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 seldom 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 (from a π- or p-orbital to an empty σ*-orbital) than it does in the more common hyperconjugation (from a σ-orbital to an empty p-orbital). See also
Conjugated system

References

Physical organic chemistry