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Volume 103, Issue 6, 08 August 1995

Molecular dynamics simulation of infrared spectra for potassium palmitate B‐form crystal
View Description Hide DescriptionThe intra‐ and intermolecular potentials of potassium palmitate B‐form crystal were estimated by a normal modeanalysis and a molecular dynamics simulation. Based on these potentials, we calculated the time history of the dipolemoment in the nine unit cells (3a×3b) containing 18 molecules and obtained the polarized infrared spectra by a Fourier transformation. The frequencies and the intensities of the intense bands agreed well with the observed ones, and were consistent with the crystal structure.

Dipole‐bound excited states of the I^{−}⋅CH_{3}CN and I^{−}⋅(CH_{3}CN)_{2} ion–molecule complexes: Evidence for asymmetric solvation
View Description Hide DescriptionDipole‐bound excited states are reported for the I^{−}⋅CH_{3}CN and I^{−}⋅(CH_{3}CN)_{2} cluster ions, located just below their vertical electron detachment energies (determined using negative ion photoelectron spectroscopy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent molecules in the I^{−}⋅M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole‐bound state results exclusively in the dipole‐bound fragment ion, M^{−}. The photoelectron spectrum of the CH_{3}CN^{−} fragment was also recorded by sequential two‐photon absorption in the I^{−}⋅CH_{3}CN parent, indicating that the excess electron is indeed weakly bound (≤10 meV) with very little intramolecular distortion evident upon electron detachment. The I^{−}⋅(CH_{3}CN)_{2} cluster displays two absorption bands, one below each of the two features in the photoelectron spectrum. The most intense band correlates with the weaker, lower binding energy photoelectron band. Excitation of this strong absorption band results in production of both CH_{3}CN^{−} and (CH_{3}CN)^{−} _{2} fragments, while excitation of the weaker absorption band only results in electron detachment. We interpret these results in the context of two structural isomers for I^{−}⋅(CH_{3}CN)_{2}: one with the solvent molecules surrounding the I^{−} and another with both solvent molecules on the same side, asymmetrically solvating I^{−} in a configuration with a large electric dipole moment in the neutral cluster over this anionic geometry.

Dynamics of hydrogen‐bonded liquids confined to mesopores: A dielectric and neutron spectroscopy study
View Description Hide DescriptionIn this paper we present and discuss experimental results on molecular mobility in propylene glycol and its three oligomers confined to the ∼100 Å pores of a controlled porous glass. The objective is to elucidate the finite size effects on the dynamics of hydrogen‐bonded liquids of different molecular weights but identical chemical composition. The methods of dielectric and neutron spectroscopy have been employed to investigate both the low‐ and high‐frequency features as a function of temperature. We find that all fluids in pores separate into two distinct liquid phases. (i) molecules physisorbed at the surface which exhibit a dramatic frustration of their mobility related to a substantial positive shift of the glass transition temperature T _{ g } by up to ΔT _{ g }≊+47 K; and (ii) relatively ‘‘free’’ molecules in the inner pore space subject to only moderate retardation of the α and normal mode relaxation and substantial broadening of the distribution of relaxation times. The shift in T _{ g } for the α process with ΔT _{ g }≊+5 K is maximal for the monomer liquid and gradually diminishes with increasing molecular weight or decreasing intermolecular hydrogen bonding. The inelastic neutron spectrum of confined propylene glycol shows the boson peak as expected in bulk strong and intermediate glass formers in the vicinity of T _{ g }. This effect can be attributed to the finite‐size induced crossover from long wave vibrations characteristic of a continuous medium to localized vibrations in a confined geometry.

