Dipole properties, dispersion energy coefficients, and integrated oscillator strengths for SF6

Long-range dispersion C6 coefficient for SF6 dimer: Experimental and theoretical study

The long-range dispersion C₆ coefficient for the SF₆ dimer is experimentally measured using a technique that uses the expansion of a supersonic pulse jet into a vacuum. A dynamic model of the jet enables us to correlate the position of the maximal peak in the time-of-flight spectrum with the initial conditions of the experiment and the parameters of the intermolecular interaction potential. Due to the low temperature of the jet target, the C₆ coefficient can be extracted directly from the experimental results. Theoretical calculation of the C₆ dispersion coefficient is also performed by using linearly approximated explicitly correlated coupled-cluster singles and doubles (CCSD(F12)) method with the subsequent utilization of the Casimir-Polder formula. Good agreement of experimental and theoretical results confirms the reliability of results.

Contributions of Multipolar Polarizabilities to the Isotropic and Anisotropic Light Scattering Induced by Molecular Interactions in Gaseous Sulfur Hexafluoride

The binary isotropic and anisotropic collision-induced light scattering spectra of the gaseous sulfur hexafluoride at room temperature are analyzed in terms of recent intermolecular potential and interaction-induced pair polarizability models, using quantum line-shapes computations. The spectra at relatively low frequencies are determined largely by the effect of bound and free transitions. At intermediate frequencies the spectra are sensitive to both the attractive part of the potential and the short-range part of the polarizability trace and anisotropy. The high frequency wings are discussed in terms of the collision-induced rotational Raman effect and estimates for the dipole–octopole polarizability E, is obtained and checked with recent ab initio theoretical value. Absolute zeroth moment of both spectra have been measured and compared with theoretical calculations using different models of the intermolecular potentials. Also, the temperature dependence of the pressure second virial coefficient, viscosity and thermal conductivity are also discussed for the proposed potentials. The results show that (M3SV) is the most accurate potential yet reported for this system.

Dipole oscillator strength distributions, properties, and dispersion energies for ethylene, propene, and 1-butene

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the ethylene molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength (DOS) data; the DOS data employed are recent experimental results not available at the time of the original constrained DOSD analysis of this molecule. The constraints are furnished by molar refractivity data and the Thomas–Reiche–Kuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for ethylene. Pseudo-DOSDs for this molecule, and for propene and 1–butene, which are based on an earlier constrained DOSD analysis for these molecules, are developed. They are used to obtain reliable results for the isotropic dipole–dipole dispersion-energy coefficients C6, for the interactions of the alkenes with each other and with 47 other species, and the triple-dipole dispersion-energy coefficients C9 for interactions involving any triple of molecules taken from ethylene, propene, and 1–butene.Key words: alkenes, dipole properties, pseudo-states, dipole–dipole and triple-dipole dispersion energies, long-range additive, non-additive interaction energies.

A post-Hartree-Fock model of intermolecular interactions

Intermolecular interactions are of great importance in chemistry but are difficult to model accurately with computational methods. In particular, Hartree-Fock and standard density-functional approximations do not include the physics necessary to properly describe dispersion. These methods are sometimes corrected to account for dispersion by adding a pairwise C6R6 term, with C6 dispersion coefficients dependent on the atoms involved. We present a post-Hartree-Fock model in which C6 coefficients are generated by the instantaneous dipole moment of the exchange hole. This model relies on occupied orbitals only, and involves only one, universal, empirical parameter to limit the dispersion energy at small interatomic separations. The model is extensively tested on isotropic C6 coefficients of 178 intermolecular pairs. It is also applied to the calculation of the geometries and binding energies of 20 intermolecular complexes involving dispersion, dipole-induced dipole, dipole-dipole, and hydrogen-bonding interactions, with remarkably good results.

Vibrational Properties of Disordered Mono- and Bilayers of Physisorbed Sulfur Hexafluoride on Au(111)

We have examined the low-energy single-phonon vibrations of disordered mono- and bilayers of sulfur hexafluoride physisorbed on Au(111) with inelastic helium atom scattering. At monolayer coverages, SF6 exhibits a dispersionless Einstein mode at 3.6 +/- 0.4 meV. We observed two distinct overtones of this vibration as both creation and annihilation events at 7.1 +/- 0.7 meV and 10.9 +/- 1.4 meV, respectively. The overtones are harmonic multiples of the fundamental Einstein oscillation. Bilayers of SF6 exhibit a softer fundamental vibration with an excitation energy of 3.3 +/- 0.3 meV. This softening, due to the weaker SF6 binding, also results in reduced overtone energies of 6.6 +/- 0.7 meV and 9.8 +/- 0.6 meV. The disordered bilayer does not exhibit dispersion, indicating that the molecules are still behaving like Einstein oscillators and not beginning to act as bulk crystalline SF6. The results have improved our understanding of the adsorbate-substrate and interadsorbate interactions which govern the properties of this model molecular physisorption system.

Dipole Oscillator Strength Distributions and Properties for Methanol, Ethanol and Propan-1-ol and Related Dispersion Energies

Recommended isotropic dipole oscillator strength distributions (DOSDs) have been constructed for the methanol and ethanol molecules through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength (DOS) data; the DOS data employed are recent experimental results not available at the time of the original constrained DOSD analysis of these molecules. The constraints are furnished by molar refractivity data and the Thomas-Reiche-Kuhn sum rule. The DOSDs are used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecules. Pseudo-DOSDs for these molecules, and for propan-1-ol based on an earlier constrained DOSD analysis for this molecule, are also presented. They are used to obtain reliable results for the isotropic dipole-dipole dispersion energy coefficients C6, for the interactions of the alcohols with each other and with 36 other species, and the triple-dipole dispersion energy coefficients C9for interactions involving any triple of molecules involving methanol, ethanol and propan-1-ol.

Article

Average energy approximations for the anisotropic triple-dipole dispersion energy coefficients are tested using reliable results for these coefficients, which are available for all interactions involving the rare gases, H2, N2, CO, O2, and NO. The original average energy approximation does not reproduce any of the anisotropic coefficients to within their estimated uncertainties. More recently derived average energy approximation formulae, requiring the isotropic and anisotropic polarizabilities and average energies for the interacting species as input, reproduce all but 69 of the 680 isotropic and anisotropic coefficients considered to within their estimated uncertainties.Key words: nonadditive, three-body interactions, dispersion energies.