Isomerization energies of tetrahedranes to 1,3-cyclobutadienes: A challenge for theoretical methods

The gas phase (298.15 K, 1 atm) isomerization energies (ΔisomE(g)) of various tetra-substituted (hydro, chloro, bromo, methyl, ethynyl, cyano, tert-butyl, and tetrakis(trimethylsilyl)) tetrahedranes to their corresponding 1,3-cyclobutadienes were investigated with a broad range of model chemistries (Hartree–Fock, density functional, Moller–Plesset perturbation, composite, coupled cluster, and quadratic configuration interaction methods) and Pople-/Ahlrichs-/Dunning-type basis sets. Substantial model chemistry dependent ΔisomE(g) variability was found for all tetrahedrane/1,3-cyclobutadiene derivatives. Basis set influences on ΔisomE(g) variability were modest and less influential than the choice of model chemistry. Several density functionals previously found to provide excellent ΔisomE(g) prediction performance for a broad range of small and large organic compounds demonstrated poor capability when applied to the tetrahedrane/1,3-cyclobutadiene isomerizations.