Calculated lifetimes and linewidth temperature dependence (1.5–150 K) of four internal vibrations of naphthalene crystal

The linewidths of four totally symmetric vibrations of naphthalene have been calculated as a function of temperature between 1.5 and 150 K on the basis of a population relaxation mechanism. The anharmonic coefficients coupling internal and lattice phonons are obtained from an intermolecular potential that includes atom-atom and quadrupole-quadrupole contributions. The agreement with the experimental data is good for two of the modes considered, while it is unsatisfactory for the others. The results show that the population relaxation (T₁ decay) can contribute substantially to the observed dephasing also at relatively high temperature. The internal and external modes responsible for the vibrational decay are individualized, showing that several distinct two-phonon processes with different weights are involved in the relaxation of the optical modes.     

Phonon dynamics and electron-phonon coupling in pristine picene

The paper reports a complete analysis of the phonon structure of crystalline picene, a recently announced organic semiconductor. Both lattice and intramolecular vibrations are investigated. An exhaustive assignment of lattice phonons is obtained through polarized Raman spectra assisted by lattice dynamics calculations based on a well tested atom-atom potential model. Raman, infrared spectra and density functional (DFT) calculations are used for the characterization of intramolecular modes. Coupling between low-frequency molecular vibrations and lattice phonons is accounted for. Molecule-to-molecule transfer integrals, as well as the Peierls and Holstein (non-local and local) coupling constants, are evaluated through the semiempirical method INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization).     

Coupling of ion and water fluxes in synthetic membranes*

The behaviour of ion-exchange membranes has, in most circumstances, been interpreted in terms of the fixed charge theory of Teorell, Meyer and Sievers. Generally it has been recognised that some allowance should be made in the theory for coupling between ion and water fluxes. In making approximate corrections for this, it has usually been assumed that close coupling between all ion fluxes and the osmotic or electro-osmotic water flux occurs. In this paper the specific nature of ion/water flux coupling is explored and it is shown that close coupling is a good approximation for co-ion and water fluxes but that the coupling of the counter-ion and water fluxes is less strong. p]The interactions between ions, water and membrane matrix in a cation exchange membrane are revealed by determining the values of the pairwise molecular friction coefficients. These findings support long standing qualitative ideas about the interplay of electrostatic, hydration and hydrophobic interactions in membranes. p]The picture that emerges shows that an exact extension of the usual equations of the fixed charge theory to include flux coupling would be too complex to be useful. An approximate procedure is to divide conceptually the counter-ions into two groups, those balancing the fixed charges and those accompanying the sorbed co-ions, i.e. constituting sorbed electrolyte. If coupling of the former group of counter-ions with the water flux is ignored whilst close coupling is assumed between the sorbed electrolyte and the water flux, a very good correction is achieved for the overall effects of coupling.     

Elektronenstruktur und NMR-Parameter von substituierten 5-Phenyl-2,4-pentadiensäuren

The ¹H and ¹³C NMR chemical shifts as well as vicinal HH coupling constants of substituted 5-phenyl-2,4-pentadienoic acids Ar? CH?CH? CR?CR? COOH are reported and discussed in connection with the molecular structure. The ¹³C chemical shift values show an alternation along the chain and can be linearly correlated to the π-electron charge densities as calculated by use of the PPP-method. The effect of para-substituents and solvents upon the ¹³C chemical shifts can be described in terms of the mutual atom-atom polarizabilities.     

Lattice frequencies of a single crystal of benzylidenemalononitrile

Far i.r. and the Raman spectra of benzylidenemalononitrile have been recorded at room and liquid nitrogen temperatures. Lattice frequencies are calculated using an intermolecular potential of the atom-atom type, involving interactions between all pairs of atoms on neighboring molecules. The experimental frequencies are compared with those calculated.     

Recombination of photodissociated iodine: a time-resolved x-ray-diffraction study

A time-resolved x-ray-diffraction experiment is presented that aims to study the recombination of laser-dissociated iodine molecules dissolved in CCl4. This process is monitored over an extended time interval from pico- to microseconds. The variations of atom-atom distances are probed with a milliangstrom resolution. A recent theory of time-resolved x-ray diffraction is used to analyze the experimental data; it employs the correlation function approach of statistical mechanics. The most striking outcome of this study is the experimental determination of time-dependent I-I atom-atom distribution functions. The structure of the CCl4 solvent changes simultaneously; the solvent thus appears as a reaction partner rather than an inert medium hosting it. Thermal expansion of the system is nonuniform in time, an effect due to the presence of the acoustic horizon. One concludes that a time-resolved x-ray diffraction permits real-time visualization of solvent and solute motions during a chemical reaction.     

The effects of anisotropic atom-atom interactions on the crystal structure and lattice dynamics of solid CS2

Good agreement with the experimental structure and with the observed lattice frequencies of crystalline CS₂ is obtained using an intermolecular potential consisting of an electrostatic part and of an anisotropic atom-atom potential. The role of anisotropic short-range interactions in the statics and dynamics of linear-molecule crystals is discussed.     

