Detailed analysis of the S-RESPDOR solid-state NMR method for inter-nuclear distance measurement between spin-1/2 and quadrupolar nuclei.

We present a detailed analysis of the Symmetry-based Resonance-Echo Saturation-Pulse DOuble-Resonance (S-RESPDOR) method in order to measure the inter-nuclear distances between spin-1/2 and quadrupolar nuclei. This recently introduced sequence employs a symmetry-based recoupling scheme on the observed spin-1/2 channel and a saturation pulse on the quadrupolar channel. This method requires a low radio-frequency (rf) field, is compatible with high MAS frequency and allows a rapid determination of inter-nuclear distances by fitting the experimental signal fraction to an analytical expression. Here, we analyze in detail the influence of the various experimental and spin-interaction parameters on the S-RESPDOR signal fraction and the measured distance. We show that the S-RESPDOR signal fraction only depends on the quadrupole interaction and the inter-nuclear distance. We demonstrate that the required rf-field on the quadrupolar channel is smaller than that required for an adiabatic-passage pulse in REAPDOR-type experiments. The only limitation of the method is the requirement of accurate rotor synchronization between the two parts of the dipolar recoupling sequences. Using S-RESPDOR, we have quantitatively measured a (31)P-(51)V distance of 357 pm in a mono-vanadium-substituted polyoxo-tungstate, K(4)PVW(11)O(40), from the Keggin family and a (13)C-(67)Zn distance of 286 pm in [80%-(67)Zn]zinc [1-(13)C]acetate. These results show that S-RESPDOR can be employed in the challenging cases of quadrupolar nuclei exhibiting a high spin number and either large chemical-shift anisotropy ((51)V) or low gyromagnetic ratio ((67)Zn).