The double bond flipping interconversion reaction between the two equivalent ground state structures of cyclobutadiene (CBD) is examined by the multireference average quadratic-coupled cluster (MR-AQCC) approach, which is capable of optimizing the structural parameters of the ground, transition and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal/mol, which places theoretical benchmark result in the middle of the range of the measured values (1.6 - 10 kcal/mol) [1]. The adiabatic singlet-triplet energy gap between the ¹B_1g transition state (TS) and the first ³A_2g triplet state is calculated to be 7.4 kcal/mol. A low interconversion barrier height and a small ΔE(³A_2g, TS) gap seem to bear some relevance on the highly pronounced reactivity of CBD together with a large angular strain. It is shown that tetracyano substitution of the CBD lowers the barrier of the automerization reaction, which will be briefly discussed.

[1] a) D. W. Whitman, B. K. Carpenter, J. Am. Chem. Soc. 1982, 104, 6473. b) B. K. Carpenter, J. Am. Chem. Soc. 1983, 105, 1700.

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1. Design of Neutral Organic Superacids and Superbases Ex Machina