Coupled-cluster calculations were performed for cyclobutane-1,3-diylidene dicarbenes 2 at the CCSD(T)//CCSD/ cc-pVDZ level of theory, in which the ground-state spin multiplicity and the structures of unique molecules were investigated in detail. The closed-shell singlet state 2(Sσπ ) with a bicyclo-[1.1.0]but-1(3)-ene (BBE) structure found to be the groundstate was much lower in energy than the corresponding singlet dicarbene structure 2(S∗∗ ), the quintet state 2(Q), and the triplet state 2(T), suggesting that the hitherto experimentally unknown BBE structure can be synthesized by the intramolecular dimerization of two carbene units. The energy gap between the BBE structures 2(Ssigma;π) and corresponding quintet states 2(Q) with electron-withdrawing substituents (X = F) at the C2 and C4 positions was found to be larger than that with electrondonating substituents (X = SiH3 ), i.e., ca. 100 kcal mol1 for2b (X = F) > ca. 85 kcal mol1 for 2a (X = H) > ca. 70 kcal mol-1 for 2c (X = SiH3 ). Two unique structures, 2(Tσ ) with a C1σC3 bond and 2(Tπ ) with a C1πC3 bond, were found to be the equilibrium structures for the triplet state of cyclobutane-1,3-diylidene dicarbenes 2.
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