engleski

The Izomerization Barrier in Cyanocyclobutadienes - An ab initio MR-AQCC Study

The problem of isomerization of mono, di- and tetracyano-substituted cyclobutadienes (CBDs) is addressed at the MR-AQCC(4, 4)/CASSCF/6-31G(d) level of theory. It was found that the energy barrier heights for the automerization reaction are lowered relative to that in CBD [1]. Consequently, it follows that bond-stretch isomerism does not take place in cyanocyclobutadienes. Instead, these compounds exhibit fluxional oscillations at room temperature between two equivalent structures, obtained in turn by the double bond flipping. The reason behind the decrease in the barrier heights is identified as a slightly enhanced resonance effect at the saddle points separating two equivalent bond-stretch isomers, compared to that in the equilibrium structures. It is also shown that the energy gap between the singlet ground state saddle point structure and the first triplet equilibrium geometry decreases upon multiple substitution by cyano groups.

cyclobutadiene ; cyano substitution ; mr-aqcc calculations

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