Chemical shift upheld

The carbon chemical shifts of the azolium salts can be found at the downheld end of the aromatic range at 5 = 140-160 ppm and the carbenes themselves about A5 = 100 ppm downheld of the imidazolium salts. Coordinahon to transihon metals brings the carbon chemical shift upheld from the value of the free carbene. Whereas the resonance in [Cp Ru(NHC)Cl] complexes are typically around 8 = 200 ppm [116], the same signal in [Ag(NHC)Cl] complexes can be found at 8 = 170-190 ppm [50] (see Figure 1.23). 

Conformations of oligo(pyridine-aA-pyrimidine)s 119 have been studied. On the basis of NMR analysis and the fluorescence spectrum in solution, the oligomers were found to take a helical conformation.203 The conformation was characterized by distinct chemical shifts (upheld shift), NOE effects, and excimer emission arising from the overlap of aromatic groups. The helical structure was confirmed for 120 in the... 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

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. 

Some available data on H NMR spectra of non-aromatic azoles containing two ring-double bonds are given in Table 10. Here there is no ring current effect and the chemical shifts are consequently more upheld. 

The chemical shifts of the pyrazolate anion (62) represented an upheld effect of 0.3 p.p.m. compared with pyrazole itself (68JA4232). 

Sparse data on the pyrazole isomers, pyrazolenines and isopyrazoles, are presented in Table 12. Besides the obvious upheld effect on the chemical shift due to the suppression of the ring current, these compounds behave normally. Data on pyrazolidinones and their salts show the behaviour of cyclic hydrazides (66T2461,67BSF3502). 

The Si NMR chemical shifts for several Tg derivatives are shown in Table 2. The alkyl derivatives fall in the range —54 to —57ppm, while the aryl derivatives are more upheld from —66.9 to —68.9 ppm, as expected for aryl versus alkyl substituents. Bassindale et al. have derived a relationship, = 0.82 x Tg/ allowing an estimation of the chemical shift for unknown Tg derivatives from the known values for the much more common Tg analogs. The solid-state Si NMR spectrum for Tg[OSiMe3]g shows five signals for the silsesquioxane silicon atoms. 

However, other data such as the small difference observed in the Si NMR chemical shift (0.9 ppm upheld from TgPhg) and the absence of any measurable Si-F coupling show that the interaction between the huoride ion and the silicon atoms is small. Studies to evaluate the collision cross section of TgPhg using Na show that the cation attaches itself to the outside of the POSS cage and does not significantly distort the structure. 

H NMR data has been reported for the ethylzinc complex, Zn(TPP—NMe)Et, formed from the reaction of free-base N-methyl porphyrin H(TPP—NMe) with ZnEti. The ethyl proton chemical shifts are observed upheld, evidence that the ethyl group is coordinated to zinc near the center of the porphyrin. The complex is stable under N2 in the dark, but decomposed by a radical mechanism in visible light.The complex reacted with hindered phenols (HOAr) when irradiated with visible light to give ethane and the aryloxo complexes Zn(TPP—NMe)OAr. The reaction of Zn(TPP—NMe)Et, a secondary amine (HNEt2) and CO2 gave zinc carbamate complexes, for example Zn(TPP—NMclOiCNEti."" ... 

Another way in which to gain structural information concerning the N-terminal residue of glycophorins A" and A is to study the N-terminal, mono[ C]methyl derivatives these are produced by using limited amounts of [ C]formaldehyde. There are distinct differences between the N, N -di[ C]methylamino and N -mono[ C]methylamino species (i) a significant, chemical-shift difference exists between the N-terminal dimethyl and monomethyl species (43 and 34 p.p.m.) (li) all of the C resonances of the N-terminal dimethyl species move upheld as the pH is increased (if they move at all), whereas all of the C resonances of the N-terminal, monomethyl species move downfield as the pH is increased and (in) A for the N-terminal monomethyl species tends to be much larger than that for the N-terminal dimethyl species. Point (in) would tend to indicate that it may be more advantageous to study the N-terminal monomethyl species. However, because of allowable protein concentrations, detection limits on available instruments, and technical difficulties, it has thus far... 

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and trans sulfoxide isomers of trans-1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upheld from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upheld from those of the supposed equatorial conformers. For trans-3, trans-5-dimethylthiane-l-oxide (177) with the oxygen axial, the 0 signal was 21 ppm upheld from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

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. 

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