Methyl addition to acetylene and ethylene from a density functional approach

The addition of methyl radical to ethylene and acetylene has been studied by different density functionals and basis sets. The results show that only inclusion of some Hartree-Fock exchange in a particular gradient-corrected functional (B3LYP model) leads to reliable results for geometries, reaction and activation energies. Although quite large basis sets are apparently needed to obtain converged results, explicit consideration of the basis set superposition error significantly improves the results provided by medium size basis sets. The results are further interpreted in terms of the deformation energy of the fragments and of a partitioning of their interaction energy on the basis of the so-called natural bond orbitals.