Principal chemical shift tensor

Principal Values of 29Si Chemical Shift Tensors for Disilenes (ppm)... 

The nature of the chemical shift tensor is a potential source of complications in relaxation studies. For sugar carbons, the CSAs are around 40 ppm and their contribution to relaxation of protonated carbons is nearly negligible. On the other hand, CSA values of the protonated carbons of the bases are between 120 and 180 ppm, the tensors deviate quite significantly from axial symmetry and none of their principal components is colli-near with the C-H bond. This makes interpretation of the relaxation rates in terms of molecular dynamics prohibitively complicated or, if neglected, introduces an error whose magnitude has not yet been evaluated. 

The DD-CSA cross-correlated relaxation, namely that between 13C-1H dipole and 31P-CSA, can also be used to determine backbone a and C angles in RNA [65]. The experiment requires oligonucleotides that are 13C-labeled in the sugar moiety. First, 1H-coupled, / - DQ//Q-II CP spectra are measured. DQ and ZQ spectra are obtained by linear combinations of four subspectra recorded for each q-increment. Then, the cross-relaxation rates are calculated from the peak intensity ratios of the doublets in the DQ and ZQ spectra. The observed cross-correlation rates depend on the relative orientations of CH dipoles with respect to the components of the 31P chemical shift tensor. As the components of the 31P chemical shift tensor in RNA are not known, the barium salt of diethyl phosphate was used as a model compound with the principal components values of -76 ppm, -16 ppm and 103 ppm, respectively [106]. Since the measured cross-correlation rates are a function of the angles / and e as well, these angles need to be determined independently using 3/(H, P) and 3/(C, P) coupling constants. 

Figure 4. The eight-pulse line shape and the peak locations of the Th4Hi5 (LP) powder sample as a function of temperature using a Ca(OH)2 single crystal as reference. The reference is oriented such that the major principal axis of the proton chemical shift tensor is parallel to the external magnetic field. A shift to the left signifies an increase in the value of

Figure 6 is a plot of the proton NMR spectrum obtained from H2Os3(CO)io when using an eight-pulse cycle (5, 6,16,17,18) to suppress the effects of proton-proton dipolar interactions. The curve results from a computer fit that assumes the lineshape is caused by the chemical shift tensor. The center of the spectrum is near r = 19 ppm, and thus it agrees reasonably with that expected from the solution NMR results (r = 21.7 ppm (37)). The three principal values of the tensor, according to this fit, are at r values 5.6 ppm, 19.9 ppm, and 31.6 ppm. Since approximately one-third of the proton pairs interact with a near... 

We have prepared a number of acylium ions on metal halide powders and measured the principal components of their chemical shift tensors (43-45). Most of these cations have isotropic l3C shifts of 154 1 ppm. Often insensitivity to substituents results from opposite and offsetting variations in the principal components. The acetylium ion has an axially symmetric chemical shift tensor because of its C3 rotation axis. When the symmetry is reduced from C3v to C2v or lower, a nonzero 27 value may be observed. The sensitivity of chemical shift tensors to symmetry is a powerful means of probing molecular structure and temperature-dependent molecular dynamics. Multiple orders of spinning sidebands may offend those who seek solution-like NMR spectra of solids, but discarding most of the information inherent in the chemical shift is a considerable concession to aesthetics. 

We emphasize that attempts to correlate chemical shift and properties related to acidity should consider, whenever possible, the principal components of the chemical shift tensor and not simply their average. 

Measurements of the static 13C line shape or sideband intensities of acetone on many solid acids at room temperature underestimate the chemical shift anisotropy due to motion, but the principal components of the chemical shift tensor can be accurately measured at reduced temperature. Table V reports these data for acetone on a wide variety of Brpnsted and Lewis acids (43, 45) note that the largest contribution to the isotropic shift is <5n The shift induced by A1C13 and other Lewis acids is rationalized by... 

The isotropic chemical shift is the average value of the diagonal elements of the chemical shift tensor. Advances in solid state NMR spectroscopy allow one to determine the orientation dependence, or anisotropy, of the chemical shift interaction. It is now possible to determine the principal elements of a chemical shift powder pattern conveniently, and the orientation of the principal axes with more effort. Hence, instead of settling for just the average value of the chemical shift powder pattern, one can now aim for values of the three principal elements and the corresponding orientations in a molecular axis system. 

As mentioned above, the principal values of chemical shift tensor give information about three dimensional electronic state of a molecule. However, in order to understand behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. The orientations of the principal axis systems of the chemical shift tensors of L-alanine Cp -carbons in some peptides were calculated, whose L-alanine moieties have different main-chain dihedral-angles, (( >,v /H-57.40,-47.50)[aR-helix], (-138.8°,134.7°)[ pA-sheet], (-66.3°,-... 

Figure 1. Correlation between calculated and experimental principal values of 13C chemical shift tensors...

The principal values of the calculated chemical shift tensors are compared to experimental values and IGLO ab initio calculations taken from literature in Table I. The experimental and most of the calculated values are given in the order 5, / 522 / 833. However in cases where the orientation of the principal axes was known from symmetry considerations or ab initio calculations, and this order was not adequately reproduced in our calculations the components were sorted according to their corresponding tensor orientation in the local coordinate system. In many of these small molecules the tensor axes are defined by the symmetry of the molecules. In other cases the axes are at least loosely defined by nearly symmetric local bond surroundings. For example the 533 component of the CH3 chemical shift tensor is close to the direction of the non CH bond or the 833 component of a sp2 carbon is perpendicular to the plane of the 71 system. 

Table I. Calculated principal values of 13C chemical shift tensors compared to experimental values and ab initio results (the reference U.F. denotes a private communication from Ulrich Fleischer, Ruhr University Bochum) ...

Table n. Principal values of the chemical shift tensor calculated with uncorrected and corrected hybrids ... 

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