NMR chemical shift tensor

D. Tinet, A. M. Faugere, and R. Prost, Cd NMR chemical shift tensor analysis of cadmium-exchanged clays and clay gels, J. Phys. Chem. 95 8804 (1991). 

NMR Chemical Shift Tensor Components eor Mes3Si /HCBiiMe5Br6 and Calculated Tensor Components eor MessSi", 1. " The Orientation oe the Eigenvectors Relative to the Molecular Frame is Given in Fig. 1... 

Table 7. 13C NMR Chemical Shift Tensors of Some Cyclopropanesa...

The study of nucleic acid bases by NMR has been reported in a number of monographs (/), but very little data is available on the, 3C and, 5N NMR chemical shift tensors in these compounds. The low sensitivity of NMR spectroscopy and the long relaxation times exhibited by many of these compounds have posed the main impediments for these studies. The use of sample doping with free radical relaxation reagents, to reduce the relaxation times facilitating 2D multiple pulse experiment (2, 3), enables one to measure and analyze the principal values of the chemical shift tensors in natural abundance samples. In previous papers from this laboratory we have presented, 5N NMR chemical shift principal values for adenine, guanine, cytosine, thymine and uracil (4, 5). 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

The 29Si NMR chemical shift tensors accessible from solid state NMR spectra also provide important insights into the nature of the chemical bonds in disilenes. After the disilene 962 and some other disilenes63 had been investigated by this method in earlier studies, West and coworkers recently reported the solid state NMR spectra of seven disilenes with widely differing substitution patterns as well as of two solvates of compound 9 (Table 3). These results in combination with MO calculations on model substances support a classical jr-bonding model for the Si=Si bonds64. 

Structures, energies and spectra of cyclopropanes TABLE 8. NMR chemical shift tensors... 

Despite a first coordination sphere of four sulfur atoms, the calculated Si NMR chemical shift tensors of 1 and 2 show relatively large anisotropies of 101.7 and 77.3 ppm, respectively. These large anisotropies are mainly caused by the partial planarization of the SiS4 tetrahedron resulting in spiro angles (angle between the two SiS2 planes) of 74.4° in 1 and 83.4° in 2. 

If the Spiro angle in 1 is fixed at 90°, the calculated anisotropy Q is reduced to 27.8 ppm (see Table 2). Figures 5-7 give an idea of the spatial orientation of the Si NMR chemical shift tensors in 1 and 2. As can be seen from Figs. 5 and 6, a change of the spiro angle in 1 mainly influences the chemical shift in the z direction. 

Fig. 5. Calculated NMR chemical shift tensor (dark gray) in spiro-[C2H4(S)2]2Si (1) geometry of the crystal structure with spiro angle at Si of 74.4°. For better visualization the chemical shifts with respect to TMS = 0 ppm were used as principal values in ail tensor drawings.

Silicon Compounds. 29Si NMR chemical shift tensors were calculated by ab initio methods for 21 simple silylenes, R2Si .686 The proton NMR spectra of (Trip)SiH3 and (Trip)AsH2 (Trip = triptycene) included a characteristic... 

Two different charge models were applied to take into account intermolecular interactions to model the solid-state effects on the F NMR chemical shift tensors." The interpretation of H NMR chemical shifts, coupling constants, and coefficients of temperature dependence (6(OH), /(H,OH), and A8(OH)/AT values) evidences that, in DMSO solution, the signal of an OH... 

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