Published on the Web 07/09/2012 www.pubs.acs.org/accounts Vol. 45, No. 9 ’ 2012 ’ 1571–1580 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 1571 10.1021/ar3000635 & 2012 American Chemical Society Revealing Charge-Transfer Effects in Gas-Phase Water Chemistry DAVID CAPPELLETTI,* , † ENRICO RONCA, ‡ LEONARDO BELPASSI, § FRANCESCO TARANTELLI,* , ‡,§ AND FERNANDO PIRANI ‡ † Dipartimento di Ingegneria Civile e Ambientale, Universit a di Perugia, Via G. Duranti 93, 06125 Perugia, Italy, ‡ Dipartimento di Chimica, Universit  a di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy, and § CNR;Istituto di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, 06123 Perugia, Italy RECEIVED ON FEBRUARY 29, 2012 CONSPECTUS A n understanding of the interactions involving water and other small hydrogenated molecules such as H 2 S and NH 3 at the molecular level is an important and elusive scientific goal with potential implications for fields ranging from biochemistry to astrochemistry. One longstanding question about water's intermolecular interactions, and notably hydrogen bonding, is the extent and importance of charge transfer (CT) , which can have important implications for the development of reliable model potentials for water chemistry, among other applications. The weakly bound adducts, commonly regarded as pure van der Waals systems, formed by H 2 O, H 2 S, and NH 3 with noble gases or simple molecules such as H 2 , provide an interesting case study for these interactions. Their binding energies are approximately 1 or 2 kJ/mol at most, and CT effects in these systems are thought to be negligible. Our laboratory has performed high-resolution molecular-beam scattering experiments that probe the (absolute scale) intermolecular potential of various types of these gas-phase binary complexes with extreme sensitivity. These experiments have yielded surprising and intriguing quantitative results. The key experimental measurable is the “glory” quantum interference shift that shows a systematic, anomalous energy stabilization for the water complexes and clearly points to a significant role for CT effects. To investigate these findings, we have performed very accurate theoretical calculations and devised a simple approach to study the electron displacement that accompanies gas-phase binary intermolecular interactions in extreme detail. These calculations are based on a partial progressive integration of the electron density changes. The results unambiguously show that water's intermolecular interactions are not typical van der Waals complexes. Instead, these interactions possess a definite, strongly stereospecific CT component, even when very weak, where a water molecule may act as electron donor or acceptor depending on its orientation. CT is mediated by an asymmetric role played by the two hydrogen atoms, which causes strong orientation effects. The careful comparison of these calculations with the experimental results shows that the stabilization energy associated to CT is approximately 2À3 eV per electron transferred and may make up for a large portion of the total interaction energy. A simple electron delocalization model helps to validate and explain these findings. 1. Introduction Water plays an essential role for life to thrive on our planet, as it constitutes the environment in which most biological processes take place. So, understanding at the molecular level the intermolecular interactions involving water is one of the most important goals of science, but one which is still lamentably far from being accomplished. 1 It is certain that many biochemical processes depend on the special attributes of the water molecule and, in particular, on its ability to form directional, weak bonds which assist the