Chemical shifts substituted methanes

The chemical shift of methane is at -2.3 ppm relative to TMS and from this base value it is possible to calculate shifts for other alkanes (and even substituted alkanes provided that the appropriate base shifts for the substituted methane are available) using the increment data given in Table 1 together with the equation... 

Among the earliest research on the transmission of substituent effects was the pioneering work of Lauterbur (58) and of Spiesecke and Schneider (59). For a wide series of substituted methanes and ethanes they found an approximate correlation of >3C chemical shifts with the electronegativity (E) of the substituents (X). So they concluded that inductive effects through o-bonds play an important role in determining the chemical shifts of a- and -positioned carbon atoms, and that this effect reflects the substituent s electron-withdrawing or -releasing ability. 

Upfield shifts of multiply heterosubstituted carbon atoms are well known in the 13C NMR literature. Some representative examples are shown in Figure 9 and Table 16, where the actual l3C chemical shifts and individual a-SCSs, respectively, for each newly introduced substituent in methane derivatives are compiled. The ICSs can be calculated by subtracting any a-SCS from its respective predecessor within a given row in Table 16. The general trend is toward a distinct decrease of a-SCS with progressive substitution that is, all the ICSs are negative. 

Figure 9. Chemical shifts 8 in some substituted methanes CH<.,X .

Fig. 4.2. 13C chemical shifts of substituted methanes versus the degree of substitution. 

Cavell also reported alkyl derivatives. Tetrabenzyl zirconium was found to react separately with one molar equivalent of two parent methanes to afford complexes 135 and 136 in excellent yields.66 The 13C NMR chemical shifts of the methandiide centres in 135 (Ad = adamantyl) and 136 of 82.8 and 84.4 ppm are clearly different from the dichloride congeners and reflect the substitution of the chlorides by alkyls. The corresponding hafnium dialkyl 137 was also prepared from 133 and neopentyl lithium and the methanide resonance for this complex appeared at 71.6 ppm in its 13C NMR spectrum.63 Noteably, these alkyls displayed far greater stability than their homoleptic tetraalkyl cousins. 

Table 20-3 H NMR Chemical Shifts of Substituted Methanes CHjX...

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-... 

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