Negative hyperconjugation
In organic chemistry, negative hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital. This phenomenon, a type of resonance, can stabilize the molecule or transition state. It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.
Negative hyperconjugation is rarely observed, though it can be most commonly observed when the σ*-orbital is located on certain C–F or C–O bonds.
In negative hyperconjugation, the electron density flows in the opposite direction than it does in the more common hyperconjugation.