Tetrahedrane--dossier of an unknown.

To probe whether tetrahedrane should be isolable the thermodynamics and kinetics of C4H4 singlet and triplet structures were studied extensively at the CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVDZ, CCSD(T)/cc-pVDZ, CCSD(T)/cc-pVDZ//B3 LYP/6-311G**, and B3 LYP/6-311G** levels of theory. The reaction of cyclopropene with atomic carbon, which was previously suggested to involve tetrahedrane as a reactive intermediate, was re-examined experimentally with low-temperature matrix-isolation techniques. While experimental and theoretical results exclude the intermediacy of tetrahedrane in the above reaction, it is predicted to be an isolable molecule. Among the many C4H4 species, we pay special attention to the electronic effects on the ground state multiplicity of the respective carbenes.