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.

[1] This phenomenon, a type of resonance, can stabilize the molecule or transition state.

[2] It also causes an elongation of the σ-bond by adding electron density to its antibonding orbital.

[1] 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.

[3][4] 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).