Actinyl cation-cation interactions in the gas phase: an accurate thermochemical study

Gas phase actinyl cation-cation interactions (CCIs) were studied by an accurate composite coupled cluster thermochemical approach for the first time. A number of CCI dimers were constructed from the monomers UO22+, UO2+, NpO22+, NpO2+, PuO2+, and AmO2+. All CCI dimers studied were calculated to be thermodynamically unstable, with dissociation energies ranging from -60 to -90 kcal mol(-1), but in many cases kinetic stability was indicated by calculated local minima with well depths as large as approximate to 15 kcal mol(-1). Most of the dimers studied involved a T-shaped geometry, although one side-on dimer, (UO2+)(2), was included since it was amenable to coupled cluster methods. In the T-shaped isomers the most stable dimers were calculated to arise when the oxo-group of an An(v) actinyl cation was oriented towards the metal center of an An(vi) actinyl cation. For both mixed-valent An(vi)/An(v) and mono-valent An(v) dimers, the stability as estimated from the depth of the calculated local minimum decreased in the donor series U(v) > Np(v) > Pu(v) > Am(v). These trends correlate well with experimental trends in condensed phase CCIs. A rationale for the bonding in CCIs was investigated by carrying out charge transfer analyses using the natural bond orbital (NBO) method. Augmenting the usual Lewis acid-base explanation, CCIs are the direct result of a competition between charge transfer stabilization, which can be as much as 0.11e or 30.7 kcal mol(-1) at equilibrium, and Coulombic repulsive destabilization.