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J Phys Chem A. 2010 Jun 10;114(22):6431-7. doi: 10.1021/jp101963p.

Effects of cyano substituents on cyclobutadiene and its isomers.

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Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin, 53706-1322, USA.


The effects of cyano substitution on cyclobutadiene are explored using density functional, coupled-cluster, CASSCF, and CASPT2 calculations. An isodesmic reaction is employed to gauge the relative stabilization (DeltaH(rxn) degrees) of cyclobutadienes with varying numbers of cyano groups. Although density functional theory predicts a relatively large stabilization for the addition of four cyano substituents to cyclobutadiene (18.5 kcal/mol), coupled-cluster theory predicts a smaller stabilization (9.3 kcal/mol). The effect of the number of cyano groups on the singlet-triplet gaps is also investigated. NBO calculations lend insight into the structural trends of the triplets, and the comparison of coupled-cluster and CASSCF calculations sheds light on the multireference electronic character in these systems. The effect of tetracyano substitution on tetrahedrane and other C(4)H(4) isomers is also explored.


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