Sample spinning frequencies

The influence of the homonuclear magnetic dipole-dipole interaction on can be reduced either by an increase of the sample spinning frequency, Vjot, (Eq. (20)) or by the application of a multiple-pulse sequence causing an additional averaging of this interaction (combined rotation and multiple-pulse spectroscopy, CRAMPS 19-21 ). With today s instruments, sample spinning frequencies of up to 40 kHz can be reached using MAS NMR rotors with an outer diameter of 2.0 mm. 

An important prerequisite for the reproducibility of NMR experiments at elevated temperatures is the accurate determination of the temperature inside the sample volume of the probe. Often, there is a systematic error in the temperature displayed by the controller unit. Therefore, methods for the temperature calibration of MAS NMR probes under various working conditions, such as various heating rates, sample spinning frequencies, etc., are required for experiments at elevated temperatures. 

For the modification of a 4-mm MAS NMR probe, an injection tube with an outer diameter of 1 mm is used, and the hole in the rotor cap has an inner diameter of 1.4 mm. This 4-mm CF MAS NMR probe reaches sample spinning frequencies of 12 kHz and is suitable for the investigations of atoms in the framework of solid catalysts, such as Na, Al, and nuclei. Approximately 50 mg of catalyst powder can fit in the rotor reactor of a 4-mm CF MAS NMR probe. 

It is well known2,3 that in the presence of second-order quadrupolar interaction, high-resolution NMR spectra are not achievable by a conventional MAS method, even if the sample spinning frequency is sufficiently high compared to the spectral... 

Figure 4(B) shows the simulated 17O MAS spectra as a function of i]q from 0 to 1. These calculations were carried out using the same conditions of the 170 stationary NMR spectra. In the similar manner as in the case of the stationary NMR spectra, the MAS spectra exhibit characteristic line shapes from which the information on t]q as well as Cq can be extracted. The anisotropy of CS tensors is removed by MAS and only <5iSO will be obtained if it exists. When the sample spinning frequency is not high enough, the effect of spinning sidebands spaced at the spinning frequency around the central peak needs to be considered in the spectral simulation. The frequency contribution from the second-order quadrupolar interaction under slow/intermediate MAS conditions is given in the literature 46,47 A complicated line shape is expected to appear in the MAS NMR spectrum so that a computer simulation is not trivial (see Figure 15).

In CF MAS NMR investigations of adsorbate complexes and reactants, modified 7-mm MAS NMR probes are used. The injection tube has an outer diameter of 1.5 mm, and the hole in the rotor cap has an inner diameter of 2.5 mm. The maximum sample spinning frequency of 7-mm CF MAS NMR probes is ca. 3.5kFfz. Approximately 200 mg of catalyst powder fits in the rotor reactors of a 7-mm CF MAS NMR probe (60). 

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