engleski

Automerization of Cyclobutadiene and its Barrier Height-An Ab Initio Study

Cyclobutadiene has been fascinating experimental and theoretical chemists for many decades1. Inspite of many efforts, some of its features are still not quite understood. The barrier height of the bond flipping automerization process is among them. The experimental measurements yield for the height a large margin between 1.6 and 10.0 kcal/mol2, 3. In particular, the 13C NMR experiment indicates that the activation energy for automerization is no more than 2.5 kcal/mol for tri-t-butylcyclobutadiene3. This value is at odds with the most accurate MR-CCSD(T) calculations performed so far by Balkova and Bartlett4, giving the barrier of 6.6 kcal/mol. In order to resolve this disagreement, we have undertaken a series of multireference average quadratic coupled cluster calculations. The most sophisticated treatment involved the state averaging over four states (11Ag, 21Ag, 11B3g, and 13B3g) and the MR-AQCC computations based on the correlation consistent augmented aug'-cc-pVTZ functions, where aug' implies that the f-functions on carbon atoms and d-functions on hydrogen atoms were deleted. It appears that the barrier height, given by the total molecular energies, is 8.3 kcal/mol. The difference with the first triplet state at the squared transition state geometry is rather low, being 5.4 kcal/mol. The influence of the vibrational energy on the barrier will be discussed.

cyclobutadiene; bond flipping automerization; MR-AQCC computations

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