Modelling Charge Transfer in Weak Chemical Bonds: Insights from the Chemistry of Helium

We studied the nature of the interaction of the weakly bound Be-He adduct by means of an integrated theoretical approach based on high-level quantum chemical calculations for the characterization of the potential energy surfaces and charge displaced upon adduct formation, together with the development of a semi-empirical analytical formulation of the interaction potential. Our results show that Be is able to form a stable adduct with He when the Be(D-1) (1s(2)2s(2)1s(2)2s(0)2p(2)) excited state is involved, with a binding energy of as much as 10.2kcal/mol, an astonishingly large value for He in neutral systems. The analysis of the leading interaction components in the Be*-He adduct proves the relevance of the charge transfer to the overall stability, which contributes to decreasing the intermolecular distance, thus strengthening the induction-energy component.