Ab-initio calculations of excited rare gas alkali ions

The excimer-like ions formed by combining a rare gas ion with an alkali atom were investigated by an ab initio HF-CI calculation. The resulting four bound excited states¹Σ⁺,¹Π,³Σ⁺ and³Π were subjected to a semiempirical spin orbit (SO)-coupling yielding eight fine structure levels with angular momenta Ω=2, 1(3) and 0(4). For (NeLi)⁺, (NeNa)⁺, (ArLi)⁺, (ArNa)⁺, (ArK)⁺ and (KrLi)⁺ the wavelengths for the five allowed transitions to the ground state were calculated. The results are in close agreement with the experimental results. Also other spectroscopic properties as binding energies, transition moments and vibrational quanta are given.     

Ab initio calculations for absorption and emission energies of alkali halide crystals doped with thallium

We calculate transition energies associated with optical properties of thallium doping in alkali halide crystals via an atomic cluster of minimal size where an sp‐valence‐shell impurity enters as a substitutional defect in the model crystal. Hartree–Fock (HF), density functional theory (DFT), and configuration interaction (CI) [CIS (CI with single excitation) and QCISD (single plus double and quadruple excitation)] calculations are performed to theoretically obtain the absorption and emission energies as vertical transitions evaluated at the ground and first excited‐state optimized geometries, respectively, where the optimization is carried out separately with the HF and DFT methods. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 785–790, 2000     

Quantum calculations on the adsorption of halide ions on the noble metals

The interaction of halide ions with the three noble metals has been investigated using the B3LYP density functional method and the cluster model approximation. The results of calculations for the M—X⁻ and M₁₂—X⁻ (M = Cu, Ag, Au; X = F, Cl, Br, I) systems are presented. At the (100) surface, modeled in the present work by the M₁₂ cluster, all halide ions have been found to adsorb preferentially at the hollow site, followed by the bridge and by the top positions. The adsorption energy has been found to decrease when going from fluoride to iodide in both atom—ion and cluster—ion cases. The opposite trend is observed for the estimates of the charge transfer from the ions to the surface. When different metals are compared, the M₁₂—X⁻ interaction energies decrease in the order Au > Ag > Cu, but for the smaller ions some deviations from this line do appear. The relative values of the calculated harmonic vibrational frequencies do agree with those found experimentally, but their magnitude is much smaller as a result of the effect of the lower surface coverage.     

On the electrolytic accumulation of halide ions at hanging mercury drop electrodes

The process of electrolytic accumulation of halide ions as Hg(2)X(2), at the hanging mercury drop electrode is considered theoretically. The efficiency of this process is explained by considering various chemical reactions of mercury ions with halogens, and the adsorption of Hg(2)X(2). The agreement of these considerations with experimental results obtained by other authors is found to be satisfactory.     

Ammonium ions in alkali metal halide crystals: tunnelling and spin relaxation

We have used low energy inelastic neutron scattering spectroscopy to examine the tunnelling spectroscopy of the ammonium ion in the (NH4)0.02Rb(x)K(0.98-x)I system. The concentration of different species were varied as x increased, this was followed systematically and the first consistent assignment scheme for these features is given. Differences were also found for the relaxation rate of the spin temperature inversions that could be generated in these species. At a critical concentration--about x = 0.04 mole fraction--the relaxation rates of the species changed dramatically.     

A set of molecular models for alkali and halide ions in aqueous solution

This work presents new molecular models for alkali and halide ions in aqueous solution. The force fields were parameterized with respect to the reduced liquid solution density at 293.15 K and 1 bar, considering all possible ion combinations simultaneously. The experimental target data are reproduced with a high accuracy over a wide range of salinity. The ion models predict structural properties of electrolyte solutions well, such as pair correlation functions and hydration numbers. The force fields provide good predictions of the properties studied here in combination with different models for water.     

Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions

Accurate models of alkali and halide ions in aqueous solution are necessary for computer simulations of a broad variety of systems. Previous efforts to develop ion force fields have generally focused on reproducing experimental measurements of aqueous solution properties such as hydration free energies and ion-water distribution functions. This dependency limits transferability of the resulting parameters because of the variety and known limitations of water models. We present a solvent-independent approach to calibrating ion parameters based exclusively on crystal lattice properties. Our procedure relies on minimization of lattice sums to calculate lattice energies and interionic distances instead of equilibrium ensemble simulations of dense fluids. The gain in computational efficiency enables simultaneous optimization of all parameters for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that enforce consistency with periodic table trends. We demonstrate the method by presenting lattice-derived parameters for the primitive model and the Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting parameters successfully reproduce the lattice properties used to derive them and are free from the influence of any water model. To assess the transferability of the Lennard-Jones parameters to aqueous systems, we used them to estimate hydration free energies and found that the results were in quantitative agreement with experimentally measured values. These lattice-derived parameters are applicable in simulations where coupling of ion parameters to a particular solvent model is undesirable. The simplicity and low computational demands of the calibration procedure make it suitable for parametrization of crystallizable ions in a variety of force fields.     

Electron binding energies of aqueous alkali and halide ions: EUV photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.     

Vector coupling coefficients for calculations of transition-metal atoms and ions by the SCF coupling operator method

We derived the necessary conditions to which the vector coupling coefficients (VCC ) a and b describing atomic L,S-multiplets of the configurations d^N (1 ≤ N ≤ 9), should satisfy. Special attention is paid to the states of non-Roothaan type for which VCC depend on the choice of degenerate d-orbitals basis set determined within the accuracy up to an orthogonal transformation u. It is shown that for such states the direct sum of matrices ‖a‖ and ‖b‖ must be the non-symmetric matrix. Obtained VCC were used for the ab initio calculations (basis set (14s9p5d)/[8s4p2d] from [15]) on first-row transition atoms (from Sc to Cu) to compare to similar calculations [16], in which the Peterson's VCC have been used, and with calculations [15] carried out by the atomic SCF program [4] as well.     

Ab initio quantum-chemical calculations of interactions of ions and hydrides of alkali metals with C3H6 and C4H8 molecules

Calculations of the C₃H₆ · LiH, C₄H₈ · M⁺, and C₄H₈ · MH systems and of C₂H₂ · MH complexes (M = Li or Na) were carried out by the unrestricted Hartree-Fock-Roothaan (UHF) method with partial optimization of the geometry using fixed geometric parameters of the C₃H₆ and C₄H₈ molecules. The standard 3-21G and 6-31G* basis sets were used. Unlike the C₃H₆ · LiH structure, the C₄H₈ · M⁺ and C₄H₈ · MH systems are typical complexes. It was found that the C₄H₈ · M⁺, C₄H₈ · MH, and C₂H₂ · MH complexes are similar in coordination of M⁺ ions and MH molecules by carbon atoms in spite of considerable differences in the interatomic distances (–1 A) between these atoms in the C₄H₈ and C₂H₂ molecules. The heats of formation (Q), which were calculated in the UHF/6-31G* approximation and using second- and fourth-order Möller-Plesset perturbation theory taking into account the electron correlation energy in the MP2/6-31G*. MP4(SDQ)/6-31G*, and MP4(SDTQ)/6-31G* approximations, satisfy the following relationships: Q(C₂H₃ · MH) < Q(C₄H₈ · MH) < Q(C₄H₈ · M⁺). It was observed that in going from Li to Na the corresponding values of Q tend to decrease.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1636–1640, July, 1996.     

Ionic Solvation in formic acid,MO SCF calculations on solvated univalent ions

CNDO/2 calculations have been performed for the mono-, di- and tetra-solvates of Li⁺, Na⁺ and Cl^? with formic acid. The most stable position for the cations is found to be between the CH hydrogen and carbonyl oxygen, confirming similar conclusions from experimental results. The calculated changes in electron densities agree well with observations in ¹H-NMR spectra.Calculated solvation energies are found to show the right relative order for the cations, although absolute values are too high. For the anion, also the absolute value is in reasonable agreement with experiment. CNDO minimum geometries, charge distributions and bond indices are given for all solvates and discussed in respect to possible methodical errors.     

