Aromatic protons chemical shifts

Table 10. Aromatic Proton Chemical Shifts in Annelated Aromatics...

A tertiary base isolated from Thalictrum strictum was assigned a pavine structure based on the spectral data (27). Three methoxyl and one methylenedioxy functions were detected with the aid of mass spectroscopy. Structure 3 was proposed as the most probable representation for this new pavine alkaloid, which indeed is the first example of a pentasubstituted pavine base. However, when the reported aromatic proton chemical shifts (8 6.23, 6.36, and 6.54) were evaluated in the light of empirical rules about the H-NMR absorptions of pavine bases (Section V,B), and it seemed possible that the two upfield absorptions belong to H-4 and H-10 rather than to H-1 and H-10. Therefore, alternative structure 4 cannot presently be completely excluded from consideration. 

The spectral analyses of another new base from Thalictrum dioicum established its structure as a diphenolic isopavine, to which the trivial name (—)-thalidicine was assigned. The nonequivalency of the aromatic protons in the NMR spectrum led to the consideration of the two possible structures, 22 and 23, where 23 was apparently favored for biosynthetic considerations and also by analogy to the NMR aromatic proton chemical shifts of other known isopavine... 

The general absorption pattern of quaternary pavines strongly resembles that of the tertiary analogs with the exception of the expected downfield shifts for each of the protons. In particular, the bridgehead protons will move downfield by about 1-1.5 ppm (32,35). N,N-Dimethyl protons will be observed as a singlet between 8 3.3 and 3.7 (29,32,35). The set of empirical rules deduced for aromatic proton chemical shifts in a tertiary system has been shown to apply also to the quaternary system (29). A listing of aromatic proton chemical shifts of some quaternary pavine bases has been presented as a reference for future studies on similar compounds (29). 

All the aromaticity indices that have been calculated are summarized in Table XV. The first two indices are derived from the NMR spectra the A parameter is based on the dilution shift method 17,1 the B parameter is based on the uniformity of the methyl effects on aromatic proton chemical shifts.19 The next two indices, EAN and Julg s (J)... 

Shapiro, B. L., Mohrmann, L. E., NMR Spectral Data A Compilation of Aromatic Proton Chemical Shifts in Mono- and Di-Substituted Benzenes, J. Phys. Chem. Ref. Data 6 [1977] 919/91. 

Figure 3. Diagrams showing the chemical shift changes observed in the peptide-DNA hairpin samples relative to the isolated DNA hairpin sample. (A) (upper panel) Phosphorus-31 chemical shift changes of the 5 - P for each residue of the 1 1 peptide-hairpin sample and (lower panel) carbon-13 chemical shift changes of the aromatic C6/C8 (open bar) and the deoxyribose Cl (thatched bar) signals in the 1 1 peptide-hairpin sample. (B) Aromatic proton chemical shift changes for the 1 1 (upper panel) and the 2 1 (lower panel) peptide-hairpin samples. The changes for the H6 8 protons (open bar), the A H2 and the C H5 protons (thatched bar) are plotted against the hairpin sequence.

Fig. 12 H NMR spectrum of bfk-S conformers in toluene at room temperature. Only aromatic proton chemical shifts are shown...

Proton chemical shifts and spin coupling constants for ring CH of fully aromatic neutral azoles are recorded in Tables 3-6. Vicinal CH—CH coupling constants are small where they have been measured (in rather few cases) they are found to be 1-2 Hz. 

Proton nuclear magnetic resonance (NMR) chemical shifts of 1,2,3-thiadiazoles give another indication of the aromatic character of these compounds. Compiled in Table 4 are a number of examples of proton chemical shifts for ring-substituted 1,2,3-thiadiazoles. 

In an organic solid representative broadenings are 150 ppm for aromatic carbon chemical shift anisotropy and 25 kHz (full width at half-height) for a rather strong carbon-proton dipolar interaction. At a carbon Larmor frequency of 15 MHz, the shift anisotropy corresponds to 2.25 kHz. In high magnetic fields the forms of the respective Hamiltonians are... 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

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