Electronic structures of aromatic hydrocarbons with high spin multiplicities in the electronic ground state
Abstract It has been established experimentally that there are alternant hydrocarbons (AH) with high spin multiplicities in the electronpic ground state, the highest multiplicity known up to date being septet. Such electronic structures are rationalized by utilizing the degeneracy of non-bonding pi molecular orbitals (NBMO) predictable from the theory of Longuet-Higgins in conjunction with the non-bonding orbitals of divalent carbon atoms which are perpendicular to p-orbitals and are nearly degenerate with NBMO. The degeneracy of NBMO is determined by the topological nature of molecules which permits the ground state with high spin multiplicities according to Hund's rule in spite of their low molecular symmetries. The nature of AH having high spin multiplicities in the ground state reported so far is understood in this manner. It was found, however, that for some AH the high spin multiplet states are unexpectedly not the ground states although they lie very close to the singlet ground state. The difference between these two types of AH with high spin multiplicities is associated with the fact that the spin distribution of NBMO is alternant in the former AH, but not in the latter. To the latter AH the weak interaction model was found to apply in which a molecule is divided into two units weakly interacting with each other.