Proton Chemical Shifts and Structure

A polar bond in a molecule generates an electric field that can have an appreciable value at the position of a nearby resonating nucleus. This electric field distorts the electronic structure around the nucleus and causes a deshielding by diminishing a. Unlike the inductive effect, the electric-field effect can be derived from a polar group that is many bonds removed from the resonating nucleus. For a significant value of ag, the polar bond must be reasonably close to the nucleus, but need not be in van der Waals contact. 

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With more H NMR specroscopic data available for these complexes, further trends between the proton chemical shifts and structural types are becoming apparent. For example, in tautomerically related complexes, the methylene protons of rj -bound allenyl ligands (type VII) appear downfield of propargylic counterparts (1 < A8 2.5 ppm), allowing a... 

Agostic interactions, i. e., the three-center bonds related to structure 51 [26, 44-49], were noted earlier by Green and Brookhart and have been cited above in the methoxycarbonylation chemistry (Figure 1.9). These bonds are often characterized by low frequency (hydride-like) proton chemical shifts, and/or substantially reduced /( C, H) values. Often, it is necessary to cool the NMR sample in order to freeze the equilibrium. Complex 52 represents a nice example of an agostic C-H bond, with relevance to polymerization chemistry [47]. 

Proton chemical shifts are very valuable for the determination of structures, but to use the shifts in this way we must know something about the correlations that exist between chemical shift and structural environment of protons in organic compounds. The most important effects arise from differences in electronegativity, types of carbon bonding, hydrogen bonding, and chemical exchange. 

Methyl group proton chemical shifts in structurally related inside yohimbanes are shown in [503] and [504]. (306)... 

The H NMR data for saturated ring systems, as recorded in the original papers, lacks fine structural information and only gives a gross indication of ring proton chemical shifts and is therefore not included. 

For NMR studies of fluorinated surfactants, the most useful nucleus is F, in addition to and H nuclei. Changes ip the F chemical shift at cmc are larger than changes in the proton chemical shifts and, therefore, provide more information on fluorinated surfactants and their micellar structures. F-NMR spectra have been recorded for structural characterization of perfluorononanoic acid [125] and perfluoropolyether surfactants [126]. Micelle formation in solutions of... 

A useful empirical method for the prediction of chemical shifts and coupling constants relies on the information contained in databases of structures with the corresponding NMR data. Large databases with hundred-thousands of chemical shifts are commercially available and are linked to predictive systems, which basically rely on database searching [35], Protons are internally represented by their structural environments, usually their HOSE codes [9]. When a query structure is submitted, a search is performed to find the protons belonging to similar (overlapping) substructures. These are the protons with the same HOSE codes as the protons in the query molecule. The prediction of the chemical shift is calculated as the average chemical shift of the retrieved protons. 

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

Ruonne NMR data can be collected readily on most spectrometers, requinng only minor adjustments to mstrumentation used to run proton samples The fluonne-19 nucleus is easily detected (relative abundance, 100%, spin, 1/2) and generates a wealth of spectral information to assist in structure elucidation To take full advantage of all the spectral evidence available, H, and chemical shifts and couphng constants should be acquired and correlated... 

Another example is provided by [30] anmlene. Longuet-Higgins and Salem have shown that the observed visible and UV absorption spectrum and, in particular, the NMR proton chemical shifts of this molecule are very difficult to reconcile with the symmetrical nuclear configuration (Dg ) suggested by the superposition of the Kekule-type resonance structures. The hypothesis of a bond-length alternation of symmetry removes this difficulty. This indicates that the resonance between Kekule-type structures should be very much impeded also in this molecule. 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

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