title: Interaction of O2 with CH4, CF4, and CCl4 by Molecular Beam Scattering Experiments and Theoretical Calculations.
abstract: Gas phase collisions of O2 by CH4, CF4, and CCl4 have been investigated with the molecular beam technique by measuring both the integral cross section value, Q, and its dependence on the collision velocity, v. The adopted experimental conditions have been appropriate to resolve the oscillating "glory" pattern, a quantum interference effect controlled by the features of the intermolecular interaction, for all the three case studies. The analysis of the Q(v) data, performed by adopting a suitable representation of the intermolecular potential function, provided the basic features of the anisotropic potential energy surfaces at intermediate and large separation distances and information on the relative role of the physically relevant types of contributions to the global interaction. The present work demonstrates that while O2-CH4 and O2-CF4 are basically bound through the balance between size (Pauli) repulsion and dispersion attraction, an appreaciable intermolecular bond stabilization by charge transfer is operative in O2-CCl4. Ab initio calculations of the strength of the interaction, coupled with detailed analysis of electronic charge displacement promoted by the formation of the dimer, fully rationalizes the experimental findings. This investigation indicates that the interactions of O2, when averaged over its relative orientations, are similar to that of a noble gas (Ng), specifically Ar. We also show that the binding energy in the basic configurations of the prototypical Ng-CF4,CCl4 systems [ Cappelletti , D. ; Chem. Eur. J. 2015 , 21 , 6234 - 6240 ] can be reconstructed by using the interactions in Ng-F and Ng-Cl systems, previously characterized by molecular beam scattering experiments of state-selected halogen atom beams. This information is fundamental to approach the modeling of the weak intermolecular halogen bond. On the basis of the electronic polarizability, this also confirms [ Aquilanti , V. ; Angew. Chem., Int. Ed. 2005 , 44 , 2356 - 2360 ] that O2 can be taken as a proper reference partner for simulating the behavior of some basic noncovalent components of the interactions involving water. Present results are of fundamental relevance to build up the force field controlling the hydrophobic behavior of prototypical apolar CX4 (X = H, F, Cl) molecules.
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: David
authors.initials: D
authors.lastname: Cappelletti
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Aquilanti
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Alessio
authors.initials: A
authors.lastname: Bartocci
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.|CNR-Istituto di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Nunzi
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.|CNR-Istituto di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Tarantelli
authors.affiliation: CNR-Istituto di Scienze e Tecnologie Molecolari, Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Belpassi
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia , Via Elce di Sotto 8, 06123 Perugia, Italy.
authors.forename: Fernando
authors.initials: F
authors.lastname: Pirani