Pressure‐ and temperature‐variable viscosity dependencies of rotational correlation times for solitary water molecules in organic solvents
View Description Hide DescriptionDeuteron nuclear magnetic resonance spin‐lattice relaxation timesT _{1} have been measured for solitary water molecules (D_{2}O) at low concentrations in apolar and polar organic solvents at 30 °C at high pressures; D_{2}O (30 mM) in C_{6}H_{6} up to 90 MPa, D_{2}O (60 mM) in CHCl_{3} up to 300 MPa, D_{2}O (100 mM) in CH_{3}CN up to 300 MPa. The rotational correlation times τ_{2R } for D_{2}O in the organic solvents increase with increasing pressure. The pressure effect on τ_{2R } for D_{2}O in solution is considerably larger than that on τ_{2R } and η (viscosity) for the neat solvent. We have tested the two forms of modified Stokes–Einstein–Debye law; the linear and nonlinear forms are τ_{2R }=τ^{0} _{2R }+S(η/T) and τ_{2R }=B(η/T)^{α}, respectively. The rotational correlation times are linearly related to solventviscosity divided by temperature (η/T) with a large positive intercept (τ^{0} _{2R }≳0). It is shown that the linear form is practically better, and that the nonlinear form constrained at η/T=0 is invalid. The temperature‐variable slope (S _{ T }) and the pressure‐variable one (S _{ p }) are markedly different, the ratios of S _{ p } to S _{ T } being 0.2–0.3. The extended‐diffusion models based on isolated binary collisions cannot be used to explain the observed pressure effect because of the neglect of the attractive solute–solvent interactions.

Optical and radiationless intramolecular electron transitions in nonpolar fluids: Relative effects of induction and dispersion interactions
View Description Hide DescriptionA microscopic theory of intramolecular optical and radiationless electron transitions in nonpolar fluids is developed. The solute is modeled by a polarizable dipolar hard sphere, and the solvent by polarizable hard spheres. The effect of the induction and dispersion interactions to the spectral line shift and width are calculated as a perturbation expansion in the solute‐solvent attractions. The relative contributions of both these effects depend significantly on the solute size. Only for large solutes the dispersions are found to dominate the first order energy shift, while inductions become important if the solute size is comparable to that of the solvent molecules. If the solutedipole moment increases with excitation the dispersion and induction components of the first order spectral shift add up leading to a redshift. In the converse case (dipole moment decreasing) the two components have opposite signs, and the shift may switch from red to blue. Furthermore, both components cause the solvent reorganization energy to decrease sharply with the solute size. However, dispersions are of minor importance relative to inductions, for the parameter values used in this study. The linear correlation of the first order line shift with the solventdielectric function (ε_{∞}−1)/(ε_{∞}+2) of the dielectric constant ε_{∞} is traced back to a compensating effect of dispersions and inductions. The continuum theory is shown to overestimate the solvent response substantially. Both the solvent reorganization energy and the Stokes shift (the difference between absorption and fluorescence energies) are predicted to vary inversely with temperature. If not masked by intramolecular reorganization, this dependence can cause a maximum in the Arrhenius coordinates for electron transfer rates in the near‐to‐activationless region.

Photoelectron spectroscopy of silicon–carbon cluster anions (Si_{ n }C^{−} _{ m })
View Description Hide DescriptionPhotoelectron spectra of Si_{ n }C^{−} _{ m } cluster anions (1≤n≤7 and 1≤m≤5) were measured at the photon energies of 3.49 eV, by using a magnetic bottle electron spectrometer. The Si_{ n }C^{−} _{ m } clusters were produced either by a laser vaporization of a silicon–carbon mixed rod or by two laser vaporizations of carbon and silicon rods in a He carrier gas. The spectra of the Si_{ n }C^{−} _{1} (3≤n≤7) clusters are similar to those of pure Si^{−} _{ n+1} clusters in the peak positions and their envelopes, which is attributed to the isovalent electronic structure of Si and C atoms as well as to the similar geometricalstructure. In contrast, the similarity in the photoelectron spectra is not observed between C^{−} _{ m+1} and Si_{1}C^{−} _{ m } (2≤m≤5) clusters, which is attributed to a change in their geometry; from chain to ring. These experimental conclusions are discussed with the results of our theoretical calculations.

