Knowledge Bank

Ab initio calculations (employing an extended basis of Gaussian orbitals) are reported for cyclobutadiene, isolated for the first time by Lin and $Krantz.1$ Unrestricted Hartree-Fock calculations for the triplet ground state yield a square equilibrium geometry with $Rcc=1.4344$\AA and a symmetric stretching force constant of $\sim $6.4md/\backslash AA(scaledfrom7.3md\backslash AA).Energeticsofvariousdistortionsfromequilibriumarediscussed,withparticularattentionpaidtothetrapezoidal($E_{u}$) deformation. The ground state singlet surface, based on an $optimized2$ 2-configuration wavefunction, has a rectangular equilibrium geometry ($Rcc=1.338$ \AA,1.565 \AA, and symmetric stretching force constants 9.3 and 4.2 md/\AA, respectively), with an appreciable barrier separating the two Kekul\ ‘{e} structures. At the triplet ($\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}A2g$) equilibrium geometry, the singlet energy is $\sim$ 8 Kcal/mole above the triplet. These and other features of the potential energy surfaces are shown to make plausible the assignment of the available IR data for the parent and deuterated cyclobutadiene to the triplet state. Frequency calculations based on ab initio and standard empirical force constants are also discussed.