Non covalent intermolecular forces in systems containing hydrogenated molecules from scattering experiments

High-resolution molecular beam scattering experiments have been performed to obtain detailed information on range and strength of the intermolecular potential in various binary complexes involving hydrogenated molecules. In all cases these experiments have yielded quantitative results which for some systems (water-noble gases and water-hydrogen) are also intriguing, because different from the expectations anticipated by the phenomenology of weak van der Waals interactions, arising from the balancing of size repulsion with dispersion attraction possibly perturbed by induction. The key experimental observable is the “glory” quantum interference pattern, in the dependence of the integral cross section on the collision velocity, and its shift at higher velocity. Such finding has been attributed to a systematic and anomalous energy stabilization for the water complexes, arising from a significant role of charge transfers (CT) effects, emerging at intermediate and short intermolecular distances. For related ammonia and hydrogen sulphide complexes the stabilization by CT appears to play a less important role and in the case of methanol is found to be ineffective. In particular, the comparison of model predictions with the most relevant experimental findings suggests that in methanol-noble gas complexes the interaction is mainly due to van der Waals and induction components. To rationalize all experimental observations, very accurate theoretical calculations have been carried out in extreme detail to define the amount of electron displacement that accompanies the formation of gas phase binary adducts. The auto-ionization dynamics of water molecules induced by Ne*(3P2,0) collisions has been also studied because of its great experimental and theoretical relevance. Indeed, Ne* is an open shell atom, having an external electronic structure of sodium and the internal one of fluorine. The energy dependence of the total ionization cross section has been measured in the 0.05–0.15 eV range. The data have been analyzed by using an optical potential model, whose real part (van der Waals + induction + electrostatic components) is formulated applying a semi-empirical method, while the imaginary part, directly dependent on the CT coupling, has been characterized during the fitting procedure of the data by adjusting its pre-exponential factor. The good agreement between calculations and experiment confirms the attractive nature of the potential energy surface driving the Ne*–H2O dynamics and the selective role of CT.