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

Electronic and dynamical properties of non-covalent diatomic aggregates formed by He with neutral and ionic Li and Be

CONTEXT AND RESULTS

This work aims to study the influence of the absence and presence of permanent charges on the electronic and dynamical properties of the non-covalent bound diatomic systems involving He and Li, Be as neutral and ionic partners. The charge displacement results suggest that in the formation of HeLi[Formula: see text], HeBe[Formula: see text], and HeBe[Formula: see text], the neutral He atom undergoes, in the electric field of the ion, a pronounced electronic polarization, and the natural bond order theoretical approach indicates that in the formation of the molecular orbital He acts as a weak electron donor. The energy decomposition analysis provides the dispersion and induction components as the attractive leading terms controlling the stability of all systems, confirming that the formed bond substantially maintains a non-covalent nature which is also supported by the Quantum Theory of Atoms in Molecules (QTAIM) analysis. Finally, it was found that the HeLi and HeBe neutral systems are unstable under any condition, HeLi[Formula: see text] and HeBe[Formula: see text] ionic systems are stable below 317K and 138K, respectively, while the HeBe[Formula: see text] system becomes unstable only after 3045K.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

The potential energy curves and interactions in all systems were studied theoretically based on coupled-cluster singles and doubles method with perturbative inclusion of triples CCSD(T) method with an aug-cc-pV5Z basis set. More precisely, it was determined the potential energy curves describing the stability of the HeLi, HeLi[Formula: see text], HeBe, HeBe[Formula: see text], and HeBe[Formula: see text] systems, the charge displacement within the formed adducts, the decomposition of their total interaction energy, the topological analysis of their bonds, their rovibrational energies, their spectroscopic constants and lifetimes.

Complexes of Alkali Metal Cations and Molecular Hydrogen: Potential Energy Surfaces and Bound States

Complexes between metal cations and molecular hydrogen are systems quite amenable for precise spectroscopic and theoretical studies, and at the same time, they are relevant for applications in hydrogen storage and astrochemistry. In this work, we report new intermolecular potential energy surfaces and rovibrational states calculations for complexes involving molecular hydrogen and alkaline metal cations, M⁺-H₂ (M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺). The intermolecular potentials, formulated in an internally consistent way to emphasize differences in the properties of the systems, are represented by simple analytical expressions whose parameters have been optimized from comparison with accurate ab initio calculations. Properties of the low-lying bound states-binding energies, frequencies, and rotational constants-are compared with previous measurements or computations and an overall good agreement is achieved, supporting the reliability of the present formulation. Variations of these properties as a function of the cation size and isotopic substitution, with a proper sequence of ortho and para rotational levels, are also discussed.

Experimental investigation of the reaction of helium ions with dimethyl ether: stereodynamics of the dissociative charge exchange process

The fate of dimethyl ether (DME, CH₃OCH₃) in collisions with He⁺ ions is of high relevance for astrochemical models aimed at reproducing the abundances of complex organic molecules in the interstellar medium. Here we report an investigation on the reaction of He⁺ ions with DME carried out using a Guided Ion Beam Mass Spectrometer (GIB-MS), which allows the measurement of reactive cross-sections and branching ratios (BRs) as a function of the collision energy. We obtain insights into the dissociative charge (electron) exchange mechanism by investigating the nature of the non-adiabatic transitions between the relevant potential energy surfaces (PESs) in an improved Landau-Zener approach. We find that the large interaction anisotropy could induce a pronounced orientation of the polar DME molecule in the electric field generated by He⁺ so that at short distances the collision complex is confined within pendular states, a particular case of bending motion, which gives rise to intriguing stereodynamic effects. The positions of the intermolecular potential energy curve crossings indicate that He⁺ captures an electron from an inner valence orbital of DME, thus causing its dissociation. In addition to the crossing positions, the symmetry of the electron density distribution of the involved DME orbitals turns out to be a further major point affecting the probability of electron transfer. Thus, the anisotropy of the intermolecular interaction and the electron densities of the orbitals involved in the reaction are the key "ingredients" for describing the dynamics of this dissociative charge transfer.

Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations

Scattering data, measured for rare gas-rare gas systems under high angular and energy resolution conditions, have been used to probe the reliability of a recently proposed interaction potential function, which involves only one additional parameter with respect to the venerable Lennard-Jones (LJ) model and is hence called Improved Lennard-Jones (ILJ). The ILJ potential eliminates most of the inadequacies at short- and long-range of the LJ model. Further reliability tests have been performed by comparing calculated vibrational spacings with experimental values and calculated interaction energies at short-range with those obtained from the inversion of gaseous transport properties. The analysis, extended also to systems involving ions, suggests that the ILJ potential model can be used to estimate the behavior of unknown systems and can help to assess the different role of the leading interaction components. Moreover, due to its simple formulation, the physically reliable ILJ model appears to be particularly useful for molecular dynamics simulations of both neutral and ionic systems.

