Chemical-shielding tensor

Champmartin D and Rubini P 1996 Determination of the 0-17 quadrupolar coupling constant and of the C-13 chemical shielding tensor anisotropy of the CO groups of pentane-2,4-dione and beta-diketonate complexes in solution. NMR relaxation study/norg. Chem. 35 179-83... 

Several calculations of the chemical shielding tensor have been carried out by different methods for simple model disilenes. For H2Si=SiH2, calculated values of 300,44 291,43 and 195 ppm46 have been obtained for Act, the CSA spread. A value of 255 ppm has been calculated for Me2 Si=SiMe2,46 and a value of 120 ppm was obtained for (CH2=CH)2Si=Si... 

Fundamental constants (Cx), spatial tensors in the principal axis frame ((fi3 m,)F), and spin tensors (Tjm) for chemical shielding (a), J coupling (J), dipole-dipole (IS), and quadrupolar coupling (Q) nuclear spin interactions (for more detailed definition of symbols refer to [50])... 

Facelli, J. C., Molecular Structure and Carbon-13 Chemical Shielding Tensors Obtained from Nuclear Magnetic Resonance, 19, 1. 

The spin interactions, dipole-dipole (D), nuclear electric quadrupole (Q) and chemical shielding (C.S), may be formally written in terms of irreducible tensors of rank l34 in Hz ... 

Here, ak is the isotropic chemical shift referenced in ppm from the carrier frequency co0, SkSA is the anisotropy and tfk SA the asymmetry of the chemical-shielding tensor, here also expressed in ppm. Note that for heteronuclear cases different reference frequencies co0 are chosen for different nuclei (doubly rotating frame of reference). The two Euler angles ak and pk describe the orientation of the chemical-shielding tensor with respect to the laboratory-fixed frame of reference. The anisotropy dkSA defines the width and the asymmetry t]kSA the shape of the powder line shape (see Fig. 11.1a). 

Fig. 11.1 (a) Powder line shapes of chemical-shielding tensors, dCSA/(2 n) = 5 kHz, for three different values... 

Composite-pulse decoupling schemes like WALTZ [36, 37], DIPSI [38], or GARP [39], which are used in solution-state NMR, have failed to offer any significant improvements in the solid state compared to CW decoupling. The residual line width in CW-decoupled spectra is dominated by a cross term between the chemical-shielding tensor of the protons and the heteronuclear dipolar-coupling tensor [40, 41]. 

In order to discuss the origin of these terms we need to allow the spins to have anisotropic shielding tensors. Molecular tumbling in solution makes the chemical shielding in the direction of the external magnetic field a stochastic function of time and acts therefore as a relaxation mechanism, called the chemical shielding anisotropy (CSA) mechanism. The Hamiltonian for each of the two spins, analogous to Eq. (5), contains therefore two... 

L. B. Moran, J.P. Yesinowski, Chemical-shift selective multiple quantum NMRas probe of the correlation length of chemical shielding tensors, Chem. Phys. Lett. 222 (1994) 363-370. 

F. H. Larsen, H. J. Jakobsen, P. D. ElUs and N. C. Nielsen, Sensitivity-enhanced quad-rupolar-echo NMR of half-integer quadrupolar nuclei. Magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors. /. Phys. Chem. A, 1997, 101,8597-8606. 

Chemical shielding tensor of a substituted imidazol-2-ylidene... 

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