Methine protons chemical shifts

TABLE B.2b Observed Methine Proton Chemical Shifts of Isopropyl Derivatives. 

Again, collections of H shifts exist which can be searched manually e.g., methine and methylenes. More usefully, proton chemical shifts have also been the subject of SCS or additivity rules. Prestsch and co-workers devised... 

Figure 3-9 Proton chemical-shift ranges for common structural units. The symbol CH represents methyl, methylene, or methine, and R represents a saturated alkyl group. The range for —CO2H and other strongly hydrogen-bonded protons is off scale to the left. The indicated ranges are for common examples actual ranges can be larger.

Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small—only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O—H protons of carboxylic acids are the least shielded. 

Differential shielding effects on the proton chemical shifts of ethyl (3R)-(/ -nitrophenyl)-3-(10)5-dihydroartemininoxy)propionate and its (35) diastereomer are correlated with crystal structure. In the two isomers, the phenyl substituent adopts markedly different conformations with respect to the nearby methine protons on the dihydropyran moiety thereby producing sizable differences in chemical shift <95AX(B)1063). 

This technique was used to distinguish between the two aromatic methine carbon atoms, C(4) and C(6), in sterigmatocystin (32) (Pachler et aL, 1976a). Olf-resonance proton decoupling was of no avail, owing to the small difference in the C(4) and C(6) proton chemical shifts (see Table I). The C(4) resonance appeared as a doublet of triplets J(CH) 165 Hz, J(CH) 7 Hz]. 

The C line assignments were made from the combination of DEPT and 2D C- H correlated spectroscopy despite the complexity of the conventional C spectrum. DEPT spectroscopy allowed the multiplicity of each resonance to be determined unambiguously. Hence, C assignments were made easily from the 2D C- H correlated spectrum even in situations where overlap of methine and methylene signals occurs in the proton spectrum. Furthermore, equivalent and nonequivalent methylenes were distinguished in the 2D C- H correlated spectrum, and this allowed assignments to be made despite spectral overlap of proton resonances. Proton chemical shifts were determined more accurately from the correlated... 

Table 7.43 Estimation of Chemical Shift for Protons of —CHj— and Methine...

FIGURE 13.13 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the methyl protons in 1,1-dichloroethane. When the magnetic moment is parallel to the external field if.o (green arrow), it adds to the external field and a smaller 3 0 is needed for resonance. When it is antiparallel to the external field, it subtracts from it and shields the methyl protons. 

The physical basis for peak splitting in 1,1-dichloroethane can be explained with the aid of Figure 13.13, which exanines how the chemical shift of the methyl protons is affected by the spin of the methine proton. There aie two magnetic environments for... 

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups147-151 (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the H NMR shifts in the presence of the optically active alcohols152. 

The GASPE spectrum of vasicinone is shown. The peak at 8 126.5 is a cluster of three peaks at 8 126.3 and 126.7 representing methine carbons. Similarly, the signal at 8 160 on the positive phase of the spectrum represents two close singlets at 8 160.4 and 160.5. Predict the chemical shift values of various protonated and quaternary carbons in the structure. 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

The SECSY spectrum of the coumarin presents cross-peaks for various coupled nuclei. These cross-peaks appear on diagonal lines that are parallel to one another. By reading the chemical shifts at such connected cross-peaks we arrive at the chemical shifts of the coupled nuclei. For instance, cross-peaks A and A exhibit connectivity between the vinylic C-4 and C-3 protons resonating at 8 7.8 and 6.2, respectively. The C-4 methine appears downfield due to its )3-disposition to the lactone carbonyl. Similarly, cross-peaks B and B show vicinal coupling between the C-5 and C-6 methine protons (8 7.6 and 7.1, respectively) of the aromatic moiety. The signals C and C represent the correlation between the oxygen-bearing C-11 (85.4) andC-12 (84.6) methine protons in the side chain. These interactions are presented around the structure. 

NMR spectroscopy is a convenient method for structural study of the equilibrium between the colored and the colorless form of spirobenzopyran. In the H-NMR spectra, the chemical shifts of gem methyl groups in 3 -position, (V-methyl group,2,11 and methine protons in 3- and 4-position are important to distinguish between the colored and colorless forms. 

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