Silicon effect and related p-effects

Similar to ethane, the barriers in methylamine and methanol appear to be dominated by hyperconjugative stabilization of the anti conformation. The barrier decreases (2.9- 2.0- l.lkcal/mol) in proportion to the number of antiperiplanar C-H/C-H interactions. This is counterintuitive because n interactions are 

Furthermore, the role of hyperconjugation is expected to increase even more in anionic species. However, the difference between staggered and echpsed conformation is only slightly higher in ethyl anion (2.3kcal at the CCSD(T)/6-311+G(d,p//B3LYP/6-311++G(3df,2p) level of theory) than in methyl amine and much lower than in ethane.  

One important caveat in the application of negative hyperconjugation involves the differences between free carbanions and compounds with carbon metal bonds. These two types of strong donors have important differences. In particular, charge localization due to the presence of a metal counter-ion (or formation of partially covalent C-M bond) can result in decreased/interrupted delocalization. 

---