Spinning, sample

The sample is placed into a gas driven spinner, the rotation axis of which is inclined by an angle ij/ against the magnetic field Bo (Fig. 3.3.4). For MAS this angle is adjusted to the magic angle of 54.7°. Then the anisotropic parts of all interactions which are described by second-rank tensors can be averaged. These are the anisotropy of the chemical shift, the dipole-dipole interaction, and the first-order quadrupole interaction. 

Solid-state NMR spectra of dilute nuclei in organic molecules are broadened by chemical-shift anisotropy and dipole-dipole couplings between H and C [Vanl]. The effect of these interactions can be eliminated from the NMR spectrum by MAS in combination 

Magic angle spinning and off-magic angle spinning 

The formal description of MAS involves different coordinate frames when samples with a preferential axis of molecular order are considered (Fig. 3.3.6). On the molecular level a sample frame is introduced, the Zs-axis of which is parallel to the preferential direction of the sample. In some cases, this frame can be attached to the molecule or the crystallite 

The phase, which the spinner accumulates during sample rotation with constant rotation speed cur is given by 

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Moller,M. Cross Polarization — Magie Angle Sample Spinning NMR Studies. With Respect to the Rotational Isomeric States of Saturated Chain Molecules. Vol. 66, pp. 59 — 80. 

Cross Polarization-Magic Angle Sample Spinning NMR Studies... 

Courtieu, J., Bayle, J. P., Fung, B. M. Variable angle sample spinning NM R in liquid crystals. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26,141-169. 

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

Figure 1, Representation of the geometric arrangement for a sample spinning at the magic angle to the magnetic field vector H0. (Reproduced with permission from Ref, 40, Copyright 1982, Royal Society of London,)...

Floquet theory [60] represents a powerful method to analyze and describe NMR experiments including, in the present context, methods for dipolar recoupling. In particular the bimodal variants are promising in the sense that they may handle perturbations/interactions involving multiple time periodicities, as for example induced by combined sample spinning and rf irradiation in the case of dipolar... 

In the following section, we explain the basic protocols used for removing the second-order quadrupolar broadening based on the refocusing of the second-order quadrupolar interaction. These protocols rely on mechanical reorientation of the rotor axis (DAS) or use a combination of sample spinning and rf manipulation of the spins evolution (MQMAS and STMAS). Experimental aspects of these methods, as well as methods for data processing and analysis, are described in Sects. 5.3 and 5.4. 

At this stage, it must be asked whether or not is a chemically viable intermediate in the formyl disproportions. To be so, it must be able to abstract hydride from other organorhenium species known to be present. Accordingly, when 6 and 8 were mixed in CD2Cl2at -70 °C in a H NMR monitored reaction, the clean hydride transfer depicted in eq iii occurred immediately (i.e., within the ca. 2-3 minute lag time needed to resume sample spinning and acquire the FT NMR data). 

Experimentally, several precautions must be taken if reliable Raman data are to be obtained from solution studies. Firstly, the instrumental slit-width should be appreciably smaller than the half-width of the band to be studied. This means that slits wider than 2 cm-1 are to be avoided. Secondly, photolytic decomposition of the sample and local boiling of the solvent have also to be avoided. Careful choice of laser frequencies, use of a low incident power and, if necessary, sample spinning are indicated. The need for a relatively high solute concentration usually means that there is little choice of solvent. Particularly for coloured samples the presence of a vestigal resonance Raman effect must be tested by measurements with a variety of... 

The 13C NMR study utilized cross polarization-magic angle spinning (CP-MAS) with spin counting. The elemental and functional group analyses provided input for a series of analytical constraints calculations that yield an absolute upper limit for the amount of aromatic carbon and most probable estimates of both aromatic and non-carboxyl aliphatic carbon in each sample. Spin counting experiments demonstrate that less than 50% of the... 

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