Cross‐polarization for quadrupolar nuclei—proton to molybdenum‐95

Abstract

AbstractHigh‐field, 95Mo, solid‐state nuclear magnetic resonance spectra of (NH4)6Mo7O24 · 4H2O and (Bu4N)2Mo2O7 were obtained using solid‐echo techniques together with 1H cross‐polarization and dipolar decoupling. The results show that reliable second‐order powder line shapes can be obtained with excellent cross‐polarization enhancements of 66–86% of maximum theoretical enhancement. The lower than maximum enhancements are due to the relatively long TIS values, which are similar in magnitude to the TH1ρ values. Experiments were performed which reveal the dynamics involved in the cross‐polarization process, including measurement of the cross‐relaxation time constant and the 1H and 95Mo rotating‐frame spin‐lattice relaxation times. It was found that the principal relaxation process causing low gain in the observed signal was the 1H T1ρ, which was found to be on the order of 23–24 ms. The experimental results obtained indicate that selective cross‐polarization dynamics occur in molybdenum cluster species with inequivalent molybdenum sites. The 95Mo rotating‐frame spin‐lattice relaxation rate is long in comparison with the other relaxation processes, making molybdenum a good cross‐polarization candidate but precluding the use of multiple contact cross‐polarization.