Chemical shift parameters

In conclusion therefore it may be stated that one must be able to distinguish between motional dispersion and motional homogeneity. Furthermore, a reconstruction of the chemical shift parameters from spectra with MAS at very low temperatures is needed to obtain quantitative motional information. Similar problems to the above also arise in other linear polymers containing interspaced soft and hard segments. 

NMR is an analytical technique that has been applied to the studies of chemical reactions with promising results. The chemical shift parameter may be used as a measure of the relative proportions of different species that are present in solution. These shifts are measured relative to a standard reference sample. 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

C NMR spectroscopy of 1,7-dioxa- (314), l-oxa-7-thia- (315), and 1,7-dithiaspiro[5.5]undecane (316) and many of their derivatives, with special emphasis on stereoelectronic and steric effects governing the configurational and conformational properties of these compounds (484-486). This work showed that chemical-shift parameters developed for six-membered carbocyclic compounds in chair conformations (149) could be applied successfully only to sulfur and not to oxygen heterocycles (486). 

Accumulated NMR data support the proposal that the chemical shift for an olefinic carbon of monoenyl compounds can be calculated with ethylene as a reference [ 184]. A similar method was applied to the polyenyl Type I components with a conjugated system to estimate chemical shift parameters. Using 1,3-butadien for the base values (C 117.6 and 137.8 ppm), the chemical shift... 

Summary of Chemical Shift Parameters for the Trityl Cation 16, Phenylindanyl Cation 13, Cyclopentenyl Cation 8, Methylindanyl Cation 12, tert-butyl Cation 2, and Methylcyclopentyl Cation 17... 

Summary of Chemical Shift Parameters for the Donor-Acceptor Complex between Acetone and a Variety of Lewis and Br0nsted Acids... 

Table 14 13C Substituent Chemical Shift Parameters for Pyrrole Derivatives 1...

Valence-bond calculations of 13C chemical shift parameters of alkanes permit the following general conclusions [204]. 

A semi-empirical molecular orbital method for the correlation of charge distributions with 13C shifts in amino acids was described [95]. Plotting of chemical shift parameters versus charge density changes of a-carbon atoms relative to the corresponding atoms in the parent hydrocarbons permits prediction of the chemical shifts of the a-carbons with an accuracy of about 10%. However, the slope (280 ppm per electron) in Fig. 5.12 is... 

FIGURE 46. Correlation curves for some 29Si chemical shift parameters vs rsi-N (a) 833, (b) <5n (A), (c) 833 — <5n and (d) (5jso. Reprinted with permission from Reference 150. Copyright 1993 American Chemical Society... 

For the demonstration of the capabilities of this method we chose the Buckminster fullerene Cgo. Since this molecule consists only of carbon atoms the net atomic charges of all atoms and hence the polarization are zero. The calculated chemical shift parameters depend only on the parameters of the unpolarized bonds and the 7t-bond order. The rc-bond order determines the extent of the 7t parameters contribution. 

In addition to the glycine-containing polypeptides and peptides, we show here new results on the 170 NMR chemical shift and quadrupolar coupling constant of poly(L-alanine)s in both a-helix and P-sheet forms, in the solid state. There results help clarify the relationship between hydrogen-bond length and e2qQ/h and chemical shift parameters. 

As regards the phenyl ring motions, interesting features are obtained from measurements of 13C chemical shift anisotropy of protonated and unpro-tonated aromatic carbons. The chemical shift parameters are orthogonal to each other, with o and 072 in the phenyl plane and <733 bisecting the phenyl plane. 

C chemical shift anisotropies of the unsaturated carbons. The chemical shift parameters used in the Ar-Al-PA for the unprotonated and proto-nated aromatic carbons, as well as for the carbonyl groups, are shown in Fig. 80. 

Fig. 80 Directions of the chemical shift parameters used in Ar-Al-PA (from [58])...

Table 4. Chemical shift parameters (ppm) used for the curve fittings...

C NMR spectrum The 13C NMR spectrum of parbendazole is shown in Fig. 5.5 (full 13C NMR spectrum) and Fig. 5.6 (off-resonance spectrum). 13C chemical shift data in TFA-d, are summarized in Table 5.5, which are consistent with the 13C contents of parbendazole. The assignments for the observed bands are based on established chemical shift parameters, off-resonance spectra, and comparisons with the following... 

Bell, H.M., Bowles, D.B., and Senese, F., Additive NMR chemical shift parameters for deshielded methine protons, Org. Magn. Reson., 16, 285, 1981. 

Table 19. Experimental and theoretical 15N chemical shift parameters (ppm) for cis-and trans-azobenzene (value of acs is 90°)...

Similar to the situation for 13C, isotropic 15N chemical shifts and the principal components of 15N chemical shift tensors have been used to study N-H- -0=C hydrogen bonds in peptides. It has been shown that isotropic 15N chemical shifts of proton donors (such as N-H) are displaced downfield by ca. 15 ppm, whereas those of proton acceptors are shifted upfield by ca. 20 ppm [110-112]. Amongst the CSA components, S33 (parallel to the C-N bond) has been shown to be most sensitive to the hydrogen bond strength, as reflected by the N- -O distance [113]. Detailed studies of the principal components and orientations of 15N chemical shift tensors for amide nitrogens in simple peptides have been reported recently [114]. This work confirmed that S33 and Siso are the 15N chemical shift parameters that are the most sensitive to details of the hydrogen bonding. It was also found that N-H... 

The most generally used reference compound in NMR is tetramethylsilane (TMS). This substance absorbs at higher field than almost all organic compounds for both H (used in proton NMR) or for (used in NMR). In a relative NMR scale, TMS is set at 0 Hz at the right edge of the scale. A chemical shift parameter can be calculated using this reference. Applying rel. (2) to the reference and to a sample, it results ... 

---