Imaging the spatial distribution of radiofrequency field, sample and temperature in MAS NMR rotor

Abstract

We investigate using nutation experiments the spatial distribution of radiofrequency (rf) field, sample, temperature and cross-polarization transfer efficiency in 1.3 mm rotor. First, two-dimensional (2D) 1H nutation experiments on silicone thin cylinders in the presence of B0 field gradient generated by shim coils are used to image the spatial distribution of rf field inside the rotor. These experiments show that the rf field is asymmetrical with respect to the center of the rotor. Moreover, they show the large inhomogeneity that still remains across the sample when using spacers, and that even in this case, the rf-field close to the drive cap is decreased to ca. only 20% of its maximum value. Such 2D nutation experiment in the presence of B0 field gradient are also employed to demonstrate the migration of adamantane sample from the center of the rotor to its ends during Magic-Angle Spinning (MAS). Furthermore, 2D 1H nutation experiments on nickelocene exhibiting temperature-dependent isotropic chemical shift provides insights into the temperature distribution inside rotor. Finally three-dimensional (3D) 1H → 13C Cross-Polarization under MAS (CPMAS) nutation experiment indicates that only nuclei subject to the largest rf field contribute to the CPMAS transfer, when using rf field of constant amplitude on both channels. Such high selectivity allows the determination of accurate dipolar coupling constants in the Cross-Polarization with Variable Contact (CP-VC) experiment under fast MAS, at the expense of low sensitivity. Conversely when using ramped-amplitude on the 1H channel during the CPMAS transfer, nuclei subject to smaller rf field contributes to the transfer, which increases the sensitivity of CPMAS experiment but does not allow an accurate determination of dipolar coupling constants using CP-VC experiment.