Ion-pair Configurations Research Articles

Potential of mean force by constrained molecular dynamics: A sodium chloride ion-pair in water

The constrained molecular dynamics simulation method has been used to obtain the mean force and the mean force potential between two particles in solution. The method has been tested for a Lennard-Jones liquid and applied to the study of a Na +-Cl − ion-pair in aqueous solution. A flexible SPC model for water has been assumed. The results have been interpreted in the light of the solvent structure around the ions for separations corresponding to the maxima and minima of the mean force potential. In contrast to earlier studies using rigid water models, the solvent separated ion pair configurations are more stable than the contact ion pair configurations.

Vibrational spectra of molecular ions isolated in glassy and crystalline alkali metal oxyanion salt matrices

Molecular anions (NO−3 and ClO−3) have been isolated in glassy alkali metal oxyanion matrices (MClO3 and MClO4) at 80°K so that the splittings of the impurity molecular ion degenerate modes become a more direct measure of the anion site asymmetry in amorphous salt media. In general it is found that the individual components of the impurity ion ν3 asymmetric stretching band are much sharper and the splitting of the ν3 components significantly reduced over values for pure glasses or molten salts. Because of the band narrowing, distinct maxima are observed for both ν3 components for KNO3 and NaClO3, for which a doublet structure has been inferred but not previously observed. Of principal interest is the observation that the isolated NO−3 ν3 splitting, for a LiClO4 or KClO4 glass matrix, is unchanged by crystallization of the salt. In other words, the vibrational data show that the distortion experienced by the NO−3 anion in an amorphous salt is not appreciably different than for an asymmetric site in a perchlorate crystal. Consequently, the vibrational data are no more indicative of ion-pair configurations in the glass or molten salt phase than for the crystal phase.

Matrix isolation technique for the study of some factors affecting the partitioning of trace elements

Some of the factors that affect the preferred positions of cations in ionic-solid solutions were investigated utilizing vibrational spectroscopy. Solid solutions of the sulfate and chromate ions codoped with La 3+ and Ca 2+ in a KBr host lattice were examined as a function of the polyvalent-cation concentration. The cation—anion pairing process was found to be random for Ca 2+ whereas the formation of La 3+SO 4 2− ion-pairs with a C 2v bonding geometry is highly preferential to any type of La 3+CrO 4 2− ion-pair formation. The relative populations of ion-pair site configurations are discussed in terms of an energy—entropy competition which can be applied to the partition of trace elements during magmatic processes.

Observation of initial ion-pairs in pulse-irradiated weakly polar liquids

The formation and decay of pyrenide ions have been observed, with nanosecond time resolution, in pulse-irradiated solutions of pyrene in diethylamine (DEA) and tetrahydrofuran (THF), both weakly polar liquids. The complex decay of the pyrenide ion was interpreted in terms of the very fast “geminate” recombination of pyrenide ions with solvent counter-ions with solvent counter-ions in the original ion-pair configuration as well as the homogeneous recombination of the “free” ions. The empirical “error-function” kinetic treatment for such decays, developed by Schuler et al., has been applied to the experimentally observed rate curves. This application, which fits the experimental data quite well, leads to estimates of ionic escape probabilities of 0·03 for DEA and 0·08 for the THF. These escape probabilities correspond to “free-ion” yields ( G fi ) of approximately 0·1 ion/100 eV for DEA and 0·3 ion/100 eV for THF based on the measured ionic yields for nanosecond pulses. This implies that the total ionization yield is approximately 4 in THF and 3 in DEA.

ESR Studies of the Ion-pairs of the Acenaphthylene Radical Anion and Alkali Metal Cations

Abstract Two types of temperature dependence of the proton hyperfine splitting constants of the acenaphthylene radical anion are described. One is that for the radical anions which are in a state of rapid exchange between the ion-paired form and the free radical anion, and the other is that for the radical anions which exist as stable ion pairs with a relatively long lifetime. From the examination of the effects of the alkali ions in terms of the molecular orbital method, it is concluded that the most favored ion-pair configuration is that in which the alkali ion is placed above the five-membered ring. The equilibrium position of the alkali ion moves toward the position 11 with an increase in the radius of the alkali ion. In relation to this ion-pair structure, the temperature dependence of alkali-metal hyperfine splittings is also discussed.