Experimental and theoretical evidence of charge transfer in weakly bound complexes of water.

We report the definitive assessment of a measurable (half a kJ/mol) charge-transfer (CT) component in the interaction of water with the noble gases. The CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor/acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions. By using an original method of partial integration of the change in electron density occurring upon the formation of Ng-water complexes (computed with accurate ab initio methods and basis sets), we map the local charge displacement along the whole intermolecular region and are thus able to appraise CT effects free of the inherent ambiguity of charge decomposition models. With this, it was possible to prove that a small CT (of the order of a couple of millielectrons at most) takes place from Ng to water. Most importantly, this CT correlates quantitatively with some unexpected "glory" quantum interference effects observed in high-resolution molecular-beam Ng-water scattering experiments that indicate a stronger bond than a pure van der Waals force. The energy stabilization associated with the observed CT does not exceed half a kJ mol(-1). The Ng-water CT is found to be strongly stereoselective and is mediated by a highly asymmetric, donor-acceptor concerted role of the two hydrogen atoms, which can in fact dictate the equilibrium geometry of the adducts. This finding may be of general relevance in explaining the known peculiar orientational features of water's interactions.