Hyperconjugation

Hyperconjugation, sometimes called 'no-bond resonance', is a permanent stabilising interaction in which electron density from a filled carbon-hydrogen (or occasionally carbon-carbon) sigma bond delocalises into an adjacent, empty or partly filled orbital -- either the empty p orbital of a neighbouring positively charged carbon (a carbocation) or the pi system of a neighbouring carbon-carbon double bond. For this delocalisation to occur, the sigma C-H bond must be on a carbon directly attached ('alpha') to the unsaturated centre, and it must be roughly aligned with the empty orbital or pi system it is donating into.

The interaction is conventionally drawn as a set of 'no-bond' resonance structures: one contributing structure shows the C-H bond intact as usual, while a second contributing structure shows that same bond broken, with its electron pair moved into the adjacent pi system or empty p orbital and the hydrogen drawn with (formally) no bond to carbon at all -- hence the name 'no-bond resonance'. The more such alpha C-H (or C-C) bonds a carbon skeleton has available, the more no-bond resonance structures can be drawn, and the greater the resulting stabilisation. …