Stability and structure of carbocations

Stabilizing effect from the donation of σ bond electrons:

Carbocations are inherently unstable because they are charged and electron deficient due to the empty orbital. A carbocation can be formed only if it has some extra stabilization. For any cation the most stable arrangement of electrons in orbitals results from making filled orbitals as low in energy as possible to give the most stable structure, leaving the highest-energy orbital empty.
Extra stabilization comes to the planar structure from weak donation of σ bond electrons into the empty p orbital of the cation. Three of these donations can occur at any one time in the t-butyl cation, and it doesn’t matter if the C-H bonds point up or down, but one C-H bond on each methyl group much be parallel to one lobe of the empty p orbital at any one time.

Click the buttons to load the structures of methyl and t-butyl cations, show the vacant p orbital and the stabilisation that is possible in the t-butyl structure.

D. H. Aue, Wiley Interdiscip. Rev. Comput. Mol. Sci., 2011, 1, 487–508.