Downfield and upheld

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

Bu ) have been used to induce paramagnetic shifts in C n.m.r. spectra. Thus, all sets of shifts in 2,2-dimethylpropan-l-ol varied with concentration of the added shift reagent, except when using the Gd complex where no shift was experienced. In the study of Bu"NH2, Pr"NH2, and n-CjHjjNH2, and upheld shift of the C-2 resonance was observed, but a comparable downfield shift occurred with Bu"OH. Induced shifts of C resonances were utilized in the analyses of the spectra of piperines" and ribose-5-phosphate. ... 

Alcohol carbons are identified by spectral comparison of the alcohol with the corresponding acetate Upon acetylation of cyclohexanols the a-carbons shift about 3 to 4 ppm downfield and the /i-carbons move 2 3 ppm upheld. Esterification of axial hydroxy groups also causes a downfield shift of 1 ppm of the y-carbons [65 a]. 

NMR at 400 MHz has been used to probe the mobility of the cytochrome domain within the holoenzyme (84). The linewidths of heme resonances downfield of 4-12 ppm and upheld of -4 ppm have been compared for the holoenzyme and the cytochrome 62 core and indicate that the cytochrome domains of the holoenzyme are markedly mobile (84). NMR has also been used to investigate the nature of the interaction between flavocytochrome 62 and cytochrome c (85,86) with the ultimate aim of defining the binding site on flavocytochrome 62 used by its physiological partner. The extensive NMR studies on the interaction between cytochrome 65 and cytochrome c (87,88) provide a useful background to the work with cytochrome 62 ... 

For half the molecules, then, absorption by a secondary proton is shifted slightly downfield, and for the other half of the molecules the absorption is shifted slightly upheld. The signal is split into two peaks a doublet, with equal peak intensities (Fig. 13.9). 

NNMR spectra of polyamide/polyether block copolymers based on polyethylene oxide) and either nylon-6 or nylon-12 are dominated by peaks at approximately 119 ppm, arising from the amorphous nylon phase. In addition, both polymers also exhibit a shoulder. Downfield (about 112 ppm) and upheld (about 117 ppm) shoulders indicate that polyamide phases exist as a (nylon-6) and y (nylon-12) forms, respectively (62). 

However, there are observations which do not appear to be explained by this theory. For instance, in comparison with a phosphonium salt, the phosphonium ylide possesses considerable d -p bonding and has a less electronegative carbon atom bonded to phosphorus. It is predicted that both of these effects should deshield the phosphorus nucleus, which is contrary to the experimental observations. Further, when the carbanion of the ylide is delocalized (which may be achieved with a minimal change in inductive effect), Sp is shifted downfield, and when alkyl groups are introduced on the a-carbon atom of ylides, Sp is shifted upheld. Note that the alkyl groups would be expected to increase the +/ effect and the + M effect by way of increased /7-character of the lone pair of electrons. Similar conclusions may be drawn from a comparison of ylides with phosphine oxides. 

Since it is now clear that the chemical shifts for carbons in typical indole alkaloid environments have reliably characteristic values, it seemed worthwhile at this stage to gather together, as has been done in the Table of structures (2)—(39) and chemical shifts which follows, a relevant selection of data from those so far available on typical alkaloids, derivatives, and simple model compounds. The values given are for 0.2—2.0moll" solutions in chloroform or deuterio-chloroform unless otherwise specified, and are d values in p.p.m. downfield from tetramethylsilane. There is not yet a generally used system for reporting C chemical shifts, thus 3 values relative to benzene, downfield from chloroform, and upheld from carbon disulphide, as well as downfield from tetramethylsilane, are currently to be found in the literature. 

If the spectral width is inadequate to cover every peak in the spectrum, then some peaks in the downfield or upheld region may fold over and appear superimposed on the spectrum. How can you identify these folded signals ... 

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