Logic gates in excitable media
View Description Hide DescriptionThe interaction of chemical waves propagating through capillary tubes is studied experimentally and numerically. Certain combinations of two or more tubes give rise to logic gates based on input and output signals in the form of chemical waves and wave initiations. The geometrical configuration, the temporal synchronization of the waves, and the ratio of the tube radius to the critical radius of the excitable medium determine the features of the logic gates.

Quantum scattering studies of the Λ doublet resolved rotational energy transfer of OH(X ^{2}Π) in collisions with He and Ar
View Description Hide DescriptionThree dimensional potential energy surfaces for the collision systems OH(X ^{2}Π)+He and OH(X ^{2}Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate ^{2}Π_{ x } and ^{2}Π_{ y } states split into two states of ^{2} A′ and ^{2} A″ symmetry, respectively, when the C _{∞v } symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close‐coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet resolution show, within the same spin–orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F _{1}(^{2}Π_{3/2}) state and an e/f conserving propensity in the F _{2}(^{2}Π_{1/2}) state, while transitions between the two spin–orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen.

Statistical adiabatic channel calculation of accurate low‐temperature rate constants for the recombination of OH radicals in their ground rovibronic state
View Description Hide DescriptionAccurate low‐energy capture cross sections and low‐temperature capture rate constants for two OH radicals in their ground rovibronic states X ^{2}Π_{3/2}(v=0, j=3/2) were calculated within the statistical adiabatic channel approach. The rate constants calculated in first order provide a good approximation to the true rate constant below 4 K. The rate constants calculated in second order provide a correction of about 25% to the first order rate constant at 20 K and indicate an only weak temperature dependence at T≳20 K. At higher temperatures deviation of the potential from long‐range electrostatic interaction have to be accounted for.

Competition between energy and phase relaxation in electronic curve crossing processes
View Description Hide DescriptionWe present results from simulations of vibrational energy and phase relaxation and electronic curve crossing using a multilevel formulation of Redfield theory, which demonstrate the shortcomings of the optical Bloch approximation and the importance of coherence transfer processes in the relaxation dynamics of multilevel systems. Specifically, we show that for a harmonic well, energy relaxation can occur with retention of vibrational phase, and that for sufficiently strong electronic coupling, the product of an electronic curve crossing process can be formed vibrationally coherent even when no coherence is present in the initially excited state.

Density functional theory calculations of the reaction pathway for methane activation on a gallium site in metal exchanged ZSM‐5
View Description Hide DescriptionDensity functional theory is used to describe the reaction profile for methanedissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.

Ab initio potential energy surfaces for the two lowest ^{1} A′ states of H^{+} _{3}
View Description Hide DescriptionThree‐dimensional potential energy surfaces of H^{+} _{3} in the two lowest ^{1} A′ electronic states have been calculated by the full configuration interaction method with a [8s6p2d1f] Gaussian‐type basis set. The features of the avoided crossing of two surfaces as well as the energy minimum of the ^{1} A′ ground state have been produced by the potential calculation at 680 different spatial geometries. These surfaces should be useful for the detailed studies of charge transfer and chemical reaction in the H^{+} and H_{2} collisions.

Ab initio calculation of nonadiabatic couplings using MELD
View Description Hide DescriptionWe present a numerical differentiation technique for the calculation of directional derivatives of electronic wave functions evaluated using a multireference configuration interaction method. The technique has been implemented in the set of programs MELD, and its application is illustrated by calculations of nonadiabatic couplings for the ArH^{+} _{2}quasimolecule.

