonstrated experimentally for the quadrupolar spin of boron-11 in 4-methoxyphenylboronic acid.