Downfield shift

Positive Z values indicate a downfield shift, and an arrow indicates the point of attachment of the substituent group to the double bond. 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

The ether oxygen of tetrahydropyran (45 X = O) induces a large downfield shift of the a carbons, while the /3 and y carbons move slightly upfield, the y more noticeably. 

Information on partially and fully saturated heterocycles is much more limited and is summarized in Figure 3. As would be expected, the downfield shift of the a-carbon atom decreases with decreasing electronegativity of the heteroatom in the sequence O < NH < S < CH2. 

Downfield shifts positive. Chemical shifts from NH3 or MeN02 calculated using approximate values McN02 380 p.p.m., NOJ 376p.p.m., NH3 Op.p.m. (b-79mui70i). 

The dimensionless acceptor number, AN, ranked the acidity of a solvent and was defined for an acidic solvent A as the relative P NMR downfield shift (A3) induced in triethyl phosphine when dissolved in pure A. A value of 0 was assigned to the shift produced by the neutral solvent hexane, and a value of 100 to the shift produced by SbClj. Gutmann suggested that the enthalpy of acid-base adduct formation be written as ... 

The intramolecular Se N interactions in complexes of the type 15.5 result in a downfield shift in the Se NMR resonances the magnitude of this shift corresponds approximately to the strength of the interaction. ... 

The signal from H-4 in the parent compounds underwent a greater downfield shift than that from H-2 when the solvent polarity was increased (CDCI3 MeaCO DMSO). This was ascribed to the counterbalancing effect of preferential solvation at the N-1 position. 

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. 

Ether carbon atoms also exhibit a downfield shift in the 13C NMR spectrum, where they usually absorb in the 50 to 80 5 range. For example, the carbon atoms next to oxygen in methyl propyl ether absorb at 58.5 and 74.8 8. Similarly, the methyl carbon in anisole absorbs at 54.8 8. 

Ring current (Section 15.8) The circulation of tt electrons induced in aromatic rings by an external magnetic field. This effect accounts for the downfield shift of aromatic ring protons in the lH NMR spectrum. 

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

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