Energy decay mechanisms and anharmonic lattice dynamics: The case of solid nitrogen

The anharmonic frequencies and linewidths of the lattice phonons in -N₂ are calculated on the basis of three different intermolecular potentials which include atom-atom and electrostatic interactions. The distinction between stationary anharmonicity and decay anharmonicity is stressed and the mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in detail. The temperature dependence of the phonon self-energy is also considered. The results thus obtained for -N₂ are compared with predictions from previous lattice dynamics. SCP and molecular dynamics calculations. The calculated anharmonic effects are substantially independent of the adopted potential: the agreement with experimental data is reasonably good as far as the linewidths are concerned, while the anharmonic deformation of the potential wells (and thus the frequency shifts) is overestimated. We suggest that, while higher orders in the diagram expansion are necessary for a proper account of the stationary anharmonicity, the decay anharmonicity limits its effectiveness to two-phonon processes, thus allowing proper predictions of the phonon lifetimes by using the lowest-order diagrams. Finally, -N₂ is compared to -CO, and the role played by the translation-rotation coupling is discussed.     

The influence of an intramolecular H-bond on the adsorption of sulfur-containing amino acids on graphitized thermal carbon black

The thermodynamic characteristics of adsorption of sulfur-containing amino acids and their derivatives on the surface of graphitized thermal carbon black were calculated by the molecular statistical method. The parameters of the atom-atom potential function of intermolecular interaction between the S atom in amino acids and the graphite C atom were determined. It was shown that an intramolecular H-bond influenced the adsorption of amino acids on the surface of graphitized thermal carbon black.     

The structures of small clusters of C60 molecules

Using a pairwise additive atom-atom intermolecular potential to describe the interaction between C₆₀ molecules, we calculated the lowest-energy structures of (C60)N clusters up to N = 15 and compared the results with predictions derived using Girifalco’s spherical potential. The cluster binding energies calculated on the basis of the former potential are in all cases significantly higher than those obtained from the latter. Moreover, the atom-atom potential predicts that small fullerene clusters have structures based on icosahedral packing, a finding which, for N = 14 and 15, contrasts with the results obtained using Girifalco’s approximation.     

Resonances in the electronic quenching of O(1D) by N2. A numerical quantum mechanical study for the collinear collision

The close coupled equations for the collinear collision O(¹D) + N₂ (¹Σ⁺_g) → O(³P) + N₂(¹Σ⁺_g) have been solved numerically for a model of two crossing potential curves assuming a constant spin-orbit coupling. Comparison between the results of an atom-atom like model and the converging results reveals a substantial (factor of ≈ 40) enhancement of the electronic quenching at room temperature together with high vibrational excitation of N₂. These results, and the study of the peaks appearing in the quenching probabilities as a function of the incident energy, clearly confirm that the high efficiency of this reaction is mainly due to resonances (quasibound states), as has been lately suggested.     

Diffusion Monte Carlo simulations on uracil-water using an anisotropic atom-atom potential model

We have developed an anisotropic atom-atom intermolecular potential model for the interaction of uracil with water. The potential consists of a distributed multipole analysis (DMA) model for the electrostatic energy, and a 6-exp potential to represent the repulsion-dispersion term. The repulsion-dispersion potential parameters are adjusted to yield good agreement with accurate ab initio data on the minima and transition states of the uracil-water complex. We have used this potential in diffusion Monte Carlo simulations of uracil-water, uracil-(water)2 and uracil-(water)3. The uracil-water simulations show that the theoretically based potential gives a qualitatively different picture of uracil hydration than that provided by a standard isotropic atom-atom point charge model, which is shown to underestimate the delocalized motion of the water hydrogen atoms. Plots of the vibrational probability density of the hydrogen atoms show the delocalized motion of the water hydrogen atoms that are not involved in hydrogen bonding.     

Interatomic voids in analysis of computer model structures of liquids and glasses

To analyze the structure of noncrystalline systems (liquids, glasses), it is suggested that the functions to be calculated should include not only the conventional atom-atom radial distribution function and structural factor, but also the same functions for a system of points representing the centers of interatomic voids. The points are located at the nodes of Voronoi polyhedra or, which is equivalent, at the centers of the spheres circumscribed around the Delaunay simplices. The system of points (system {D}) is unambiguously defined by the atomic coordinates and conveys additional information on the structure of the system because each interatomic void is determined by the relative positions of several atoms. Using a homogeneous noncrystalline packing model of atoms and inhomogeneous models derived from it as an example, we demonstrate that system {D} is sensitive to the structural features on medium scales and may be used along with the conventional atom-atom functions to study structural correlations other than short-range order correlations.     

