Fernando Pirani

Generalized correlations in terms of polarizability for van der Waals interaction potential parameter calculations

General correlations between van der Waals interaction potential parameters and polarizabilities of the interacting neutral–neutral partners of any nature are presented and discussed. To ensure the full applicability of the correlations, an evaluation of the long‐range interaction constants is performed in terms of the Slater–Kirkwood approximation whose numerical coefficients, having the meaning of effective electron numbers, are estimated interpolating the values deduced by theoretical considerations. The values of the long‐range constants so obtained are compared satisfactorily with the available experimental ones. The correlations are tested successfully over practically all systems characterized experimentally. Their use to predict the parameters of unknown systems is suggested.

Range, strength and anisotropy of intermolecular forces in atom–molecule systems: an atom–bond pairwise additivity approach

Abstract A new method is proposed to calculate bond energies and equilibrium distances in atom–molecule van der Waals complexes which arises from a balancing between long-range attraction and asymptotic tail of the repulsion. The method, based on correlation formulas between the polarizability of the interacting partners and the main interaction parameters, is an extension of an approach originally developed for atom–atom cases. The basic idea exploits the concept of bond polarizability additivity to represent both the molecular repulsion, in terms of a size which is mainly ascribed to the molecular bonds nearest to the probe atom, and the molecular attraction as due to multi-dispersion centers delocalized on the molecular frame. The method, mainly tested on hydrocarbon–rare gas complexes, can be considered as the starting point for the study of systems of higher complexity.

Generalization to ion—neutral systems of the polarizability correlations for interaction potential parameters

Abstract Correlations in terms of particle polarizabilities are developed for both positive and negative ion—neutral interactions extending a general scheme previously proposed. For a proper assessment of the parameters, the ratio of the dispersion to the induction energy is evaluated in terms of the polarizability of the interacting partners. The correlations are successfully tested for a variety of ion—neutral systems. Their use to predict the potential parameters of unknown systems is suggested.

Molecular beam studies of weak interactions for open‐shell systems: The ground and lowest excited states of ArF, KrF, and XeF

Absolute integral cross sections for scattering of ground state fluorine atoms by argon, krypton, and xenon have been measured in the thermal velocity range. Information has been obtained on the long range interaction and using a technique for magnetic analysis of substates of F atoms, a characterization is given for the bonding in the ground and the two lowest excited states of these rare gas fluorides. The potentials are represented as a spherical part and an anisotropic component, which have been obtained in an adiabatic decoupling treatment, including also information from other scattering data. Nonadiabatic coupling matrix elements and other general features of these interactions are also presented.

Experimental benchmarks and phenomenology of interatomic forces: open-shell and electronic anisotropy effects

This article gives a perspective view of some representative experimental information available on interatomic forces. They play a role in gaseous properties, but modern quantitative information comes from spectroscopy and molecular beam scattering. This latter technique is emphasized here: recent experimental results and consideration of physical properties of interacting species is complementary to progress of modelling based on ab initio or other quantum chemical calculations. Interactions involved in closed-shell–closed-shell species are considered to be typical of the so-called ‘non-covalent’ forces, although additional effects of a ‘chemical’ nature are demonstrated to be non-negligible in some cases. The partition of the interaction into van der Waals (repulsion + dispersion) and possibly electrostatic and/or induction components is analysed. Interactions involving open-shell species offer a most interesting phenomenology, because electronic anisotropy often provides further strength to the bonds, which are usually weaker than ordinary chemical bonds. Again, the focus is on experimental information (especially on scattering of magnetically analysed open-shell atoms) and on the understanding that comes from the analysis of the ample phenomenology accumulated. Additional terms such as those of specific ‘covalent’ nature appear in the partition of the interaction, besides those already mentioned. The extension of this approach for describing molecular anisotropies is also outlined. Contents PAGE 1.  Introduction  1.1. Motivation and dedication 166  1.2. Scope and outline of the paper 167 2.  Isotropic interactions and van der Waals forces 168  2.1. 2 S +1S atom– 168  2.2. Combination rules and correlation formulas 170  2.3. 2 S +1S Ion– 172 3.  Anisotropic interactions and open-shell effects 175  3.1. General 175  3.2. 2 S +1P atom– 176  3.3. Charge transfer and bond stabilization 178  3.4. 2 S +1P ion – 180  3.5. Dications 181 4.  Final remarks 182  4.1.  Towards atom–molecule and molecule–molecule interactions 182  4.2.  Prospects for future work 184  4.3.  From van der Waals interactions to chemical bonds 185 Acknowledgements 185 Appendix A– Interatomic forces by molecular beam scattering 185 Appendix B–Basic contributions to the interatomic interactions and their dependence on physical properties f involved species 188 Appendix C–Electronic anisotropy and orbital alignment 192 References

Regularities in van der Waals forces: correlation between the potential parameters and polarizability

Abstract An analysis of regularities in the van der Waals forces has been performed for over fifty systems. Correlations are found between features of the interaction, namely the well depth and its location, and atomic properties such as the polarizability. These correlations can be extended to the spherical part of the interaction of anisotrophic systems, and can be used to estimate potential parameters for species that are difficult to study experimentally.

The N2Ar potential energy surface

Abstract The potential energy surface for the N 2 Ar system has been obtained assuming a spherical average interaction previously reported from this laboratory. The angular dependence has been assessed by a combined analysis of the integral and differential scattering cross sections and sonic spectroscope data. The potential energy surface is given via a parametric model. A similar potential energy surface for O 2 Ar has been obtained with the same procedure. This surface is an improvement of an earlier one, because it reproduces the differential total cross sections recently measured.

Scattering of magnetically analyzed F (2P) atoms and their interactions with He, Ne, H2 and CH4

Abstract An improved source of a beam of fluorine atoms has been characterized by magnetic analysis of sublevels for F( 2 P). Ground state and lower lying excited state potential energy curves for the interaction of fluorine atoms with He, Ne, H 2 and CH 4 have been obtained from measurements of integral scattering cross sections at thermal energies. The results for the rare gas systems are discussed in connection with previous work on Ar, Kr, and Xe fluorides; those for the H 2 and CH 4 systems provide information of fine structure effects on the long range pan of entrance channels for reactive potential energy surfaces.

Molecular beam studies of weak interactions for open‐shell systems: The ground and lowest excited states of rare gas oxides

Integral cross sections as a function of velocity for scattering of ground state oxygen atoms by the rare gases have been measured at thermal energy. Analysis of atomic sublevels by a Stern–Gerlach magnet allows a control of the relative contribution from different fine structure scattering channels. The results are analyzed using an adiabatic decoupling scheme to derive the interaction as a spherical part and an anisotropic component, from which information is obtained on the six lowest states of the rare gas oxides and on nonadiabatic coupling terms.

The N2–N2 system: An experimental potential energy surface and calculated rotovibrational levels of the molecular nitrogen dimer

An accurate new representation for the potential energy surface for the N2–N2 dimer has been obtained from the analysis of scattering experiments from our laboratory, and of available second virial coefficient data. A harmonic expansion functional form describes the salient geometries of the dimer and accounts for the relative contributions to the intermolecular interaction from components of different nature. The equilibrium geometry is a T conformation with well depth 13.3 meV (107.14 cm−1) and at a distance of 4.03 A. In order to assist in the analysis of spectra, we calculated the bound rotovibrational states for the (N2)2 system for J⩽6 by solving a secular problem over the exact Hamiltonian, considering the N2 monomers as rigid rotors, and where the Coriolis coupling is included.