Ab initio model potential embedded‐cluster study of V^{2+}‐doped fluoroperovskites: Effects of different hosts on the local distortion and electronic structure of ^{4} T _{2g }–^{4} A _{2g } laser levels
View Description Hide DescriptionIn this paper we present the results of ab initio model potential (AIMP) embedded‐cluster calculations on the ground ^{4} A _{2g } and excited ^{4} T _{2g } state levels of V^{2+}‐doped KMgF_{3}, KZnF_{3}, KCdF_{3}, and CsCaF_{3}. Complete active space SCF (CASSCF) and averaged coupled‐pair functional (ACPF) calculations are performed on the (VF_{6})^{4−} embedded cluster. The AIMP embedding potentials represent both static and relaxed/polarized lattice effects which are found to be an important refinement due to the large local distortions produced by the V^{2+} impurity. The calculated local distortions are found to be considerably large, but much smaller than expected in terms of the mismatch of ionic radius of the impurity and the substituted cation. The host dependency of the crystal field splitting, which was found to violate the simple ligand fieldR ^{−5}‐law in a wide family of V^{2+}‐doped halide crystals, if R is the metal–ligand distance in the host, is also examined and is found to be quite close to the simple ligand fieldtheory prediction, as long as the calculated impurity‐ligand distances are considered. The results of the ^{4} A _{2g }→^{4} T _{2g } absorptions are in close agreement with available experimental data. The comparatively high values of the calculated ^{4} T _{2g }→^{4} A _{2g }fluorescence indicate the need of the inclusion of intracluster Jahn–Teller coupling of the excited state. However, the host variation of fluorescence is, again, well reproduced.

Interchain interactions in one‐dimensional periodic systems: An analysis of second‐order effects causing deformation
View Description Hide DescriptionThe role of second‐order perturbations in interchain interactions of one‐dimensional electronic systems is examined. The general features of such interchain interactions are deduced from a simple two‐chain model. From a perturbation‐theoretic analysis, the second‐order term originating from two different bands near the Fermi level leads to an important out‐of‐phase coupling of charge‐density‐waves on neighboring chains. The preferred distortion is predicted for various electron counts, using a transition‐density analysis. Specific geometrical conclusions are derived for polyacene systems.

Nanophase coexistence and sieving in binary mixtures confined between corrugated walls
View Description Hide DescriptionThe grand canonical Monte Carlo method is used to study a binary mixture of Lennard‐Jones atoms confined to a corrugated slit micropore which is in thermodynamic equilibrium with its bulk phase counterpart. The micropore has atomically structured walls; one of the which possesses nanoscale structure in the form of rectilinear grooves (corrugation). The grooved surface divides the confined fluid film into two strip shaped regions, that inside and that outside the grooves. Transverse solidlike order in the film gives rise to shear stress. Transverse order coupled with packing restrictions give rise to a difference between the pore and bulk fluid mixture compositions. Solidlike order may appear within the grooves only, outside the grooves only, or in both regions simultaneously. As the relative alignment of the walls is shifted the pore fluid undergoes freeze–thaw cycles in one or both regions with associated changes in the shear stress and pore fluid composition. The degree of transverse order in the film is less than would be expected in a pure Lennard‐Jones film and fluid‐solid phase transitions are gradual as opposed to sudden as seen in pure Lennard‐Jones films. The magnitude of the shear stress is greatest when a fluid–solid phase transition occurs in both regions of the pore.

Self‐consistent integral‐equation theory of chain‐molecular liquids: Structure and thermodynamics
View Description Hide DescriptionSelf‐consistent integral equations for the pair intramolecular and intermolecular correlation functions are derived from a general hierarchy of integral equations for chain‐molecular liquids. These coupled equations are obtained by using superposition approximations for the triplet correlation functions, an approximate translational symmetry for the site–site intramolecular correlation functions and the equivalence of sites for intermolecular correlation functions. In addition to this self‐consistent set of integral equations, the polymer reference interaction site model (PRISM)integral equation is also made self‐consistent by coupling this intermolecular equation to the equations for the intramolecular correlation functions derived in the present theory. The intra‐ and intermolecular correlation functions of the self‐consistent schemes considered in this work obey integral equations, and they are different from the other self‐consistent schemes proposed in the literature. Self‐consistent solutions for the structuralproperties, such as intra‐ and intermolecular correlation functions and structure factor, and macroscopic properties, such as chain expansion factor and thermodynamic functions of athermal polymer melts, are compared with available Monte Carlo results and other theories. For the properties examined, self‐consistent solutions yield better results than the non‐self‐consistent calculations with ad hoc, ideal Gaussian inputs for the intramolecular correlation functions.

