The Barrier Height for Automerization Reaction of Cyclobutadiene - The Ab Initio Benchmark Study
|Mirjana Eckert-Maksić 1, Mario Vazdar 1, Hans Lischka 2, Zvonimir Maksić 1|
1. Rudjer Boskovic Institute, Bijenicka cesta 54, Zagreb HR-10000, Croatia
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) . The adiabatic singlet-triplet energy gap between the 1B1g transition state (TS) and the first 3A2g triplet state is calculated to be 7.4 kcal/mol. A low interconversion barrier height and a small ΔE(3A2g, 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.
 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.
Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 2, by Mirjana Eckert-Maksić
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-05-15 11:47 Revised: 2009-06-07 00:44
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