Upheld shift

Substituent effects are often considerable, particularly when strongly electron-donating effects (to the aza-nitrogen) are present, when upheld shifts of up to 60 p.p.m. (2-NH2) may be observed. 

Hydrogen bonding to the nitrogen lone pair leads to an upheld shift, the extent of which depends on the proton-donor ability of the solvent, and the acceptor ability of the base shifts of some 20 p.p.m. are commonly found. 

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. 

The NMR spectral properties of the parent heterocycles are summarized in Table 12. The signal for the pyrrole a-carbon is broadened as a result of coupling with the adjacent nitrogen-14 atom (c/. Section 3.01.4.3). While the frequencies observed for the /3-carbon atoms show a fairly systematic upheld shift with increasing electronegativity of the heteroatom, the shifts for the a-carbon atoms vary irregularly. The shifts are comparable with that for benzene, S 128.7. 

Exocyclic conjugation causes a small upheld shift of the ring hydrogen resonances, as can be seen in Table 6. The increase in tr-bond hxation also results in an increase in the 4,5 coupling constant to about 6.0 Hz. The use of coupling constants for the investigation of tautomerism is discussed in Section 4.17.5. 

NMR experiments where undertaken to determine the site of coordination of the two other complexes. The thioether complex showed an upheld shift for the N-atom in the thioether gold complex and a similar result was obtained in the case of the thiazole compound, confirming imine coordination. 

Chemical shift difference in ppm. Negative values mean upheld shifts. Source Ref 46. Copyright 1999 American Chemical Society. 

Chemical shift difference (ppm) with an uncertainty of 0.01 ppm. Negative values designate upheld shifts. 

FIG. 10 Chemical shift change of the NMR signals of EPC induced by the injection of DBC H and PRC H with a molar ratio of 1/1. Negative value means an upheld shift. 

FIG. 13 Chemical shift change of the EPC H NMR signals induced by the ring current effect of BPA. Negative value means upheld shift. (From Ref. 47. Copyright 1999 American Chemical Society.)... 

Chemical shift change (ppm) induced by the BPA delivery. Positive and negative values mean downfield and upheld shifts, respectively. 

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