Chemical shielding tensor components

Table 21 Comparison Between Experimental and Calculated Absolute C and 0 Chemical Shielding Tensor Components (ppm) for the CO Molecule and Group 6 Transition Metal Carbonyls... 

The starting point for NCS analysis is the GIAO expression for the anisotropic NMR chemical shielding tensor components Oafi as a sum of unperturbed (u) and induced (i) contributions,... 

Not all 9 components of a are typically reported when describing the chemical shielding tensor. Instead, the 3 principal components (or eigen values) in a principal axis system (PAS) are reported. The CSA tensor can also be described by three additional parameters 1) the isotropic value (or trace), aiso, of the shielding tensor and is defined as... 

These effects of changes in geometry on the shielding tensor in PH3 provide the framework for discussion of trends observed in the simple phosphate systems described below. In theory, the difference in the variation for the isotropic and individual principal components of the chemical shielding tensor can be used... 

From a detailed analysis of the rotation pattern of a single crystal of NH4CIO4 at room temperature, the 35C1 quadrupolar and chemical shielding tensors were deduced.92 The extreme principal components of the CSA tensor differed by 18.1 ppm. Other 35C1 measurements of perchlorates included one-pulse observation of the phase changes in a range of -methylammonium... 

Isotropic chemical shifts, obtained from solution studies, are useful indicators of the electronic environment around the nucleus, but they provide only a fraction of the available information. The shielding tensor components, which can be extracted from spinning sidebands in solid-state spectra, also contain useful information on bonding and structure, because they are sensitive to bond type. Techniques other than nitrogen NMR, such as spin-rotation 165) or relaxation 166) measurements, can be used to determine the nitrogen shielding tensor. 

The orientational dependence is present in the chemical shielding tensor which in its principal axis system (PAS) is diagonal with principal components axx, o yy and ctzz-In a crystalline powder sample all orientations of the PAS system of the shielding tensor are present and for most orientations the PAS does not coincide with the laboratory (LAB) frame where the z-axis is aligned with the direction of the applied magnetic field Bq. Hence the tensor representation gives in the PAS... 

If the spectra are too crowded with sidebands, 2D NMR spectroscopy can advantageously spread the overlapping CSA patterns across a second dimension. An array of these techniques is currently available (PASS, MAT, VACSY, FIREMAT, etc.), and they are described in excellent review articles. However, the analysis of powdered samples with the natural abundance of C and is not capable of providing information about the orientation of individual shielding-tensor components in the molecular frame. At present, the orientation of the principal components in the molecular frame is available from quantum chemical calculations. This topic will be discussed briefly in Section 5. 

One potential problem with chemical shift anisotropy lineshape analysis (or indeed analysis of lineshapes arising from any nuclear spin interaction) is that the analysis results in a description of the angular reorientation of the chemical-shielding tensor during the motion, not the molecule. To convert this information into details of how the molecule moves, we need to know how the chemical-shielding tensor (or other interaction tensor) is oriented in the molecular frame. A further possible complication with the analysis is that it may not be possible to achieve an experiment temperature at which the motion is completely quenched, and thus it may not be possible to directly measure the principal values of the interaction tensor, i.e. anisotropy, asymmetry and isotropic component. If the motion is complex, lack of certainty about the input tensor parameters leads to an ambiguous lineshape analysis, with several (or even many) possible fits to the experimental data. 

The N CSA principal components an < a22 < < 33) of the chemical shielding tensor can be experimentally determined by observing the powder pattern arising from a randomly dispersed sample. Then the N CSA tensor is rotated from the PAS through the FAS to the LAB frame of reference as follows ... 

The observed chemical shielding corresponds to the component of the transformed chemical shielding tensor that is parallel to Bq, which is the (3,3) component of the resulting [Pg.316]

Several reports [12-14,17-19] provide data on F shielding tensor components for PTFE, obtained from multiple-pulse methods. These have been summarised by McBrierty and Packer [20]. The values reported lie in the chemical shift ranges (in order from highest to lowest) 5n = - 27 to -53 ppm, S22 = -141 to -156 ppm, 833 = -156 to -205 ppm, with 22 = 33 at... 

Solid-state Hg NMR can clearly resolve several issues raised by solution NMR studies. If the solid-state isotropic shift is equal to the solution shift, then the solution chemical shift does not represent an average of several species in rapid exchange. As has been shown with Cd NMR (186), correspondence between solution and solid-state chemical shifts greatly increases the ability of the inorganic chemist to use solution spectra to classify molecular structure and bonding. Equally important, analysis of the solid-state chemical shift and the shielding tensor components can provide information about coordination number and asymmetry at the metal center in solids, even when other structural information is lacking. 

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