The four-membered cyclic C4H2resulting from electrocyclization of butadiyne has been found 75.2 kcal/mol above the latter species. In its ground state, it is a singlet non-planar biscarbene of C2symmetry. The electronic structures of the singlet and triplet have been studied using ELF analysis. The opening into butadiyne needs ca. 20 kcal/mol activation and follows a pathway which does not preserve the C2symmetry. Breaking the carbene-carbene bond results in a bicyclo-biscarbene, a degenerate isomerization which offers a reaction path for the scrambling of C2C3atoms in butadiyne. Breaking the HC-C bond results in a sigmatropic shift yielding cyclopropylidene. © 2004 Elsevier B.V. All rights reserved.
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