[CKabes]

Why do SN1 reactions have energy barriers higher than their subsequent carbocation intermediates?

Unlike an SN2 reaction, the transition state in SN1 is about halfway between starting material and carbocation. You might expect this to have energy that's also about halfway between. Why is this less stable than the pure carbocation?

[Babcock_Hall]

Of course it is true by definition that the transition state is higher in energy, but that doesn't really explore the question of why.  I wonder if the question has to do with the geometry of the transition state.

[nakhimov]

It may be worth consideration of what types of molecules are most prone to S_N1 reactions! 3° > 2° >> 1° cations; do you suppose something might be disrupting that stabilization effect?

Hint: consider the orbitals of the molecules undergoing the reaction.