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Hydrogen, downfield shift

Hydrogens on carbon next to an ether oxygen are shifted downfield from the normal alkane resonance and show U-f NMR absorptions in the region 3.4 to 4.5 8. This downfield shift is clearly seen in the spectrum of dipropyl ether shown in Figure 18.4. [Pg.671]

The following table given by Olah and J. Denovan (J. Amer. Chem. Soc. 1977.90.5026) shows that substitution of an ethyl for a methyl group or a methyl for hydrogen causes a downfield shift showing that the central carbon becomes somewhat more positive. [Pg.7]

The addition of tetrabutylammonium chloride to H solutions of [68] and [69] in deuteriated acetonitrile resulted in remarkable nmr shifts of the respective proton signals of both receptors. Of particular note were the substantial downfield shifts of the amide protons (AS = 1.28 ppm for [68] and 1.52 ppm for [69]) on addition of one equivalent of chloride. These results suggest that a significant —CO—NH-Cr hydrogen-bonding interaction contributes to the overall anion complexation process. Subsequent nmr titration curves suggesting 1 1 stoichiometry with anion complexes of [68] and [69] were found in all cases. Negligible shifts were observed under identi-... [Pg.51]

Such an arrangement is intrinsically associated with a downfield shift of the signals of the hydrogen-bearing carbon atoms. The physical meaning of this interaction, however, was not specified (146). On the basis of this reinterpretation steric crowding appears to cause downfield rather than upfield shifts (146), a hypothesis that is confirmed by the yg-SCS(Br) reductions cited in Tables 13 and 14 and by the appearance of a downfield-shifting 8M-SCS(X) (cf. Section III-E) (184). [Pg.248]

An important tool for the investigation of carbocation structure is measurement of the C nmr chemical shift of the carbon atom bearing the positive charge.66 This shift approximately correlates with electron density on the carbon. 13C chemical shifts for a number of ions are given in Table 5.2.67 As shown in the table, the substitution of an ethyl for a methyl or a methyl for a hydrogen causes a downfield shift, indicating that the central carbon... [Pg.172]

The H NMR spectra of the parent heterocycles (cf. Table 7) each consist of two multiplets, of which the one at lower field is assigned to the a-hydrogens. Comparison of the chemical shifts of the vinylic protons of furan, H-2 7.46 and H-3 6.41, with those observed for 4,5-dihydrofuran, H-2 6.31 and H-3 4.95 (66JCS(B)127), indicates that there is ca. 1-1.5 p.p.m. downfield shift attributable to the presence of an aromatic ring current in furan. The same effect is observed for thiophene, H-2 7.35 and H-3 7.13,and 4,5-dihy-drothiophene, H-2 6.17 and H-3 5.63. The similar range of chemical shifts observed for... [Pg.7]

Substitution of a methyl group for hydrogen on pyrrole N or C atoms leads to upfield shifts (ca. 0.1-0.3 p.p.m.) in the remaining ring proton resonances. Electron-withdrawing substituents (e.g. formyl, acyl, alkoxycarbonyl) on N cause substantial downfield shifts of the a-proton resonances and much smaller downfield shifts for those of the -protons (Table 4). The downfield shifts experienced by the ring proton resonances in pyrroles... [Pg.165]


See other pages where Hydrogen, downfield shift is mentioned: [Pg.7]    [Pg.134]    [Pg.271]    [Pg.395]    [Pg.441]    [Pg.225]    [Pg.113]    [Pg.106]    [Pg.395]    [Pg.441]    [Pg.123]    [Pg.132]    [Pg.190]    [Pg.95]    [Pg.347]    [Pg.35]    [Pg.260]    [Pg.71]    [Pg.258]    [Pg.317]    [Pg.318]    [Pg.320]    [Pg.157]    [Pg.49]    [Pg.124]    [Pg.128]    [Pg.148]    [Pg.92]    [Pg.145]    [Pg.194]    [Pg.233]    [Pg.286]    [Pg.80]    [Pg.104]    [Pg.337]    [Pg.528]    [Pg.364]    [Pg.134]    [Pg.121]    [Pg.146]    [Pg.616]    [Pg.26]    [Pg.636]    [Pg.134]   
See also in sourсe #XX -- [ Pg.7 ]

See also in sourсe #XX -- [ Pg.7 ]




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