title: Effects of cyano substituents on cyclobutadiene and its isomers.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin, 53706-1322, USA.
authors.forename: Jessica L
authors.initials: JL
authors.lastname: Menke
authors.forename: Eric V
authors.initials: EV
authors.lastname: Patterson
authors.forename: Robert J
authors.initials: RJ
authors.lastname: McMahon