To probe whether tetrahe-drane should be isolable the thermody-namics and kinetics of C_4H_4 singlet and triplet structures were studied exten- sively at the CCSD(T)/cc-pVTZ// CCSD(T)/cc-pVDZ,CCSD(T)/cc-pVDZ,CCSD(T)/cc-pVDZ//B3LYP/6-311G**,and B3LYP/6-311G** levels of theory.The reaction of cyclopropene with atomic carbon,which was previ-ously suggested to involve tetrahedrane as a reactive intermediate,was re-ex-amined experimentally with low-tem-perature matrix-isolation techniques.While experimental and theoretical re-sults exclude the intermediacy of tetra-hedrane in the above reaction,it is pre-dicted to be an isolable molecule.Among the many QH_4 species,we pay special attention to the electronic ef-fects on the ground state multiplicity of the respective carbenes.
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