The structure of alkali halide dimers: A critical test of ionic models and new ab initio results

Abstract

The structure of alkali halide dimers: A critical test of ionic models and new ab initio results. T. Torring, S. Biermann, J. Hoeft, R. Mawhorter, R. J. Cave, and C. Szemenyei, J. Chem. Phys. 104, 8032 (1996) In semiempirical ionic models a number of adjustable parameters have to be fitted to experimental data of either monomer molecules or crystals. This leads to strong correlations between these constants and prevents a unique test and a clear physical interpretation of the fit parameters. Moreover, it is not clear whether these constants remain unchanged when the model is applied to dimers or larger clusters. It is shown that these correlations can be substantially reduced when reliable information about dimers is available from experiments or ab initio calculations. Starting with Dunham coefficients of the monomer potential determined from microwave measurements, we have calculated the monomer to dimer bond expansion and the bond angle without any additional adjustable parameter. Assuming that the overlap repulsion between nearest neighbors remains unchanged, the bond expansion is mainly determined by the simple Coulomb repulsion between equally charged ions and depends only very little on the effective ion polarizabilities. Deviation of the bond angle from 90°sensitively tests the difference of effective polarizabilities of the two ions. A comparison with previously available data and new ab initio MP2 results presented here for the heavy-atom containing dimers shows that bond angles can be modeled reasonably well with SeitzRuffa corrected Pauling polarizabilities while calculated bond expansions are much too long. This shows that changes of the overlap repulsion term must be considered for reliable predictions of the structure of dimers and larger clusters.

Holding onto Electrons in Alkali Metal Halide Clusters: Decreasing Polarizability with Increasing Coordination

“…For clusters, a decisive point is also what kind of crystal structure they adopt in the process of self-assembling from the gas-phase monomers to (molecules). In several earlier works it has been suggested that alkali metal halide clusters were cuboids (rectangular parallelepipeds with an even number of atoms along at least one of the edges). ,, During the later time it has been theoretically shown that for the ionic clusters with the number of atoms below 100 the ratio of the cation and anion radii, r C / r A , defined whether a tubular or cubic geometry would be energetically favorable. ,, If r C / r A ≤ 0.5, tubular structures occur to be more stable. Potassium chloride belongs to those alkali metal halides for which this criterion allows assuming cubic geometry for all cluster sizes because r C ≈ r A in this case.…”

“…The corresponding calculations show that though in a class of cases the cluster properties are close to those of the solid, for some types of clusters with ionic bonding the electronic and geometric structure may deviate significantly from those adopted at macroscale. For example, for the alkali metal halide clusters containing up to10 2 atoms the possibility of a structure with tubular geometry minimizing the energy to a degree higher than the solid-like cubic geometry continues to be a subject of theoretical investigation − since Aguado et al suggested it.…”

“…More recently, another important reason has emerged: the study of the geometric and electronic structures of alkali metal halide clusters can give information on the formation and dynamics of sea-spray-originating nanoparticles in atmospheric marine aerosols. In the studies of the alkali metal halide cluster geometry and energetics, various theoretical and experimental approaches have been used, such as molecular dynamics − and ab initio methods − in theory and electron diffraction, mass spectroscopy, photoelectron spectroscopy, − etc. in experiment.…”

Equilibrium Gas-Phase Structures of Sodium Fluoride, Bromide, and Iodide Monomers and Dimers

“…27 Matrix isolation studies of the vibrational frequencies of Cs 2 Br 2 and Cs 2 I 2 have also been recently published. 28 The literature is peppered with many theoretical studies 16,[29][30][31][32][33] which have some difficulty in reproducing the highly accurate microwave monomer distances.…”

Temperature-Gradient-Driven Aging Mechanisms in Alkali-Bromide- and Sulfate-Containing Ash Deposits

“…Molecule-Dependent Constants for Determining the Saturation Pressures (A i and B i ) of Alkali Bromides, Effective Molecular Diameters in the N 2 Atmosphere (d i,eff ), Effective Mass in the N 2 Atmosphere (μ i ), and Density at Room Temperature (ρ i )30,41,42 …”