Chemical structure effects on the equilibrium and under shear properties of thin films in confined geometries: A molecular dynamics simulation study
View Description Hide DescriptionWe have made a comparative study of confined thin fluid films, composed of either n‐decane or 4‐propyl‐heptane. The films are studied in equilibrium and under shear using molecular dynamics (MD) simulations. The films composed of linear chains present density profiles of methylene subunits with higher degree of layering than those composed of branched molecules. There are no significant differences in the diffusion coefficients of the two molecules studied in bulk, or in confined geometries. The diffusion coefficients for the confined films are strongly dependent on the strength of the frictional forces exerted by the wall, rather than on the density of the films. They also indicate that the confined films remain in a fluidlike state in all the simulations. The bulk values of the diffusion coefficient of n‐decane are in excellent agreement with the experimental data. When the confining walls move in opposite directions, the fluid films develop shear flow with a very weak shear rate. Superimposed to the shear flow, the films seem to perform an oscillatory motion, where they alternately flow following the direction of motion of either wall. The steady state values of the shear stress increase linearly with the pressure normal to the confining walls, as also found experimentally. The films composed of linear chains exhibit higher resistance to the displacement of the walls than those composed of branched chains. This is because the films composed of linear chains have higher density of methylene subunits in the region of the pore where the fluid molecules exert frictional forces on the walls.

On the application of instantaneous normal mode analysis to long time dynamics of liquids
View Description Hide DescriptionWhile the applicability of instantaneous normal mode (INM) analysis of liquids to short time dynamics is in principle obvious, its relevance to long time dynamics is not clear. Recent attempts by Keyes and co‐workers to apply information obtained from this analysis to self‐diffusion in supercooled liquid argon is critically analyzed. By extending the range of frequencies studied we show that both imaginary and real branches of the density of modes are represented better, for large ω, by ln[ρ(ω)]∼ω^{2}/T than by ln[ρ(ω)]∼ω^{4}/T ^{2} as advocated by Keyes [J. Chem. Phys. 101, 5081 (1994)]. However, since in the relevant frequency range the two fits almost overlap, the numerical results obtained by Keyes, showing good agreement with the simulation results for self‐diffusion in supercooled liquid argon, remain valid even though implications for the frequency dependence of the barrier height distribution change. We also explore other possibilities for extracting information from the INM analysis: (1) The density of ‘‘zero force modes,’’ defined as the distribution of normal modes found at the bottom or top of their parabolic potential surfaces, can be computed with no appreciable additional numerical effort. This distribution provides a better representation than the total density of modes for the normal mode distribution at well bottoms and at saddles, however, we find that it makes little difference in quantitative analysis. (2) We suggest that the ratio ρ_{ u }(ω)/ρ_{ s }(ω) between the density of modes in the unstable and stable branches provide an estimate for the averaged barrier height distribution for large ω. Using this estimate in a transition state theory calculation of the average hopping time between locally stable liquid configurations and using the resulting time in a calculation of the self‐diffusion coefficient yields a very good agreement with results of numerical simulation.

Frequency spectra of two‐band fluids and fluid mixtures: Mean spherical approximation and beyond
View Description Hide DescriptionIn the framework of a recently proposed approximation, we investigate here the frequency spectra of two‐band fluids (fluids composed of particles with two independent Drude oscillators embedded) as well as fluid mixtures of particles with one Drude oscillator. Both cases are analyzed in the low density regime where departures from the linear theories are more evident. Our theory, which goes beyond the mean spherical approximation (MSA), reproduces the correct low density spectra while retaining the proper qualitative behavior of the MSA at fluid densities.