Interaction of alkene radical cations with solvent molecules as described with density functional theory

Structures and energetics of several distonic radical cations, in particular the adducts between ethylene radical cation and water, ammonia, and hydrogen fluoride, are investigated using various density functional and post Hartree-Fock methods. Both the structure and the energetics of the [CH2CH2 . NH3](.+) and [CH2CH2 . HF](.+) adducts with pronounced covalent and electrostatic bonding, respectively, are well described using the gradient-corrected BLYP functional. The intermediate bonding situation of [CH2CH2 . OH2](.+) in particular the structure, is not well described with BLYP, whereas the binding energy is in surprisingly close agreement with post Hartree-Fock methods. The reason for this behavior is analyzed in terms of the different contributions to the total energy, and it is shown that the deficiency is remedied when the nonlocal hybrid functional BHLYP is used. Finally, it is found that the performance of BLYP improves on inclusion of additional water molecules. Thus the weakness of BLYP to describe the structure of the [CH2CH2 . OH2](.+) radical cation is a specific exception rather than typical for the large class of distonic radical cations. One can therefore expect BLYP to provide a realistic description of alkene radical cations in aqueous solution.