Determination of the water geometry in violuric acid monohydrate with a Monte Carlo simulation

The positions and orientations of water molecules in violuric acid crystals have been determined with the Metropolis Monte Carlo method. The interaction potentials between water and violuric acid needed in the simulation have been developed using ab initio calculations corrected for the basis set superposition error with the C.P. technique and fitted to atom-atom pairwise potentials. The agreement between experimental data and simulation results is good, proving that the interaction potentials can be used with confidence.     

Thermodynamic characteristics of the adsorption of isomeric molecules of fluoroadamantanes on the surface of the basal face of graphite

The thermodynamic characteristics of adsorption (TCA) for isomeric molecules of mono-, di-, tri-, and tetrafluoroadamantanes were determined for the first time experimentally and by molecular statistics on the surface of graphitized thermal black (GTB). The parameters of atom-atom potential function of pair intermolecular interaction (φ(r)) for F atoms included in fluoroadamantanes with C atoms on the basal face of graphite were calculated for the first time within an atom-atom approximation of the semi-empirical molecular statistical theory of adsorption. The adsorption non-equivalence of F atoms in various positions of an adamantane framework, a consequence of the mutual effect of atoms in a framework molecule, was determined. On the basis of the definite TCA values, the conclusion was drawn as to the possibility of isolation of isomeric fluoroadamantanes under the conditions of gas-adsorption chromatography on GTB.     

Transition moment directions of some important in-plane vibrations of uracil,thymine and cytosine. A fixed partial charge model calculation

Transition moment directions for the double-bond region vibrations of uracil, thymine and cytosine are calculated using the fixed partial charge model approximation. The coupling between the two carbonyl bonds and the carbon—carbon double bond in uracil and thymine is sensitive to the choice of the corresponding stretching force constants. The transition moment directions are correspondingly changed, thus, in principle, enabling the quantitative determination of coupling between the vibrations from linear dichroism measurements.     

The method of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption

Different methods of introducing correction factors into parameters of atom-atom potentials of intermolecular interaction used for calculation of thermodynamic characteristics of adsorption by the semiempirical molecular statistical theory were compared. A method of isostructural fragments based on the application of the atom-atom potential corrected for the molecular fragment was suggested for introducing the correction factors. The advantages of this method were demonstrated for chlorobenzenes, chlorodioxines, and chlorobiphenyls. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 681–687, April, 2000.     

Calculation of henry constants for the adsorption of isomeric phenylenediamines on graphitized thermal carbon black

Henry constants for the adsorption of o- and p-phenylenediamines on the surface of graphitized thermal carbon black within the temperature range 433–479 K were calculated by the molecular statistical method. The parameters of the atom-atom potential function of intermolecular interaction between the nitrogen atom in aniline and isomeric phenylenediamines and the carbon atom of the basal face of graphite were determined. It was shown that an intramolecular H bond influenced the geometry and adsorption properties of o-phenylenediamine.     

Density functional theory for Yukawa fluids

We develop an approximate field theory for particles interacting with a generalized Yukawa potential. This theory improves and extends a previous splitting field theory, originally developed for counterions around a fixed charge distribution. The resulting theory bridges between the second virial approximation, which is accurate at low particle densities, and the mean-field approximation, accurate at high densities. We apply this theory to charged, screened ions in bulk solution, modeled to interact with a Yukawa potential; the theory is able to accurately reproduce the thermodynamic properties of the system over a broad range of conditions. The theory is also applied to "dressed counterions," interacting with a screened electrostatic potential, contained between charged plates. It is found to work well from the weak coupling to the strong coupling limits. The theory is able to reproduce the counterion profiles and force curves for closed and open systems obtained from Monte Carlo simulations.     

New theoretical insight into the interactions and properties of formic acid: development of a quantum-based pair potential for formic acid

We performed ab initio quantum-chemical studies for the development of intra- and intermolecular interaction potentials for formic acid for use in molecular-dynamics simulations of formic acid molecular crystal. The formic acid structures considered in the ab initio studies include both the cis and trans monomers which are the conformers that have been postulated as part of chains constituting liquid and crystal phases under extreme conditions. Although the cis to trans transformation is not energetically favored, the trans isomer was found as a component of stable gas-phase species. Our decomposition scheme for the interaction energy indicates that the hydrogen-bonded complexes are dominated by the Hartree-Fock forces while parallel clusters are stabilized by the electron correlation energy. The calculated three-body and higher interactions are found to be negligible, thus rationalizing the development of an atom-atom pair potential for formic acid based on high-level ab initio calculations of small formic acid clusters. Here we present an atom-atom pair potential that includes both intra- and inter molecular degrees of freedom for formic acid. The newly developed pair potential is used to examine formic acid in the condensed phase via molecular-dynamics simulations. The isothermal compression under hydrostatic pressure obtained from molecular-dynamics simulations is in good agreement with experiment. Further, the calculated equilibrium melting temperature is found to be in good agreement with experiment.