Proton chemical shifts for

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. 

Figure 8. Parameters used to calculate the charge dependence of the proton chemical shift for methyl groups on carbonium ion centres.

Figure 12. Correlation of carbon-13 and proton chemical shifts for trigonal carbon atoms in a number of cations, anions, and neutral molecules.

Figure 14. Schematic presentation of correction of the proton chemical shifts for the triphenylmethyl cation and anion.

Fig. 83 Proton chemical shifts for organic compounds in dilute solutions of CC14 or CDC13. R and Ar stand for alkyl and aiyl groups.

Table 6 lists proton chemical shifts for a selection of monosubstituted furans including some with a metal or other less common heteroatom attached to the ring, but perhaps it should be noted that values are not closely comparable unless obtained in closely comparable conditions. Solvents can have a substantial effect upon chemical shifts (66CR(B)(263)1227>, especially benzene (72JA8854). Table 7 lists some ring interproton coupling constants and Table 8 some shift values for disubstituted furans. Polysubstituted furans are covered in the reviews and compilations mentioned above. 

Table 15. Proton Chemical Shifts for Benzylic Anions 202 205)...

Representative proton and carbon chemical shifts and coupling constants of benzyl fluorides and fluoromethyl naphthalenes are given in Scheme 3.78. Note that the proton chemical shifts for fluoromethyl naphthalenes are significantly larger than those of fluoromethyl benzenes. 

As was the case for the monofluoro series, halogens attached directly to the CF2 carbon deshield the fluorine nuclei (Tables 4.2 and 4.3). Iodine has the greatest deshielding effect on fluorine chemical shifts I > Br > Cl > F. In contrast, iodine has its usual shielding effect upon carbon chemical shifts. When considering proton chemical shifts for the fluoromethanes, again one must keep in mind the significant solvent effects observed for all di- and trihalomethanes. 

Figure 13 NMR and structural data supporting a hydrogen bonding network in cyclotides linking a conserved Glu residue in loop 1 (Glu7) with backbone amides of loop 3. (a) pH titration of selected residues in kalata Bl. Proton chemical shifts for the amide protons of Asnl5 and Thrl6 are plottted as a function of pH. These residues display a marked pH dependence as a result of the protonation of Glu7. (b) The hydrogen bond network is highlighted in the NMR structure of kalata Bl.

When magnetic nuclei other than protons are present, it should be recalled that some values of J might be as large as many proton chemical shifts. For example, in Fig. 13.3, 2 /HF = 48 Hz, accounting for the widely spaced 1 3 3 1 quartets due to the CH that is coupled to both the fluorine and the adjacent methyl group. Because 3Jhf = 21 Hz and 3JhH = 7 Hz, the CH3 resonance is a doublet of doublets. 

For complexes of the type [Me SiX4 ] it has been reported that 7( C— H) is an additive property of the a substituent, provided that the a substituent is not very electronegative (71). Data have also been reported for [SiH4 n(NMe2)n] (36). An empirical linear relationship has been shown to exist between 7( C— H) and the proton chemical shift. This has been used to determine nonlocal contributions to the proton chemical shift for [Me Ge], [Me4Sn], and [Me4Pb] (67). 

In the nmr spectrum of oxirane, the protons 3 to the oxygen display a small chemical shift compared to that of the j3 protons of larger cyclic ethers this can only be explained by the shielding effect of the abnormal electron density, which assumes a low electron density around the oxygen, as suggested earlier on theoretical grounds. The proton chemical shifts for various substituted oxiranes are given in a number of reviews and handbooks. ... 

Proton Chemical Shifts for /V-Amino Group in JV-Aminoazoles... 

From the H CRAMPS NMR spectra, therefore, it was possible to determine the NH proton chemical shift value for [Ala ]n-2 (a-helix 6 = 8.0) which is identical with that determined using BR-24 (2.0 kHz). Further, it was possible to determine the " NH proton chemical shift for [Ala ]n-1 (/3-sheet, 6 = 8.6) using the MREV-8 pulse sequence at 3.5 kHz. However, unfortunately, the NH proton chemical shift values for [Ala]n-2 and [Ala]n-t could not be determined because the line shapes of the " NH signals exhibit an asymmetric doublet pattern in this system also. Thus, it is found that determination of the true NH chemical shift of poly(L-alanines) can be achieved to measure fully N-labelled samples at higher MAS speed (3.5 kHz) and that these chemical shifts depend on conformation (a-helix 6 = 8.0 /3-sheet S = 8.6). This is the first determination of the true NH proton chemical shifts of poly(L-alanines) by H CRAMPS NMR. 

Figure 13.11 Proton chemical shifts for solutions showing how these occur at lower field strengths than for bulk water.

The proton chemical shifts for the fluorenyl carbanion (92, 129) are given in Table XII. Both FLi(DME)n in DME and FLi(THF)n in THF exist in solution as solvent-separated ion pairs (130, 131). This is consistent with the observation that the fluorenyl proton chemical shifts are nearly identical in these solvents. The 1 1 dimethoxyethane (DME) adduct in benzene probably has the same structural configuration as the 2 1 quinuclidine adduct (Figure 20) although a rapid equilibrium will... 

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