David Cappelletti

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.

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

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.

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.

Range and strength of interatomic forces: dispersion and induction contributions to the bonds of dications and of ionic molecules

Abstract An application of previously introduced correlation formulas in terms of polarizabilities of interacting species permits an assessment of the relative role of dispersion and induction contributions to describe bond lengths and energies in some doubly charged ion-neutral systems (the dications of alkaline earth atoms and rare gases). An extension to ion-ion systems is also presented, on the basis of an empirical analysis of available data on alkali halides, in order to establish the role of both dispersion and induction interactions as compared to Coulomb attraction. An improved version of the Slater-Kirkwood formulation with an appropriate choice for the effective electron numbers is suggested to estimate the long range effective dispersion coefficients. The present approach provides a unifying framework for the characterization of bonds for systems which greatly vary in size and strength and recipes are suggested for interatomic potential energy curves. It is concluded that this approach is also relevant as a starting point for extensions to include anisotropies due both to orbital overlap for open shell partners and to mutual orientation when molecules are considered.

Scattering of aligned molecules. The potential energy surfaces for the Kr-O2 and Xe-O2 systems

Total integral cross sections for scattering of oxygen molecules on krypton and xenon atoms were measured in the thermal energy range, as a function of the collision energy and under a controlled alignment of the rotational angular momentum of the molecules [Aquilanti et al., Nature, 371, 399 (1994)]. Data obtained with a “hot” effusive molecular beam, which contains fast rotating and randomly oriented O2 molecules, mainly probe the spherical component of the potential energy surfaces. Experiments with supersonic seeded beams, where the oxygen molecules are cooled at the K=1 rotational level and selectively aligned [Aquilanti et al., Phys. Rev. Lett. 74, 2929 (1995)], probe the anisotropy of the potential energy surfaces. The analysis of the experimental results, based upon close-coupling exact quantum mechanical calculations of the cross sections, provides an accurate characterization of the interactions at intermediate and large intermolecular distances for the Kr-O2 and Xe-O2 systems. It is found that ...

Charge-Transfer Energy in the Water-Hydrogen Molecular Aggregate Revealed by Molecular-Beam Scattering Experiments, Charge Displacement Analysis, and ab Initio Calculations

Integral cross-section measurements for the system water-H(2) in molecular-beam scattering experiments are reported. Their analysis demonstrates that the average attractive component of the water-H(2) intermolecular potential in the well region is about 30% stronger than dispersion and induction forces would imply. An extensive and detailed theoretical analysis of the electron charge displacement accompanying the interaction, over several crucial sections of the potential energy surface (PES), shows that water-H(2) interaction is accompanied by charge transfer (CT) and that the observed stabilization energy correlates quantitatively with CT magnitude at all distances. Based on the experimentally determined potential and the calculated CT, a general theoretical model is devised which reproduces very accurately PES sections obtained at the CCSD(T) level with large basis sets. The energy stabilization associated with CT is calculated to be 2.5 eV per electron transferred. Thus, CT is shown to be a significant, strongly stereospecific component of the interaction, with water functioning as electron donor or acceptor in different orientations. The general relevance of these findings for waters chemistry is discussed.

An intermolecular potential for nitrogen from a multi-property analysis

New measurements are reported for the velocity dependence of the integral cross-section for N2–N2 scattering showing the glory oscillations. These data are analysed along with recent high precision second-virial coefficient data and traditional transport property measurements to yield an improved N2–N2 potential energy surface by adjusting certain parameters in the fit to this surface by van der Avoird et al. The new potential gives improved agreement with the integral cross-section, virial and transport data. Transport and relaxation cross-sections sensitive to anisotropy are very close to those obtained previously using the van der Avoird et al. surface. New comparisons with rotational relaxation and NMR relaxation data are reported.