Methyl group, resonances

Nuclear magnetic resonance studies on spin labeled derivatives are not extremely useful due to the paramagnetism of the molecule. However, the NMR spectrum of spin labeled methylcobinamide confirms that the nitroxyl function is coordinated to the cobalt. It is possible in this compound to obtain good resolution of the methyl group resonance. 

For quantification of TMA and TMA N-oxide, the area under the curve of the TMA and TMA N-oxide resonances (both nine equivalent protons) and of the creatinine methyl group resonance (three equivalent protons) is calculated by peak integration. Correction for the number of contributing protons and dividing the TMA and... 

H NMR spectra have been recorded for several 6-substituted l-alkyl-277-thiopyranium fluoroborates 75 in mixed solvent systems (CD3CN, CDCI3) the J-methyl group resonates at ca. 52.8 and the 2-methylene protons are equivalent and show coupling to H-3 and H-4 <2001J(P 1 )2269>. 

A bridging methyl group may also show a large shift to high field. In the case of the ( 3-l,3-dimethylallyl)nickel methyl dimer (14), the structure of which has been confirmed by X-ray crystallography (26), the bridging methyl groups resonate at —20.3 ppm [ J(CH) = 118 Hz]. 

The resonance appearing near 20.0 ppm consists of two peaks, the one at 20.03 ppm is the methyl group resonance from methyl branches and the other at 20.14 ppm is associated with C-2 methylene of propyl branches. An integration of all methyl peaks indicates the presence of 24 methyl carbons per 1000 carbons, comparable to a value previously obtained of 19.8 (16). 

The cyclobutenium di-cation, 59, is formally derived from the (as yet unisolated) anti-aromatic cyclobutadiene by the removal of two n-electrons. This di-cation, like the cyclopropenyl cation, represents a member of the aromatic sub-group possessing just two 7t-electrons. The tetramethyl derivative of 59, 64 (see Table 14), has been studied 206), and its n.m.r. spectrum appears to possess the expected aromatic characteristics. The methyl groups resonate at t 6.32 in SbFs SO2 solutions and at t 5.89 in SbF5—SO2CIF. A reasonable model for this... 

The stereochemistry of sy/i- and a/fr/-13 diols in the polyene macrolide antibiotics and polyketides can be determined from the chemical shifts of the acetal carbons and methyl carbons of the corresponding acetonides. The acetonides of jy/i-l,3-diols [Scheme 3.36] occupy a low energy chair conformation displaying an acetal resonance at b 98.5 and methyl resonances at 6 30.0 (equatorial) and b 19.6 (axial). In both possible chair conformations of the a/i/i-l,3-dioI acetonides. there are severe 13-diaxiaI interactions that are relieved in the corresponding twist-boat. In the twist-boat conformation, the acetal carbon resonates at 6 100.6 whereas both methyl groups resonate at b 24.6 because the methyl groups are in nearly identical environments. The twist-boat conformation is more stable than the corresponding chair conformations by about 2 kcal/mole. 

Guarea thompsonii has been reported" to contain 6,12a-diacetoxymethyl angolensate (74). The 12a-acetate methyl group resonance appears at S 1.50 as a result of shielding by the furan ring. 

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