Chemical shift variation

In cases of complexes bearing an exocyclic double bond directly coordinated to the metal center, the carbons of the double bond usually exhibit coupling with NMR-active metal centers and/or auxiliary ligands.6 14 18 19 The chemical shifts of the quaternary carbon atom vary from 66.976 to 82.2818 ppm, while the methylene group gives rise to signals at 29.16,14 41.91,6 or 51.3418 ppm in the 13C 1H NMR spectra. As one can see, the chemical shift variation is relatively broad and significantly affected by the nature of the metal center. 

Fig. 2. 7Li NMR chemical-shift variation as a function of the DMSO/ LiC104 molar ratio in y-butyrolactone solution at 25°C (92).

In trans 2-butene, the chemical shift variations are almost identical, which could correspond to a general intermolecular interaction. However, if the comparison is made with liquid trans 2-butene, specific interactions are clearly shown again by the greater change in the methinic C carbon atom. 

The chemical shift variations, the linewidths and the longitudinal relaxation times are all in agreement in favor of two different adsorbed species, at high and low temperature, respectively. At high temperature the 1- and 2-butene molecules are more stabilized as 7r-complexes on the NaGeX zeolite surface, while the cyclic type adsorbed species is preferred at low temperature. 

Other enzyme-substrate or inhibitor interaction studies80 82 have been addressed, using a combination of STD and trNOE NMR experiments, in order to collect details on the substrate bound conformation (ligand perspective). In other cases, the availability of a labelled protein receptor83 have permitted to follow the induced chemical shift variations of the protein resonances upon ligand addition to the NMR tube by HSQC methods (protein perspective). 

Requirement 2. Self-consistency The peak lists must be faithful representations of the NOESY spectra, and the chemical shift positions of the NOESY cross peaks must be correctly calibrated to fit the chemical shift lists within the chemical shift tolerances. The range of allowed chemical shift variations ( tolerances ) for 1H should not exceed 0.02 ppm when working with homonuclear [1H,1H]-NOESY spectra, or 0.03 ppm when work-... 

Finally, chemical-shift variations originating in van der Waals compression are noteworthy, although very few reports have come to our attention discussing signal shifts of sp3 carbon atoms in terms of van der Waals interactions. Schill and co-workers (89) found downfield shifts of up to 1 ppm in the [2]-catenane 5 when its 13C resonances are compared with those of the two isolated subunits. From the increase of these chemical-shift differences with increasing distance from the nitrogen atom, these authors concluded that the strongest van der Waals interactions occur in that part of the heterocycle which is opposite to the amide moiety in space (cf. Scheme 4). 

Intraactions between elections in three-membered rings and unsaturated groups in the same molecule have been detected via 13C chemical-shift variations in a number of instances. Thus, introduction of the carbonyl function in tricy-clo[3.2.1,02,7]decane (e.g., 274) leads to significant downfield shifts of the signals of C(l) (+8.0), C(2) (+15.5), and C(7) (+7.7) (385), whereas corresponding effects in bicyclo[3.1.0]hexan-2-one (275) are smaller (385,386). A corresponding dependence was reported for 276 and 277 and related to more effective electron withdrawal in 276 (387). An even more pronounced deshielding effect was observed by Murata and co-workers (388,389) in the ketone 278 when they compared it with 279. 

In our studies of the TGF-P3 dimer in solution, we detected the direct interaction of several metallic cations, such as Ca ", Na" ", and Mn " with the aid of NMR spectroscopy, hi particular, chemical shift variations in response to changes of Na and Ca ion concentrations and hne broadening due to the interaction with paramagnetic Mn + ions were observed in the heteronu-clear two-dimensional HSQC spectra and mapped onto the spatial... 

The introduction of isotopes into a compound alters the coupling pattern and the chemical shifts of the observed spectrum. As shovm in Eigure 1.18, deuterium-induced chemical shift variations have allo ved the estimation of the ratio of isomers 90a-d formed in Eq. (2) vhen R = Ph, R = CHjCOOH, and DCOOD/ Et3N is used for the hydrogen transfer [136]. The three sp -carbons Cl, C2 and C3 each afford a distinct singlet for the four possible isotopomers 90a-d (replacement of by shifts the resonances of the adjacent carbon nuclei to lo ver frequency) [137]. 

Gutmann acceptor numbers were determined in the "usual" way via the chemical shift variation of triphenylphosphine oxide by Osteryoung et al. [26]. While, again, the donor numbers were concentration- and composition-independent for basic melts, the acidic melts showed a strong composition dependence. Nonetheless, the acidity range was comparably small and was found around 100 (which compares to the acidity of trifluoroacetic acid). The donor number for basic melts was found to be 98, which was, of course. 

Other reports concerning chemically modified amino acid as metal chelating agents used for the carbonic anhydrase active site model reconstruction are in close agreement with the small contribution of Zn(II) binding to the proton chemical shift variation discussed above. NMR experiments carried out in DMSO-rfg, at 300 K, and the observed... 

Chemical shift variations among N-methyl cations have been related to electron densities and, in the case of fV-methylpyridinium ions, to Hammett substituent constants.126 Upfield shifts of H,126-128 13C,126 and 14N nuclei126 are observed when electron-donating groups increase the electron density at the quaternized site. In instances where shift differences are large, application of these correlations to molecules giving isomeric products will enable conclusive identification of product structures to be made. 

By plotting the chemical shift variation against the acidity, one observes a typical acid-base titration curve (Figure 1.5) and the p/fBH+ of the indicator can be determined this way. This NMR method, which was first proposed by Grunwald et al.,41 has been applied by Levy et al42 using various ketones and a-haloketones for the determination of ketone basicity and evaluation of medium acidity. 

Figure 1.5. Acidity-dependent H NMR chemical shift variations protonation curve for acetaldehyde.42 CF3C00H-H2S04, A CF3COOH-CF3SO3H.

In contrast, approach to the structure determination by solid-state NMR [86], has proposed a more defined geometry of both the macrocycle and the side chain. In fact, the 10 solid-state NMR structures derived were very well defined, while the average RMSD of the heavy atom to the X-ray structure was only 0.75A. When comparing assignments of the solid-state NMR with the previously reported solution-state NMR data [84], a qualitative correlation was found between the chemical-shift variations observed and the possibility of intermolecular hydrogen-bonding [86],... 

The same authors26 have also studied the effect of pH on the 33S line width of a solution of Na2S04. In the pH range 0.9-6.4, the overall line width change is 190 Hz, and the chemical shift variation is 3 ppm. The origin of these variations is not due to exchange phenomena but probably to the interaction of the unprotonated species with sodium ions. Ion pairs should relax faster than not complexed species. 

In order to further extend our understanding of the origin of chemical shift variations in zeolites, we have examined the effects of gallium substitution for aluminum in the framework on the silicon NMR spectra of sodalite and faujasite. Gallium substitution in zeolites is well known though most preparations... 

The peaks in the spectra shown in Figs 5 3 1 and 5 3 2 are very broad, as is typically found for the H NMR spectra of lignins Whether this is merely due to chemical shift variation or is also related to the polymeric nature of the sample is a question which deserves further attention... 

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