Internuclear <sup>31</sup>P−<sup>51</sup>V Distance Measurements in Polyoxoanionic Solids Using Rotational Echo Adiabatic Passage Double Resonance NMR Spectroscopy

We report the first results establishing rotational echo adiabatic passage double resonance (REAPDOR) experiments for distance measurements between a spin-<sup>1</sup>/<sub>2</sub> (<sup>31</sup>P) and spin-<sup>7</sup>/<sub>2</sub> (<sup>51</sup>V) pair in a series of vanadium-substituted polyoxoanionic solids from the Keggin and Wells−Dawson families. We have quantitatively measured <sup>31</sup>P−<sup>51</sup>V distances in monovanadium substituted K<sub>4</sub>PVW<sub>11</sub>O<sub>40</sub>, 1-K<sub>7</sub>P<sub>2</sub>VW<sub>17</sub>O<sub>62</sub>, and 4-K<sub>7</sub>P<sub>2</sub>VW<sub>17</sub>O<sub>62</sub>. Numerical simulations of the experimental data yield very good agreement with the averaged P−W/P−V distances determined from the X-ray diffraction measurements in the same or related compounds. REAPDOR is therefore a very sensitive P−V distance probe anticipated to be especially useful in the absence of long-range order. Our results suggest that REAPDOR spectroscopy could be broadly applicable for interatomic distance measurements in other spin-<sup>7</sup>/<sub>2</sub>−spin-<sup>1</sup>/<sub>2</sub> nuclear pairs.