Hyper-conjugation

In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma orbitals (e.g. C–H or C–C) with an adjacent empty (or partially filled) non-bonding orbital, antibonding σ or π orbital, to give an extended molecular orbital. Increased electron delocalization associated with hyperconjugation increases the stability of the system. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by direct hyperconjugation. However, extended versions of hyperconjugation (such as double hyperconjugation) can be important as well. The Baker–Nathan effect, sometimes used synonymously for hyperconjugation, is a specific application of it to certain chemical reactions or types of structures.

Hyperconjugation can be rationalizing a variety of other chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred radicals. Hyperconjugation is proposed by quantum mechanical modeling to be the correct explanation for the preference of the staggered conformation rather than the old textbook notion of steric hindrance.

...
Wikipedia