Shielding tensor analysis

Hg(CN)2 in the solid state has a structure (I42d neutron diffraction), completely different from that of Cd(CN)2 Almost-linear molecules (r(Hg—C) 201.9, r(C—N) 116.0pm (corrected for thermal motion) a(C—Hg—C) 175.0°) are arranged such that four secondary bonds N" Hg (274.2 pm) yield the often-occurring 2 + 4 coordination around Hg.103 Analysis of the 199Hg MAS NMR spectrum of Hg(CN)2 has yielded the chemical shift and shielding tensor parameters.104... 

Si shielding tensor information may reflect the structure of organosilicon compounds while the structure might have been determined by X-ray crystallographic analysis. Thus,... 

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... 

Eleven of the 12 crystallographically independent sites are resolved. They are clearly split into two sets of axial and equatorial carbonyls with a separation of 20.3 ppm between the centres of the two sets. Furthermore the analysis of the SSB intensities gave the values of the principal elements of the shielding tensors which afforded an average value for the csa of 358 ppm. 

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. 

Of particular importance in chemistry is the response of a molecular system to an external magnetic field as applied in routinely performed NMR experiments for the identification of compounds, the analysis of reaction mechanisms, and reaction control. Theoretical tools must provide spin-spin coupling constants and shielding tensors in order to calculate quantities, which can be related to experimental data. Needless to say, coupling constants and chemical shifts calculated from shielding tensors can only be obtained from accurate four-component methods for heavy nuclei. The theory of relativistic calculations of magnetic properties has recently been analysed in great detail (Aucar et al. 1999). 

Why is the displacement of the an different for [Ala ] and [Leu ] The reason for this is not clear at present, but this is a very important question in reaching and understanding of the correlation between structures and the l5N shielding tensor. The 15N shielding tensor may be useful for conformational analysis of solid polypeptides, if the origin of the chemical shift displacements can be elucidated. 

Some early attempts were made to determine anisotropies from second-moment analysis in which use is made of the anisotropic contribution to the observed second moment.5 6 Limitations of this method are the need for large shifts and fields and the often-required assumption of axial symmetry of the shielding tensor. In 1968 a method was reported that uses coherent averaging techniques7 to effectively narrow the dipolar-broadened lines of powdered solids. The observed spectrum is curve-fitted using a computer-broadened theoretical chemical shift distribution to give the principal components of the shielding tensor. 

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

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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