Chemical shifts considerations

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

The proton spectra analysis of thietane, thietane oxide and thietane dioxide at 100 and 300 MHz in the temperature range — 140 to 190 °C confirmed the puckered structure for the oxide (5a) with the sulfinyl oxygen in the equatorial orientation, as inferred from chemical-shift considerations . It appears that the repulsive-type 1,3-interactions between the oxygen and the 3-substituents are operating between oxygen and the axial proton on C-3 in the unsubstituted thietane oxide (5a). For the thietane dioxide (5b ... 

From simple chemical-shift considerations, it would be expected that one / -D-Gal residue will give rise to an anomeric chemical-shift of 104.2 p.p.m., together with one / -D-Man residue whose substituted signal C-3 occurs at 75-80 p.p.m., and one terminal a-D-Man residue whose C-l signal occurs at 103.66 p.p.m. Although these SRR data do not define a unique structure for this glycopeptide, they do indicate the types of residues that are present. The glycopeptide structure discussed had previously been determined to be that depicted in 11. It may, indeed, be... 

Because fluorine is relatively sensitive to its environment and has such a large range of chemical shifts, considerable changes in chemical shift can be observed when a nearby atom is replaced by an isotope. For example, replacement of 12C by 13C for the atom to which the fluorine is attached, gives rise to a quite measurable shift, usually to lower frequency. A consequence of this isotope effect is the observation that the 13C satellites in a fluorine spectrum are not symmetrical about the 12C—F resonance. 

The full paper on the synthesis of pollinastanol from cycloartanol has been published.21 The revision of stereochemistry at C-4 in the Buxus alkaloids, suggested by 13C chemical shift considerations,22 has been confirmed by X-ray analysis of the derivative (18) of cyclobuxidine F.23 Assignments of the 13C resonances of a series of cycloartanes have been reported.24... 

Although chemical shifts do not always correlate with bond strength due to other contributions (for example, the paramagnetic part of the chemical shift), consideration of this factor is often appropriate since the diamagnetic part of the chemical shift interaction, in the molecular systems under discussion, is dominating. For example, a definite difference in the C=0 bond strength was argued to be the main reason behind the difference in the C chemical shifts of C 0 and C 0 molecules [58]. 

One of the entries in Table II enabled an initial assignment error in ref. (J) (for the cubic octamer) to be corrected in Table I, leaving one unassigned unsplit line at A634 = -25.60 ppm. It is possible that this peak is due to a 5- or 6-membered ring, or to a double -5- or double -6- membered cage (j[2), but chemical shift considerations make all these possibilities seem somewhat unlikely. 

Chemical shift considerations for the gem. dimethyl groups have been successfully used to assign relative configuration to carotenoids containing 2,6-substituted e-rings such as the Cso-carotenoid decaprenoxanthin (8) by comparison with synthetic 2,6-cis and 2,6-tram model C42-... 

Chemical shift considerations have also proved adequate for assignment of geometrical isomerism in the phytoene (21) series 115), for configurational assignment of the hydroxylated isopropylidene end group in decaprenoxanthin (8) and lycoxanthin (22) after allylic oxidation to the corresponding aldehydes 133, 156), as well as for analysis of 13-cw and 3-trans mixtures of 20-substituted carotenoids related to reniera-purpurin-20-al (23) 49). 

Studies of individual carbon sites of hen egg-white lysozymes by C n.m.r. spectroscopy have been reported. Chemical shift considerations, the effect of pH, and partially relaxed Fourier transform n.m.r. spectra were used to assign each resonance to one of the seven types of non-protonated aromatic carbons of amino-acid residues. Spectra of chemically modified lysozyme samples yielded various assignments to specific residues in the sequence. 

Chemical shifts can be observed for atoms in both organic and inorganic materials. Generally, E increases as the oxidation state of the atom increases. This dependence can be rationalized on the basis of simple electrostatic considerations in terms of an increased attraction for the electron by the atom making it that much harder to remove. These chemical shifts can range in magnitude from <1 eV to >10 eV. 

Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

When the lone electron pair is protonated, the nitrogen chemical shift moves by ca. 100 p.p.m, to higher field. Large upheld shifts are also found when a compound exists in a tautomeric form with a proton on the nitrogen. The nitrogen NMR spectrum is often of considerable value in studies of tautomerism of this type. 

First-order spectra (mulliplels) are observed when the eoupling constant is small compared with the frequency difference of chemical shifts between the coupling nuclei This is referred to as an A n spin system, where nucleus A has the smaller and nucleus X has the considerably larger chemical shift. An AX system (Fig. 1.4) consists of an T doublet and an X doublet with the common coupling constant J x The chemical shifts are measured from the centres of eaeh doublet to the reference resonance. 

NMR chemical shifts reported for the fluonde ion vary considerably [759, 160, 161, 162, 163]. The electronic environment associated with a fluonde ion is much more exposed to the medium than a covalent C-F moiety. Chemical shifts... 

The study of the NMR spectra of thiophenes has attracted considerable interest, 22,24-3sb partly because the spectra of substituted thiophenes containing only a few ring hydrogens are quite suitable for complete analysis and partly because in a series of related compounds the chemical shifts observed are related to differences in the electron distribution about chemically nonequivalent hydrogens (for review, see reference 39), especially for hydrogens far removed from the substituent. 

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