Chemical shift causes

Partial plots extracted from HMBC spectra recorded with the five different low-pass J filters are shown in Figure 7. Clearly, ambiguities may arise, particularly if accidental degeneracy in chemical shifts causes true long-range correlations and residual 1Jch correlations to overlap. 

Together, the published data show characteristic changes in chemical shift, caused by sulfation, which may be used as an indicator in elucidating... 

The protonation of 1-methyl-1,2,4-triazole [88] leads to a cation stabilized by amidinium type resonance [89] only if the proton adds on to N-4. Chemical shifts caused by protonation confirm this. 

C-4 protonation would be expected to occur preferentially in 3-pyrazolines. It is in fact found that the thermodynamically stable conjugate acid is protonated on C-4 (Aubagnac et al., 1967a and 1969), but a transitory existence of an N-protonated form could also be demonstrated (Aubagnac et al., 1967b). This was identified as N-1 protonated by measurements of chemical shifts caused by protonation of l,2,4-trimethyl-3-phenyl-3-pyrazoline [158]. The signal of... 

The chemical shifts caused by different substituent groups closely resemble those observed for benzene derivatives [76T2339, 76ZN(B)I641]. 

By extrapolation of the changes in the chemical shifts in the NMR spectrum of epi-inositol, the stability constant of the calcium complex has been calculated to be ca. 3M"1—i.e., about 70% of the cyclitol is present as a cationic complex in 1M calcium chloride solution. However, the extrapolation cannot be accurately carried out because there are secondary changes in chemical shifts caused by weaker complexing at two oxygen atoms. 

The transverse relaxation rate enhancement / 2P could be treated in the same way if it were not for the difference in chemical shift A com between the paramagnetic and the diamagnetic species. As has already been shown, the difference in chemical shift causes a line broadening — an increasing in R2M — even when the... 

Table A3.12 Effect on 15C chemical shifts caused by replacing a methyl group by a polar substituent ( values, TMS)...

Change in chemical shift caused by the methylsulfonyl group. 

The C-3 chemical shifts show low sensitivity to changes in structure and charge of the molecule. The amplitude of changes of C-4 and C-5 chemical shifts caused by the earlier transformation of I-III is practically the same (Table 3.17). 

Nuclear spins are sensitive to the chemical environment of a nucleus. Electrons moving near the nucleus establish an internal magnetic field that modifies the local effective field felt by each proton to a value different from that of the externally applied field. The resulting chemical shift causes protons within different structural units of the molecule to show NMR peaks at different values of magnetic field. All protons in chemically equivalent environments will contribute to a single absorption peak in the spectrum. The relative area under each absorption peak is proportional to the number of protons within each equivalent group. In order to standardize procedures, chemical shift values are recorded relative to the selected reference compound tetramethylsilane (TMS) by adding a very small amount of... 

As the final example in this section, we show how an investigation of the changes in H chemical shifts caused by aromatic ring currents provides insight into the nature of the supramolecular host—guest complex, 4 5.29 The receptor 4 belongs to a family... 

An interesting approach of structure refinement is based on the use of NOEs plus chemical shifts that are caused by intermolecular interactions. Iwadate et performed calculations on the geometry of a melittin tetramer at dilferent temperatures. The structural models rely on a relatively small number of intermolecular chemical shifts caused by the interaction of special groups such as tryptophan rings with protons. There are even a number of examples where aggregation chemical shifts are used to build and refine molecular models.For example, van Rossum et used the shift... 

A similar, but less pronounced chain length vs chemical shift dependence is observed for a series of nylon-6,6 type polyamides based on aliphatic or aromatic a,co-diamines and aliphatic or aromatic diacids in trifluoroacetic acid. However, the downfield chemical shift caused by increasing the chain length of aliphatic diacid residues is less pronounced relative to that of aliphatic diamines [12]. 

In summary, the phenomenon of chemical shift enables the chemist to obtain some fundamental information about electronegativities, bond anisotropies, and so on. Above all, the ability to observe the chemical shift causes hydrogen atoms (and to some extent other nuclei) to become functional groups that can be qualitatively and quantitatively estimated. 

Actinomycin (. J 5), when examined by H, and T n.m.r. in the presence of deoxyoligonucleotides, revealed intercalation by the chemical shifts, caused by the ring-current fields of the phenoxazone nucleus, in the purine-pyrimidine resonance signals of the nucleotide. This led to detailed understanding of the solution structure of the intercalated components, and established that intercalation occurred at the site of the central GC base-pairs (Patel, 1974). This work was extended, using N-enriched actinomycin (Brown, Shafer and Mirau, 1982). 

Haggis, G.H., Hasted, J.B., and Buchanan, J.T. 1953. Temperature behavior of chemical shift caused by hydrogen bonds. J. Chem. Phys. 21 1688-1693. 

Figure 2 Effect on nitrogen chemical shifts caused by replacing hydrogen atoms by alkyl groups in an aliphatic chain.

It may be noted that refocusing occurs only in the heteronuclear system, i.e., when a selective 180° pulse is applied. Field inhomogeneities or differences of chemical shift cause an additional fanning out of the magnetic vectors which can be eliminated by the application of the 180° pulse to the X nucleus. This is important for practical purposes since the Larmor frequency of the X nucleus is not normally identical to the carrier frequency Vq, as assumed in the above discussion. 

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