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... [Pg.212]

Comparison of carbon-13 shifts of methyl phosphines and methyl substituted methanes... [Pg.10]

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. [Pg.323]

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