Methyl carbon shieldings

The neighbor anisotropy term <7 of eq. (3.2) plays an important role in proton shielding, permitting, for example, a distinct differentiation between aromatic and olefinic protons due to the ring current effect. However, this contribution is small in 13C NMR (<2 ppm). A comparison of the methyl carbon shieldings in methylcyclohexene and toluene shows that the ring current effect often cannot be clearly separated from other shielding contributions ... 

Table 4.7. (b) Increments A, for Prediction of Axial and Equatorial Methyl Carbon Shieldings [87]. 

Methyl carbon shieldings also offer conformational insight in cyclohexanones. In cis-3,5-dimethyl- and 3,3,5-trimethyl-cyclohexanones the equatorial 3-methyl carbon absorbs at ca. 22.3 p.p.m. whereas an equatorial 2-methyl carbon experiences an upfield shift to 14.6 p.p.m. ascribed to a y-interaction between methyl and oxygen. From related compounds these shifts have been estimated as 19.3 and 17.4 p.p.m. respectively. In the former case the upfield shift of 3 p,p.m. is less than the corresponding cyclohexane value on account of the smaller number of syn-axial interactions. In 3-methylcyclohexanone the methyl shielding indicates about 10% axial methyl conformer, corresponding to —AG of ca. 1.3 kcal mol . In cis-2,5-dimethylcyclo-hexanone (67) the observed methyl shieldings of 15.3 and 19.8 p.p.m. are in accord with an equilibrium which contains ca. 80% (67a). 

The first series involves cc-methyl-/Miydroxycarbonyl compounds where C-3 (carbinol) and the methyl carbon arc always shielded in the syn- compared to the aw/i-diastcrcomcr the chemical shift difference A d being 1.1 to 5 ppm. The results are less clearcut for C-2 (methine). The results can be explained after an inspection of the conformations for the diastereomers. In the syw-form the a-methyl group is partly in the axial position leading to a smaller chemical shift due to a higher number of gauche interactions. This is not the case for the w//-form because anti-1 (p 330) is the dominating conformation. 

The H-NMR spectrum of the perhydro-3a,6a,9a-triazaphenalene 353 (R = H) shows absorption for the methine proton at 5 2.31, indicative of the trans-trans-trans conformation 354.299,300 The shielding of the methine proton is shown by comparison300 with the absorption (5 3.67) of the methine proton in 355.30 The, 3C-NMR spectrum of 353 (R = Me) at ambient temperatures shows a chemical shift of the methyl carbon at 5 — 6.6, consistent with the predominance of the trans-trans-trans conformation. It has been estimated that the equilibrium contains 14% of the ds-cis-trans conformation 356.302... 

Due to reduction or lack of hydrogen bonding, carbonyl carbon nuclei of amides [313-315], anhydrides [316], esters [310-312], and halides [317] display smaller shift values relative to the parent acids (Tables 4.34 and 4.35). Methyl esterification shieldings are about — 6+1 ppm for mono- and — 1.5 + 1 ppm for diesters, reflecting weaker hydrogen bonding in dicarboxylic acids [316] Taking the methoxy carbon as a /i effect... 

Remember that McaSI, tetramethyl silane, in Garb on NMR resonates at 0.0 p.p.m, This is another cOn equence of siltaanf being more electropositive than carbon—the methyl carbons of TMS are more shielded and So resonate at a smalter chemical shift than other saturated carbons. 

First, the carbon atom is much closer to the substituent dian the proton. In the compounds in Table 15.2, the methyl carbon atom is directly bonded to the substituent, while the protons are separated from it by the carbon atom of the methyl group. If the functional group is based on a large electron-withdrawing atom like sulfur, the protons will experience a simple inductive electron withdrawal and have a proportional downfield shift. The carbon atom is close enough to the sulfur atom to be shielded as well by the lone-pair electrons in the large 3sp3 orbitals. The proton shift... 

V. Allenes. Allenes form a unique class of compounds because of the extremely low field shift of the central allenic carbon C2 (200 to 220 ppm). Table 5 presents representative data for a number of substituted allenes. For a given alkyl substituent, there is a linear relationship between the number of substituents and the chemical shift of the central carbon. The shielding is regarded as an additive property, a methyl group shields that carbon by 3.3 ppm, an ethyl group by 4.8 ppm and a. sec-alkyl group by 7 ppm. Carbons Cl and C3 are shielded by some 30 ppm relative to corresponding ethylene carbons but otherwise display similar substituent effects. Strain in cyclic allenes appears to have little effect. 

Fig. 11.6. (a) C CP/MAS NMR spectra of i-PP with a-form and (b) peak simulations based on the calculated shielding for NMR chemical shift in methylene and methyl carbons regions. 

These chemical shifts, however, may be changed significantly if further substituents are close-by. Especially, substituents on neighbouring carbons atoms of the coumarin system may strongly shield the methyl carbons. Here again, methyl carbon at 3 or 8 position are particularly sensitive. On the other hand, substituents in peri position deshield a methyl carbon due to a 8-syn effect (60, 85), an influence which is exceptionally strong (4-5 ppm). Substituents further away do not have effects larger that a few tenths of a ppm. 

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