Deshielded

Figure Bl.11.7. chemical shifts in [10]-paracyclophane. They have values on either side of the 1.38 ppm found for large polymethylene rings and, thus, map the local shielding and deshielding near the aromatic moiety, as depicted in the upper part of the figure.

The spectrum of 5-ethylthiothiazole compared to that of thiazole shows a slight deshielding effect on H-Cq (9 versus 9.1 ppm) and a slight shielding effect on H-C4 (7.93 versus 7.88 ppm) (270). 

Hydrogens that are directly attached to double bonds (vinylic protons) or to aro matic rings (aryl protons) are especially deshielded... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Protons on carbons adjacent to a carbonyl group are deshielded slightly more than allyhc hydrogens... 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

NMR Like other carbonyl groups the carbon of the —CO2H group of a car boxylic acid is strongly deshielded (8 160-185) but not as much as that of an aldehyde or ketone (8 190-215)... 

NMR Compared with C—H the carbon of C—O m a phenol is deshielded by about... 

Further aza substitution ortho or para in the same ring deshields the nitrogen the effect is moderate for a para-, and large for an ort/io-nitrogen. The latter is probably a special azo effect , since the nitrogens of a simple azo group absorb at still lower field (-130 p.p.m., in ether). 

Chemical shifts of pyridine and the diazines have been measured as a function of pH in aqueous solution and generally protonation at nitrogen results in deshielding of the carbon resonances by up to 10 p.p.m. (73T1145). The pH dependence follows classic titration curves whose inflexions yield pK values in good agreement with those obtained by other methods. 

Af-Oxidation of pyrazines appears to result in increased shielding of the a and a carbon resonances by 6-11 p.p.m., whereas the /3 and /3 carbon atoms are deshielded by 3-4 p.p.m., a trend similar to that observed with substituted pyridines. These results have been qualitatively explained in terms of resonance polar effects (80OMR(l3)l72). 

The most characteristic coupling constant in indazoles is the cross-ring Vs, present both in indazoles and in isoindazoles unsubstituted in positions 3 and 7. 2-Methyl isomers show an additional Vmc.h coupling which can serve to identify an isoindazole unsubstituted in position 3. In 3-azidoindazole, as in 3-azidopyrazole (56), the prototropic exchange is slowed down sufficiently to allow the measurement of a zig-zag /i,4 coupling constant. The deshielding effects observed in A-acetyl derivatives, e.g. 1-acetyl (60) on H-7 and 2-acetyl (61) on H-3, are related to a preferred E conformation (Section 4.04.1.4.3). 

Figure 1.2. H NMR spectrum of ethyl dichloroacetate (CDCI3, 25 °C, 80 MHz). The proton of the CHCh group is less shielded (more strongly deshielded) in comparison with the protons of the CH2 and CHj residues...

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