H and 13C Chemical Shifts

H and 13C chemical shifts and coupling constants of some selected representative four-membered ring sulfoxides and sulfones are given in Table 7. ... 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

NMR investigations of some carbohydrate-substituted tetrazolo[l,5-h]pyrimidin-5(6//)ones as nucleotide analogues have been studied by Chu et al. <1997JOC7267>. The H and 13C chemical shifts of these derivatives are shown in Table 3. 

In many cases it is not easy to identify individual effects or blends unequivocally, because they are not independent from each other in how they influence nuclear shieldings. Compilations of such effects on H and 13C chemical shifts have been presented24,2S, and the following discussion will provide further instances for these and other nuclei. 

The orientation of lone electron pairs at heteroatoms can affect H and 13C chemical shifts substantially. This has been studied in conformationally rigid cyclic amines. Protons in piperidines are shielded significantly if they are antiperiplanar to the nitrogen lone pair70-72. 

In addition to H and 13C chemical shifts, vicinal 1H,1H coupling constants provide another tool for the stereochemical assignment of disubstituted cyclopropanes, as the cis or trans relationship of the substituents can be determined from the absolute values, often without the need to compare diastereomers. The relationship is ... 

H and 13C chemical shifts have been reported for many 1,2-disubstituted oxiranes432 436-439. Similar to cyclopropanes, oxirane ring carbon chemical shifts are smaller in the cis- than in the frans-isomers, and an increment rule using pairwise substituent interaction cross-terms has also been developed439. 

Chapter 13, Spectroscopy, has been supplemented by an expanded discussion of H and 13C chemical shifts and a new section on 2D NMR. A new box, Spectra by the Thousands, points the way to websites that feature libraries of spectra and spectroscopic problems of every range of difficulty. 

Section 16.18 An H—C—O—C structural unit in an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency in its infrared spectrum and the H—C chemical shift in its H NMR spectrum. Because sulfur is less electronegative than oxygen, the H and 13C chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C. 

It has been reported in a follow-up study,324 that the product 3-arylindenones underwent double protonation in strong superacids (HSO3F, CF3SO3H) and the stability of formed species allowed their observation by NMR spectroscopy at room temperature. Although fast proton exchange prevented to observe (9-protonation, H and 13C chemical shifts unequivocally showed the presence of dication 93. 

In the absence of sequence-specific protein NMR resonance assignments, the data used for NOE matching are limited to the unassigned experimental protein H and 13C chemical shifts, the predicted protein H and 13C chemical shifts, and the experimental and predicted... 

H and 13C chemical shifts of dioxetane ring atoms (ppm with respect to TMS). 

The theoretical background for these four nuclei has already been presented in Chapters 3 and 4. Our treatment of spin, coupling, NOE, Fourier transformation, etc. can be applied to these nuclei without modification. The concept of chemical shift we also use without modification, but we must avoid exercising the predictive skills that we have developed for H and 13C chemical shifts to these nuclei (with some exceptions as noted). 

H and 13C chemical shifts are very useful in determining the position of cyclometallation on aromatic rings in cases where several possible positions exist, e.g. in 66 and similar arylazonaphthalenes,139-142 as well as in the characterization of reaction products (ref. 135 and references therein). 

H and 13C Chemical Shifts for the Dihydropyrazine Fragment in the H- and l3C-NMR Spectra of the Methoxy Adducts 42-44 ... 

Selected H- and 13C-Chemical Shifts (in Parts Per Million) of 1V-(1-Haloalkyl)Hetarylium Halides 7, 33, and 43... 

Several elaborate algorithms have been developed to relate H and 13C chemical shifts to structural fragments in organic molecules and are incorporated in several of the large spectral databases mentioned in Section 4.11. These treatments are very effective for 13C. As we saw in Section 4.5, H chemical shifts are influenced drastically by neighboring molecular moieties, hence are more difficult... 

The dynamics of surfactant molecules in micelles can also be studied by various NMR methods such as H and 13C chemical shift, band shape, relaxation... 

Jacobs et al. [1] used two-dimensional NMR spectroscopy to assign the H and 13C chemical shifts of cucurbitacin B, establishing these values as a benchmark for analyzing the spectra of other cucurbitacins isolated from the same source. In fact, the characteristic groups present in all cucurbitacins are easily detectable by means of NMR and typically include the presence of carbonyls at C-ll and C-22, which appear between 212-213 ppm, as well as hydroxyls at C-16(a) and C-20( 3), which appear at approximately 71-73 ppm and 77-79 ppm, respectively. Other characteristic patterns of substitution such as the presence of acetoxyl groups at C-2 and C-25 have also been observed. In this case, the acetylation of the hydroxyl at C-2 modifies both the C-l and C-3 values. Indeed, when a carbonyl is present, this latter change involves a major shift, namely from 213 to 205 ppm. In the case of a second double... 

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