Chemical shift, and conformations

C Chemical Shifts and Conformational Changes Observed on Protonation of Amino-Glycosides. R. U. Lemieux, S. Koto, and B. Y. Chung, Abstr. 2nd Joint Conf. CIC/ACS, Montreal, May 30-June 2 (1977), CARS 4. 

Barbarella, G., Dembech, P., Garbesi, A., and Fara, A., 13C NMR of organosulfur compounds. II. 13C chemical shifts and conformational analysis of methyl substituted thiacyclohexanes, Org. Magn. Reson., 8,469, 1976. 

It should be noted that the negative sign of the shielding constant cr indicates deshielding, and so shielding variations can be compared with the experimental chemical shift 8 where a positive sign denotes deshielding. Such a chemical shift map successfully predicts the chemical shifts and conformations of the L-alanine (L-Ala) residues in polypeptides and proteins. For example, the experimental chemical shift for the a-helix form appears at lower frequencies by —5.5 ppm from that for the j8-sheet form. The calculated map reasonably predicts the experimental data. 

Conformational Analysis.— The major effort in this area has been directed towards empirical correlations of chemical shift and conformation. Tonelli and co-workers have examined the stereosequence dependence of C chemical shifts in polypropylene (PP) poly(vinyl chloride) (PVC), and polystyrene (PS) using the rotational isomeric-state scheme of polymer statistical mechanics. The structural variation of C shifts was shown to correlate well with the occurrence of three-bond gauche (y) interactions between carbon atoms together with y interactions between carbon and chlorine atoms in PVC and ring-current shifts in PS. It has been... 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

Chemical shifts and coupling constants have been used for structural, conformational and stereochemical assignments and preferences and for the establishment of the four-membered ring sulfone effect . ... 

The isomerization shifts conform to those that are described in literature (Englert and Vecci 1980, Englert 1995). All chemical shifts and isomeric shift differences of the olehnic region are listed in Table 4.1. 

Chemical shift and line width are related and can be used to describe conformational changes. The 831P value of 5 ppm and the line width of 20 Hz indicate that dianionic 5 phosphate is very tightly bound to the active site and tumbles with the protein. Lower values of chemical shift indicate more loosely bound phosphate. Higher values of the line width indicate the presence of two conformers in equilibrium. If the open and close form undergo slow interconvertion, two signals are observed in the 31P NMR spectra. 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

The 60-MHz, p.m.r. spectra of compounds 25, 26, and 33 cannot be fully analyzed, inasmuch as, in parts of the spectra, the magnitudes of the differences of the chemical shifts and that of the spin-spin coupling-constants (splittings) are very similar. In order to avoid possible conformational changes by O-protection82 in the compounds to be analyzed, compounds 25, 26, and 33 were investigated84 in85 pyridine-d5 by 220-MHz, p.m.r. spectroscopy. 

Makriyannis, A., and Knittel, J. J. (1979) The conformational analysis of aromatic methoxyl groups from carbon-13 chemical shifts and spin-lattice relaxation times. Tetrahedron Lett., 2753-2756. 

Further doubt about the validity of the original Grant-Cheney model was expressed by Seidman and Maciel (185), whose INDO calculations of proximity effects in hydrocarbons revealed that there is no simple correlation between carbon chemical shifts and calculated electron-density increases caused by steric C-H bond polarization they report the conformational relation of interacting bonds and groups to be at least equally important, if not more so (185). 

The preceding prediction rules are largely restricted to acyclic compounds. But there is also a considerable need for parameter sets enabling the spectroscopist to calculate I3C chemical shifts of conformationally defined cyclic molecules, especially of the cyclohexane type. Methyl-group effects in methylcyclohexanes (100,101) that are to be added to the basic value for cyclohexane itself (8 = 27.3) are listed in Table 29. Analogous methyl-group parameters in tetralins and tetra-hydroanthracenes have been reported (403). 

For the a-SCS(OH) values in cyclohexanol derivatives Wray (406) has suggested that a further parameter is necessary to account for four-bond interactions, as indicated in 298 and 299. He remarked that significant deviations from the experimental chemical shifts indicate conformational distortion of the parent compound, and that such SCS values cannot be calculated for five-membered ring alcohols (406). 

There is no doubt that the most powerful tool for studying molecular structures in solution at the present time is NMR. Given the high sensitivity of chemical shifts and coupling constants to conformation and steric environment, an obvious expedient is to use MM-... 

There is some evidence for the population of a turn conformation similar to the bound conformation in the free peptide, however this is expected to be only one of many conformations adopted by the free peptide, as other NMR parameters (chemical shifts and values) indicated conformational... 

Chemical shifts and chemical-shift differences between pairs of ano-mers in the monodeoxymonofluoro-D-hexopyranose series were observed to be strongly dependent on the stereochemistry of electronegative substituents in the molecule. In consequence, no a priori distinction between anomers can be made on the sole basis of their respective fluorine shifts. The use of fluorine shifts as a probe for structural and conformational... 

Conformational Flexibility Detected bv NMR Chemical Shifts and Coupling Constants... 

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