F 19 / 23 Na multiple quantum cross polarization NMR in solids

Abstract

F 19 → 23 Na triple quantum (TQ) cross polarization (TQCP) experiments and numerical simulations have been performed on the oxyfluoride NaMoO3F. Due to the orientation dependence of the quadrupolar tensor and thus the TQ nutation frequency (ωs,nut), only a fraction of the spins in the powder can match the Hartmann–Hahn condition at the same time, for a fixed F19 rf field strength (ω1I). Numerical simulations of the static TQCP process, for different single crystallite orientations, demonstrate that the most efficient TQCP occurs for parts of the powder where the quadrupolar splitting, Q′, is largest, and where the TQ coherences are pure eigenstates of the system, even though ωs,nut is smallest for these orientations. Under magic angle spinning (MAS) conditions, ωs,nut becomes time dependent and again, efficient TQCP is observed at times during the rotor period where Q′ is largest. TQCP intensities for different crystallites were calculated as a function of ω1I, for fixed Na23 rf field strengths, to obtain TQCP matching profiles and to determine optimum Hartmann–Hahn conditions. The TQCP matching profiles vary significantly with crystallite orientation and with spinning speed, due to the different time dependences of the quadrupolar interaction for the different orientations. Centerband matching conditions are observed at F19 rf fields, close to the minimum values of ωs,nut observed during the rotor period. Two different centerbands are observed for crystallite orientations where there are two different local minima for ωs,nut. The time dependence of ωs,nut results in higher-order (>2) sideband matching conditions. The experimentally observed static and MAS TQCP NMR matching profiles, and changes in the Na23 static and MAS second-order quadrupolar NMR lineshapes, could be rationalized on the basis of the calculated distributions of ωS,nut that occur within the powder. Optimum TQCP intensities were obtained with high Na23 rf power, to maximize ωs,nut, and slow MAS, to ensure efficient spin-locking; a further increase in intensity could be achieved by ramping the F19 rf field during the contact time. The efficiencies of the single quantum cross polarization (SQCP) multiple quantum (MQ) MAS and TQCP-MQMAS experiments were compared and were found to be very similar for NaMoO3F. Finally, the two-dimensional TQCP-MQMAS experiment was illustrated.