A Theoretical Investigation of the Geometries, Vibrational Frequencies, and Binding Energies of Several Alkali Halide Dimers

Results are presented from ab initio calculations on the symmetrical alkali halide dimers made up of Li, Na, K, F, and Cl. We examine the sensitivity of representative monomer and dimer geometries to the variation of the basis set with and without polarization and diffuse functions. The geometries are then compared with available experimental results. We have also calculated vibrational frequencies at the restricted Hartree–Fock level and examined the changes in geometry brought about by correlation using second‐order Møller–Plesset perturbation theory. It is found that Hartree–Fock theory in a modest basis set with diffuse and polarization functions yields results comparable to much larger sp basis sets and that the theoretical results are in good agreement with the experimental results for the Li and Na dimers. Our best results for the K‐containing species tend to have bond lengths that are too long for the monomers and this error is carried over for the dimers. We also find a nearly uniform expansion of the M–X bond length in proceeding from monomer to dimer of 0.16±0.03 Å, independent of the alkali or halide involved. The calculated dimer dissociation energies are in excellent agreement with experiment. Inclusion of correlation appears to have a minimal effect on the computed geometries and a modest effect on the binding energies. The vibrational frequencies for the monomers are in excellent agreement with experimental gas‐phase results and reasonable agreement is obtained with the available experimental frequencies for the dimers. Finally, a reanalysis of the electron diffraction data for Na2F2 in light of the current ab initio results leads to a significant change in the experimental value of the bond angle

The Structure of Alkali Halide Dimers: A Critical Test of Ionic Models and New Ab Initio Results

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 Seitz–Ruffa 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

Erratum: The Structure of Alkali Halide Dimers: A Critical Test of Ionic Models and New Ab Initio Results

It has come to our attention that some of the ab initio results presented are incorrect due to errors in the Cs and C1 basis sets, and a small error in the binding energy of Rb2F2. The corrected results are presented below for the species that were affected, modifying the results in Table III of the original paper. Only those values which are different from the results of the original Table III are included. Note that some of these results are used for comparison with the ionic models in later tables. In addition, some HF data quoted in Tables V and VI is affected, and the correct values are given in Table II. All the changes in quoted values are small and none of the conclusions drawn in the article are affected, nor are the comparisons with the ionic models significantly affected. However, the error in the C1 basis is what gave rise to the anomalously short M–Cl bond lengths, and the results presented here lead to longer bonds, in somewhat poorer agreement with the experimental results for Cl containing species

Equilibrium gas-phase structures of sodium fluoride, bromide, and iodide monomers and dimers

The alkali halides sodium fluoride, sodium bromide, and sodium iodide exist in the gas phase as both monomer and dimer species. A reanalysis of gas electron diffraction (GED) data collected earlier has been undertaken for each of these molecules using the EXPRESS method to yield experimental equilibrium structures. EXPRESS allows amplitudes of vibration to be estimated and correction terms to be applied to each pair of atoms in the refinement model. These quantities are calculated from the ab initio potential-energy surfaces corresponding to the vibrational modes of the monomer and dimer. Because they include many of the effects associated with large-amplitude modes of vibration and anharmonicity, we have been able to determine highly accurate experimental structures. These results are found to be in good agreement with those from high-level core-valence ab initio calculations and are substantially more precise than those obtained in previous structural studies

Electric dipole moments and the search for new physics

Static electric dipole moments of nondegenerate systems probe mass scales for physics beyond the Standard Model well beyond those reached directly at high energy colliders. Discrimination between different physics models, however, requires complementary searches in atomic-molecular-and-optical, nuclear and particle physics. In this report, we discuss the current status and prospects in the near future for a compelling suite of such experiments, along with developments needed in the encompassing theoretical framework.Comment: Contribution to Snowmass 2021; updated with community edits and endorsement

THE SALT MOLECULE LiKF2LiKF_{2}

S. Biermann, et al., J. Chem. Phys. 105, 9754-9761 (1996).R.J. Mawhorter, et al., J. Mol. Struct. 413-414, 415-422 (1997).Author Institution: Gottfried-Wilhelm-Leibniz-Universitat, Institut fur Physikalische Chemie, Callinstra{\ss}e 3A, 30167 Hannover, Germany; Pomona College, Department of Physics and Astronomie, 610 North College Avenue, Claremont, CA 91711The mixed alkali halide dimer $LiKF_{2}$ has been observed and characterized using LASER-ablation supersonic-jet Fourier transform microwave spectroscopy. This is the first step in extending the earlier successful studies}} of $LiNaF_{2}$ to heavier mixed dimers. Fifteen transitions of 3 isotopologues of $LiKF_{2}$, $^{6}Li^{39}KF_{2}$, $^{6}Li^{41}KF_{2}$, and $^{7}Li^{39}KF_{2}$, have been observed. This allows a determination of the kite-shaped $LiKF_{2}$-structure as well as the leading centrifugal distortion constants, which in turn provide more insight into the nature of these ionic bonds. Measurements of the hyperfine splitting of these spectral lines provides another constraint on molecular charge density, i.e. a further detailed check of theoretical calculations

A Harmonic Potential Function for Lithium Sodium DiFlouride

A harmonic force field for the mixed alkali halide dimer LiNaF2 is reported based on microwave centrifugal distortion coefficients and matrix-isolation vibrational frequencies for both 6LiNaF2 and 7LiNaF2, and an ab initio force field. It is compared to RHF and MP2 ab initio calculations, with a particular emphasis on determining reliable general alkali halide mean amplitudes of vibration l. Detailed comparisons between ionic model, RHF, MP2, and CCSD ab initio dipole moment values and the experimental value of 2.64(2) D are also made