Quantum chemical studies of the chemisorption of water and of unhydrated and hydrated halide ions on mercury

Local structures on electrode interfaces can be explored by quantum chemical investigation of medium-sized systems consisting of a cluster of substrate (metal) atoms, one or several solvent molecules, and/or at least one ion to be adsorbed at the interface. For the study of water adsorption and halide ion adsorption (unhydrated as well as hydrated) on a mercury surface, we have used the standard CNDO method together with geometrical optimization of the atom positions.In this paper, the following topics have been treated: (a) adsorption of a single water molecule in different positions on a close-packed plane cluster of seven mercury atoms; (b) adsorption of unhydrated halide ions (Cl^?, Br^?, I^?) in the “on-top” or hollow position on the mercury surface; (c) adsorption of monohydrated halides on the mercury surface. Further studies including solvation by six water molecules are discussed.The calculations provide information about minimum-energy geometries, energetic data, and local charges. Furthermore, they allow some conclusions about water mobility and reorientation on a close-packed metal surface, water orientation under the combined influence of an adsorbed ion and the metal surface, and trends of charge distribution in the halide series to be drawn. Calculations are critically discussed in the light of experimental and other quantum chemical data.     

X- (X = O, S) ions in alkali halide lattices through density functional calculations. 1. Substitutional defect models

Monoatomic X- (X = O, S) chalcogen centers in MZ (M = Na, K, Rb and Z = Cl, Br, I) alkali halide lattices are investigated within the framework of density functional theory with the principal aim to establish defect models. In electron paramagnetic resonance (EPR) experiments, X- defects with tetragonal, orthorhombic, and monoclinic g-tensor symmetry have been observed. In this paper, models in which X- replaces a single halide ion, with a next nearest neighbor and a nearest neighbor halide vacancy, are validated for the X- centers with tetragonal and orthorhombic symmetry, respectively. As such defect models are extended, the ability to reproduce experimental data is a stringent test for various computational approaches. Cluster in vacuo and embedded cluster schemes are used to calculate energy and EPR parameters for the two vacancy configurations. The final assignment of a defect structure is based on the qualitative and quantitative reproduction of experimental g and (super)hyperfine tensors.     

SCF CI MO calculations on the base–base interactions in dinucleoside monophosphates

In order to elucidate further details of RNA conformations we have studied base stacking in dinucleoside monophosphates (DNP's) using UV difference spectra and the hypochromic effect. SCF CI MO calculations according to PPP and MIM approximations were carried out for six pyrimidine-containing DNP's in each of several stacking geometries. The calculated and plotted difference spectra were fitted to the experimental spectra. Different DNP's showed distinct geometries in the range of the helix angle of 35°. From our results we conclude that there is a microstructure in the helix. This would imply an additional content of information in this macromolecule, a higher precision in nucleic acid interactions, and make possible the prediction of the conformation of polynucleotides.     

Investigation of the relative stability of positive alkali halide cluster ions generated by fast atom bombardment

The stabilities of alkali halide cluster ions [M(MX)_n]⁺ (M ? Li, Na, K, Rb, Cs; X ? F, Cl, Br, I) have been studied by measuring the fragment ion yields following dissociation of the ions in the second field free region of a ZAB-2F mass spectrometer. Extractable cluster ions were observed for certain values of n. It was found that the stabilities of the neutral fragment species formed are also of importance in determining the fragmentation rates. Possible configurations of M and X in the stable ions are discussed.     

Absolute hydration free energy scale for alkali and halide ions established from simulations with a polarizable force field

A polarizable potential function for the hydration of alkali and halide ions is developed on the basis of the recent SWM4-DP water model [Lamoureux, G.; MacKerell, A. D., Jr.; Roux, B. J. Chem. Phys. 2003, 119, 5185]. Induced polarization is incorporated using classical Drude oscillators that are treated as auxiliary dynamical degrees of freedom. The ions are represented as polarizable Lennard-Jones centers, whose parameters are optimized to reproduce the binding energies of gas-phase monohydrates and the hydration free energies in the bulk liquid. Systematic exploration of the parameters shows that the monohydrate binding energies can be consistent with a unique hydration free energy scale if the computed hydration free energies incorporate the contribution from the air/water interfacial electrostatic potential (-540 mV for SWM4-DP). The final model, which can satisfyingly reproduce both gas and bulk-phase properties, corresponds to an absolute scale in which the intrinsic hydration free energy of the proton is -247 kcal/mol.