title: Catching the role of anisotropic electronic distribution and charge transfer in halogen bonded complexes of noble gases.
abstract: The systems studied in this work are gas-phase weakly bound adducts of the noble-gas (Ng) atoms with CCl4 and CF4. Their investigation was motivated by the widespread current interest for the intermolecular halogen bonding (XB), a structural motif recognized to play a role in fields ranging from elementary processes to biochemistry. The simulation of the static and dynamic behaviors of complex systems featuring XB requires the formulation of reliable and accurate model potentials, whose development relies on the detailed characterization of strength and nature of the interactions occurring in simple exemplary halogenated systems. We thus selected the prototypical Ng-CCl4 and Ng-CF4 and performed high-resolution molecular beam scattering experiments to measure the absolute scale of their intermolecular potentials, with high sensitivity. In general, we expected to probe typical van der Waals interactions, consisting of a combination of size (exchange) repulsion with dispersion/induction attraction. For the He/Ne-CF4, the analysis of the glory quantum interference pattern, observable in the velocity dependence of the integral cross section, confirmed indeed this expectation. On the other hand, for the He/Ne/Ar-CCl4, the scattering data unravelled much deeper potential wells, particularly for certain configurations of the interacting partners. The experimental data can be properly reproduced only including a shifting of the repulsive wall at shorter distances, accompanied by an increased role of the dispersion attraction, and an additional short-range stabilization component. To put these findings on a firmer ground, we performed, for selected geometries of the interacting complexes, accurate theoretical calculations aimed to evaluate the intermolecular interaction and the effects of the complex formation on the electron charge density of the constituting moieties. It was thus ascertained that the adjustments of the potential suggested by the analysis of the experiments actually reflect two chemically meaningful contributions, namely, a stabilizing interaction arising from the anisotropy of the charge distribution around the Cl atom in CCl4 and a stereospecific electron transfer that occurs at the intermolecular distances mainly probed by the experiments. Our model calculations suggest that the largest effect is for the vertex geometry of CCl4 while other geometries appear to play a minor to negligible role.
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia 06123, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Bartocci
authors.affiliation: Istituto di Scienze e Tecnologie Molecolari del CNR, Perugia 06123, Italy.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Belpassi
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia 06123, Italy.
authors.forename: David
authors.initials: D
authors.lastname: Cappelletti
authors.affiliation: Dipartimento di Ingegneria Civile ed Ambientale, Università degli Studi di Perugia, 06125 Perugia, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Falcinelli
authors.affiliation: Dipartimento per la Innovazione nei sistemi Biologici, Agroalimentari e Forestali (DIBAF), Università della Tuscia, 01100 Viterbo, Italy.
authors.forename: Felice
authors.initials: F
authors.lastname: Grandinetti
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia 06123, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Tarantelli
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Perugia 06123, Italy.
authors.forename: Fernando
authors.initials: F
authors.lastname: Pirani