Chemical shift quaternary carbons

For paraffins the chemical shift is calculated assuming that the carbon of interest is an alkyl-substituted methane molecule. First, a constant is used which nearly corresponds to that of methane (—1.87 ppm). Then chemical shift contributions are added for each carbon up to five carbons adjacent to the specified carbon. These contributions are described by constants a for the first bonded carbon, for the second carbon, which is two bonds away, / for the third, S for the fourth, and s for the fifth. If there are two a, or other carbons, the constant is multiplied by the appropriate number of adjacent carbons of the type. Values of a through s were obtained by Grant and Paul from a regression analysis of a series of paraffins. The results are shown in Table 6.2. For branched alkanes upheld shifts are observed for branched carbons and carbons next to branches, so corrective terms are required to account for chemical shifts of carbons associated with branching. Quaternary, tertiary, secondary, and primary carbon atoms are designated by 4°, 3°, 2°, and 1°, respectively. In the cor-... 

One disadvantage of the APT experiment is that it does not readily allow us to disdnguish between carbon signals with the same phases, i.e., between CH3 and CH carbons or between CH2 and quaternary carbons, although the chemical shifts may provide some discriminatory information. The signal strengths also provide some useful information, since CH3 carbons tend to be more intense than CH carbons, and the CH2 carbons are usually more intense than quaternary carbons due to the greater nuclear Overhauser enhancements on account of the attached protons. 

The GASPE spectrum of podophyllotoxin is shown. The signals at 8 56.0,108.6, and 152.0 each represent two carbons in identical magnetic environments, while the signal at 8 147.6 also represents two carbons that accidentally appear at the same chemical shift. Assign chemical shift values to various protonated and quaternary carbons in the structure. 

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 HMBC spectrum of vasicinone along with the H-NMR assignments are shown. Determine the H/ C long-range heteronuclear shift correlations based on the HMBC experiment, and explain how HMBC correlations are useful in chemical shift assignments of nonprotonated quaternary carbons. 

Like the HMBC, the COLOC experiment provides long-range hetero-nuclear chemical shift correlations. The COLOC spectrum, H-NMR, and C-NMR data of 7-hydroxyfrullanolide are presented here. Use the data to assign the quaternary carbons. 

The HMBC spectrum of vasicinone displays long-range heteronuclear shift correlations between the various H/ C nuclei. These correlations are very helpful to determine the C-NMR chemical shifts of quaternary carbons and allow the interlinking of the different substructures obtained. 

In cases of complexes bearing an exocyclic double bond directly coordinated to the metal center, the carbons of the double bond usually exhibit coupling with NMR-active metal centers and/or auxiliary ligands.6 14 18 19 The chemical shifts of the quaternary carbon atom vary from 66.976 to 82.2818 ppm, while the methylene group gives rise to signals at 29.16,14 41.91,6 or 51.3418 ppm in the 13C 1H NMR spectra. As one can see, the chemical shift variation is relatively broad and significantly affected by the nature of the metal center. 

The X-ray crystal structures of the interesting phosphonium azoniaspiroylides 7 and 8 have been published <1995TL7859, 1997T7557>. The structures were originally deduced from the 111 and 13C spectroscopy data, in particular the chemical shifts of the protons and carbons adjacent to the quaternary nitrogen atom, but were confirmed by the X-ray crystallographic structures. 

The noise-modulated broadband decoupled 13C NMR spectrum of (/)/.)-penicilla-mine is shown in Fig. 5. Two methyl carbons were found to resonate at chemical shifts of 27.7 and 30.2 ppm, in addition to a CH carbon at 64.6 ppm. Two quaternary... 

The p-carbon signals in heterocyclohexanes 52 (143) respond to the electronegativity of endocyclic substituents similarly to those of carbon atoms in p positions with respect to exocyclic substituents. The slopes of chemical shift vs. electronegativity plots for the p-methylene groups in 52 and 53 are negative (Table 10). For the quaternary P-carbon atom in 53, however, a slight positive slope is observed (164). 

C-21 methyl protons. The C-3 and C-16 melhine protons appeared at 8 3.82 and 4.99, respectively. The downfield chemical shift values of C-3 and C-6 methine protons were indicative of the presence of geminal oxygen functionahties. The C-6 resonated at 8 5.36 while the sp hybridized C-28 methylene protons resonated as two singlets, integrating for one proton each, at 8 5.56 and 6.06. A combination of H and C-NMR spectral data indicated to us that compound 11 has a steroidal skeleton. A detailed interpretation of broad C-NMR and DEPT spectra revealed the presence of three metlyl, ten methylene, eight methine and five quaternary carbon atoms in 11. The stereochemistry at various chiral centers was estabhshed with the aid of NOESY spectrum. 

Examination of the C-NMR spectra of roseadine (23) (Table XI) through comparison with vindoline (3) and leurosine (11) permitted the assignment of all carbons of the dihydroindole unit. The carbons of the indole nucleus were assigned by comparison with vinblastine (1), and the presence of three deshielded carbons, a methine carbon at 8 142.9 and two quaternary carbons at 8 133.2 and 169.2, were observed. The latter was assigned to the methoxycarbonyl carbon, which is shielded somewhat from its characteristic chemical shift of 8 174 1 ppm in the vinblastine series by attachment of an olefinic unit. The other two deshielded carbons at 8 133.2 and 142.9 could be assigned as C-18 and C-17, respec-... 

The spectrum of 6, 20 and 40 wt. % PEMA solutions at 34 C are shown in Figure 1. All of the resonances are easily discernible except for the backbone methylene at 40%. At low concentration the poljimer a-CH3, quaternary carbon, and backbone methylene carbon exhibit resolved or partially resolved chemical shifts due to the various stereochemical sequences since the pol3rmer was not stereoregular. A rough estimate indicates the polymer is essentially atactic. 

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