Singlet-Triplet Energy Separations in Some Hydrocarbon Diradicals

Abstract

A diradical is literally a molecule with two unpaired electrons. We prefer the somewhat broader definition of a diradical as a molecule in which two electrons occupy a degenerate or nearly degenerate pair of orbitals. Group theory predicts a symmetry enforced degeneracy for a pair of orbitals, occupied by a total of two electrons, in the most symmetrical molecular geometry of methylene (CH2), cydobutadiene (CBD), and trimethylenemethane (TMM) (see Figure 1). Thus, we consider all three of these molecules to be diradicals. CH2 is the simplest molecule containing a divalent carbon atom. Although a very large number of other carbenes have been �!udied (1-3), in this review we confine our discussion to the parent hydro­ carbon. CBD yields to cyclopropenyl anion (4, 5) the position of being the smallest fully conjugated cyclic hydrocarbon containing 4n pi elec­ trons; but the former annulene has received more attention. Therefore, our discussion of molecules that are diradicals by virtue of being antiaromatic annulenes focuses on CBD. We have also chosen to discuss another four pi electron system, TMM, as representative of the class of diradicals that are fully conjugated hydrocarbons but for which no classical Kekule structures can be written (6). TMM is the simplest such molecule and, again, the best studied.