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The two classes of carbenes are [[Diradical|singlet]] and [[diradical|triplet]] carbenes. Singlet carbenes are spin-paired. In the language of [[valence bond theory]], the molecule adopts an sp<sup>2</sup> [[Orbital hybridisation|hybrid structure]]. Triplet carbenes have two unpaired electrons. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and halides directly bonded to the divalent carbon.
Carbenes
For simple hydrocarbons, triplet carbenes usually have energies 8 [[kilocalorie|kcal]]/[[mole (unit)|mol]] (33 [[kilojoule|kJ]]/mol) lower than singlet carbenes (see also [[Hund's rule of maximum multiplicity]]), thus, in general, triplet is the more stable state (the [[ground state]]) and singlet is the [[excited state]] species. [[Substituent]]s that can donate [[electron pair]]s may stabilize the singlet state by delocalizing the pair into an empty p orbital. If the energy of the singlet state is sufficiently reduced it will actually become the ground state.
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