van der Waals interactions in sterically crowded disilenes

Since double bonded systems constituted by higher main group elements are necessarily substituted by bulky substituents, in order to protect these structures kinetically, van der Waals interactions are dominant on the resulting bonding energies. This aspect, studied in detail with different density functional methods for a variety of selected disilenes, is compared with the results of MP2 calculations. Throughout a triple-ζ basis set was used, in order to allow flexible bonding. The bonding energies in the disilenes were characterized by dissociation into singlet silylenes. For the parent disilene the equilibrium was studied in addition with CCSD calculations utilizing Dunning type basis sets. The corrections for dispersion energies on the functionals BP86, PBE, and B3LYP were at times evaluated according to the Grimme approach, and the results compared with the refit double-hybrid functional BP97-D and MP2. They are sizable for the bulky systems. The only known case of reversible silylene-disilene in equilibrium is studied too. This silylene is known in a cis or trans conformation, with a stronger steric stronger congestion in the cis geometry. The dispersion corrections on stability favor to more extent the more encumbered geometries. Overall the calculations indicate that the traditional density functionals, like B3LYP or BP86 are not capable to describe properly the bulky molecular structures.