The Barrier Height for Automerization Reaction of Cyclobutadiene - The Ab Initio Benchmark Study

Mirjana Eckert-Maksić 1Mario Vazdar 1Hans Lischka 2Zvonimir Maksić 1

1. Rudjer Boskovic Institute, Bijenicka cesta 54, Zagreb HR-10000, Croatia
2. Institute for Theoretical Chemistry, University of Vienna, Waehringerstrasse 17, Wien A-1090, Austria

Abstract

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 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.

[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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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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