Methanes, methyl substituted shifts

Table II shows that the observed 29) shifts 8 (taken as positive downfield) show a degree of linear additivity in the methyl-substituted methanes, with a shortfall for neopentane. Table II shows also the division of the shielding into atom-plus-ligand diamagnetic and paramagnetic parts, relative to methane for which era is 295 ppm. The diamagnetic terms were calculated by Flygare s method 25). With each substitution of hydrogen by carbon, aa for the central carbon increases by 28 ppm but o-p increases by about 37 ppm, and the line moves 9 ppm downfield. Analogous relationships have been demonstrated for shielding in methyl-substituted NH3 and NH4+ 30). Table II shows that the shortfall at neopentane is in the paramagnetic term.

Although the observed shifts are reasonably additive for methyl-substitution in methane, this is not true for other substituents, and the top half of Fig. 5 contains an up-dated version of the well-known plot 47) of shifts in substituted methanes against the number of substituents. The lower half of Fig. 6 shows that the large increase in with successive substitution accounts for most of the upheld turn of the plots. The residual shortfall in upAL increases with degree of substitution and... 

Comparison of carbon-13 shifts of methyl phosphines and methyl substituted methanes... 

There is evidence of analogous hyperconjugative interaction in F-substituted compounds. In particular, the v(C-H) for methyl fluoride is 16 cm" red-shifted relative to methane (2976 vs. 2992 cm" ). Because hybridization effects imposed by Bent s rule should increase s-character in the C-H bonds and lead to the blue-shift, the experimental red-shift observation indicates the importance of np->-o C-H interactions. Additional fluorine substituents remove most of the red-shift (v(C-H) for CH F and CHFj are 2984 vs. 2990cm" respectively) suggesting the increasing importance of rehybridization (progressive s-character increase in the C-H bond(s)) and, possibly, the importance of Up o C-F interactions that partially neutralize the donor capacity of the fluorine lone pairs. Small blue-shifts are observed for the chloro- and bromoderivatives of methane. 

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