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

Holistic approach to chemical degradation of Nafion membranes in fuel cells

The state of health of polyfluorinated sulfonic-acid ionomer membranes (e.g. Nafion) in low-temperature proton exchange membrane fuel cells (LT-PEMFCs) is negatively influenced by degradation phenomena occurring during their operation. As a consequence, the performance and durability of the membrane are decreased. In this article, we focus on simulating and predicting chemical membrane degradation phenomena using a holistic zero-dimensional kinetic framework. The knowledge of chemical degradation mechanisms is widely spread. We have collected and evaluated an extensive set of chemical mechanisms to achieve a holistic approach. This yields a set of 23 coupled chemical equations, which provide the whole cause and effect chain of chemical degradation in LT-PEMFCs (based on the Fenton reaction between Fe [sup] 2+ and H [sub] 2O [sub] 2 via the attack of hydroxyl radicals on the membrane, loss of ionomer moieties and emission of fluoride). Our kinetic framework allows the reproduction of experimentally accessible data such as fluoride emission rates and concentrations of ionomer moieties (from both in situ and ex situ tests). We present an approach, which allows estimations of the membrane lifetime based on fluoride emission rates. In addition, we outline the demetallation of Fe-N-C catalysts as a source of additional harmful iron species, which accelerate chemical membrane degradation. To demonstrate the expandability and versatility of the kinetic framework, a set of five chemical equations describing the radical scavenging properties of cerium agents is coupled to the main framework and its influence on membrane degradation is analysed. An automated solving routine for the system of coupled chemical equations on the basis of the chemical kinetic simulation tool COPASI has been developed and is freely accessible online (http://ptc-pc-139.tugraz.at/ cgi-bin/Membrane_Degradation/)

DIPOLE MOMENTS OF CALCIUM MONOHALIDES FROM MOLECULAR BEAM LASER-MICROWAVE DOUBLE RESONANCE SPECTROSCOPY

$^{1)}$ K. M\""{o}ller, H.-U. Sch\""{u}tze-Pahlmann, J. Hoeft, and T. T\""{o}rring, Chem.Phys.68, 399 (1982). $^{2)}$ W.J. Childs, D.R. Cok, and L.S. Goodman, J.Chem.Phys. 76, 3993 (1981). $^{3)}$ W.J. Childs, D.R. Cok, and L.S. Goodman, J.Mol.Spectrosc. 95, 153 (1982).Author Institution: Institut f\""{u}r Molek\""{u}lphysik, Freie Universit\""{a}t BerlinHighly precise ground-state dipole moments of CaCl and CaBr were measured by using the molecular-beam laser-microwave double-resonance method. Microwave transitions were observed with linewidths of about 20 kHz. With an electric field applied Stark shifts of several hyperfine components were recorded. The determination of the dipole moment from measured line shifts required diagonalization of the complete energy matrix. Rotational constants were taken from M\""{o}ller $et al.^{1}$) and spin rotation and hfs constants from Childs $et al.^{2), 3)}$. The fit yielded the following results (statistical error in parentheses): [FIGURE] An additional systematic uncertainty of 0.02 D arises from the limited accuracy in determining the distance between the Stark plates but this does not affect the relative differences of the values. It will be shown that the Rittner ionic model fails in the calculation of these dipole moments. A modified model is proposed which takes into account explicitely the large charge shifts in the metal ions arising from the polarization

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