Tetramethyleneethane (TME) diradical: Experiment and density functional theory reach an agreement

REKS-type (Filatov, M.; Shaik, S. Chem. Phys. Lett. 1999, 304, 429) density functional calculations were carried out for the lowest energy singlet and triplet states of tetramethyleneethane (TME) diradical. The calculations i ndicate that the ground state of TME in the gas phase is the singlet state, whereas the triplet state should be metastable at low temperatures. The tr iplet state metastability derives from the energetic preference for the tri plet state at the optimal triplet molecular geometry and from the extremely small (<0.05 cm(-1)) spin-orbit coupling between the triplet and ground si nglet states. REKS calculations predict that, in the vibrational spectra of the two states of TME, the modes corresponding to symmetric scissoring vib rations should have nearly the same frequencies of 357 cm(-1), in accord wi th the experimental observation of two modes of the same frequency of 335 c m(-1) in the vibrational structure of the photoelectron spectrum of the [TM E](-) anion.