A carbocation may be stabilized by resonance by a carbon-carbon double bond or by the lone pair of a heteroatom adjacent to the ionized carbon. In order for a carbocation to be resonance-stabilized, the molecular orbital of the donating group must have the proper symmetry, orientation, and energy level to interact with the empty 2p orbital of the carbocation. Such cations as "allyl" cation CH2=CH–CH2+ and "benzyl" cation C6H5–CH2+ are more stable than most other carbocations due to donation of electron density from π systems to the cationic center. Furthermore, carbocations present in aromatic molecules are especially stabilized, largely due to the delocalized π electrons characteristic of aromatic rings. Molecules that can form allyl or benzyl carbocations are especially reactive. These carbocations where the C+ is adjacent to another carbon atom that has a double or triple bond have extra stability because of the overlap of the empty p orbital of the carbocation with the p orbitals of the π bond. This overlap of the orbitals allows the positive charge to be dispersed and electron density from the π system to be shared with the electron-deficient center, resulting in stabilization. The doubly- and triply-benzylic carbocations, diphenylcarbenium and triphenylcarbenium (trityl) cation, are particularly stable. For the same reasons, the partial p character of strained C–C bonds in cyclopropyl groups also allows for donation of electron density and stabilizes the "cyclopropylmethyl" (cyclopropylcarbinyl) cation.