Intermolecular interactions of H2S with rare gases from molecular beam scattering in the glory regime and from ab initio calculations

Integral cross sections for collisions of rotationally hot H2S molecules with rare gas atoms (Ne, Ar, and Kr) have been measured, in the collision energy range of 10-60 kJ mol(-1), using a molecular beam apparatus operating under high resolution both in angle and in velocity. A well resolved glory pattern has been measured which permitted the accurate characterization of the intermolecular potentials both at long range (in the attractive region) and at intermediate distances (in the well region). Considering the conditions used in the experiments, the obtained potentials must be considered very close to the spherical averages of the full intermolecular potential energy surfaces. Extensive ab initio calculations have also been carried out in parallel in order to characterize energy minima in the potential energy surfaces and energy barriers associated to the motion of the rare gas atoms around H2S. An assessment of the relative role of the various interaction components has been also attempted: the combined analysis of experimental and theoretical results suggests that H2S-rare gas aggregates are mainly bound by nearly isotropic noncovalent interactions of the van der Waals type.

Molecular-beam study of the water-helium system: Features of the isotropic component of the intermolecular interaction and a critical test for the available potential-energy surfaces

We report molecular-beam measurements of the total integral cross sections for the scattering of water molecules by helium atoms. A combined analysis of the new experimental data together with available differential cross section results has allowed an accurate determination of the isotropic component of the interaction potential for this prototypical system. The potential well shows a depth of 0.265 +/- 0.010 kJ/mol at a distance between He and the center of mass of the water molecule of 0.345 +/- 0.02 nm. An effective isotropic long-range attraction constant C(LR) = (6.3+/-0.3) x 10(-4) kJ mol(-1) nm(-6), including both dispersion and induction contributions, has also been determined. The most recent and accurate ab initio potential-energy surfaces have been tested against these new experimental results.

Interaction potentials of the RG–I anions, neutrals, and cations (RG=He, Ne, Ar)

Interaction potentials of the iodine atom, atomic cation, and anion with light rare-gas atoms from He to Ar are calculated within the unified ab initio approach using the unrestricted coupled-cluster with singles and doubles and perturbative treatment of triples correlation treatment, relativistic small-core pseudopotential, and an extended basis set. Ab initio points are fit to a flexible analytical function. The calculated potentials are compared with available literature data, assessed in the I(-)-and I+-ion mobility calculations and the Ar-I(-)-anion zero electron kinetic-energy spectra simulations, and analyzed using the correlation rules. The results indicate a high precision of the reported potentials.

Penning ionization of N2O molecules by He*(2S3,1) and Ne*(P2,03) metastable atoms: Theoretical considerations about the intermolecular interactions

A theoretical investigation of the intermolecular interaction, operative in collision complexes of He*(2 3S1), He*(2 1S0), and Ne*(3P2,0) with N2O, is carried out to explain the main results of the experimental study reported in the preceding paper. The analysis is carried out by means of a semiempirical method based on the identification, modeling, and combination of the leading interaction components, including the effect of the selective polarization of the more external electronic cloud of the metastable atom in the intermolecular electric field. These and other crucial aspects of our approach have been quantitatively verified by ab initio calculations. The proposed method permits to evaluate the interaction at any configuration of the complexes and provides a useful and inexpensive representation of the intermolecular potential energy for dynamics studies. The main experimental findings can be rationalized taking into account the critical balancing between molecular orientation effects in the intermolecular interaction field and the ionization probability. These orientation effects tend to become less pronounced with increasing collision energy.

Low-lying electronic states of HBr2+

The present study describes the characterization of energy and structure of HBr(2+) in its low-lying electronic states, achieved through an extension of a new empirical method [Chem. Phys. Lett. 379, 139 (2003)] recently introduced to evaluate the interatomic interaction in the HX(2+) (X=F,Cl,Br,I) molecular dications. The method is based on identification of the main components of the interaction and their evaluation through some simple correlation formulas. Potential energy curves, given in a simple, natural, and analytical form, made possible the calculations of some important properties, such as double-photoionization energy thresholds, vibrational spacing, average lifetime, and Franck-Condon factors. The predictions, compared with data available in the literature, are of great interest for the analysis and interpretation of some new experimental results.

EXPERIMENTS ON THE DYNAMICS OF MOLECULAR PROCESSES: A Chronicle of Fifty Years

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▪ Abstract  This paper reviews the way in which, in the Italy of the years immediately after World War II, interest in the dynamics of molecular processes was awakened. The narrative begins with the work of a small number of chemists and physicists who, in the initial stage, interacted closely. In the course of the years, their interests diverged and younger people joined the newly formed groups. Even now, after half a century, a common approach can still to be seen regarding how to attack problems and perform experiments. Experimental work is discussed, bringing out the common viewpoint of fields as diverse as mass spectrometry, isotope effects, chemical kinetics, molecular beams, molecule-molecule interactions, molecule-ion interactions, molecule-surface interactions, and plasma chemistry.