bluuebunny/crossref_metadata_2025 huggingface dataset:
On the separation of static and dynamic solvent effects for electron transfer reactions
Authors: Cukier R. I. , Nocera D. G.
Published: July 2002

Recent studies of electron transfer (ET) reactions in polar solvents have shown that solvent dynamics, in addition to solvent statics, can have a dramatic effect on the rate. ET reactions were analyzed with the aim of suggesting methods that permit a separation of solvent static and dynamic effects on the rate. Two schemes that can separate these effects are (1) the measurement of ET charge separation (typically a normal regime reaction) and charge recombination (typically an inverted regime reaction) rates in the same solvent and (2) the variation of the static (transition state) rate in a given solvent, as induced by altering the electronic coupling via a series of donor–acceptor complexes at different fixed distances. To emphasize dynamical effects, the static rate contribution should be large, and therefore in the adiabatic, strong electronic coupling limit of Marcus theory. We obtain an expression for the solvent dynamically influenced rate, for the inverted ET regime, which can describe this limit as well as all degrees of electronic coupling. It has the form of a consecutive reaction scheme, the steps being diffusion along the reaction coordinate followed by surface crossing (the static rate contribution), as was obtained previously for the nonadiabatic static rate case.
Phonon relaxation processes in molecular crystals (CS2) at high pressure and low temperature
Authors: Jodl H.-J. , Jordan M. , Däufer H.
Published: July 2002

Solid CS2 is investigated in a diamond anvil cell in the pressure range of 0.4–2.4 GPa at 20 K by means of high resolution Raman spectroscopy. The pressure dependent linewidths of several selected modes are determined and discussed in the framework of anharmonic lattice dynamics considering depopulation and dephasing processes which are influenced by anharmonic terms in the crystal potential and multiphonon density of states. The relaxation of Ω− at high pressure is mainly governed by a four phonon down process via ν2 and two lattice phonons, whereas Ω+ decays in a state of the two phonon band directly.
On the use of bond functions in molecular calculations
Authors: Tao Fu-Ming
Published: July 2002

The effect of bond functions and the related problems are studied by performing the fourth-order Mo/ller–Plesset perturbation calculations for the dissociation energy of the ground state F2 molecule, using a series of basis sets systematically extended with polarization functions and bond functions. The results show the usefulness of bond functions if the basis set superposition errors (BSSE) are corrected by the counterpoise method. A new interpretation of BSSE effect and the counterpoise method is given.
The structure and dynamics of solid benzene. I. A neutron powder diffraction study of deuterated benzene from 4 K to the melting point
Authors: Craven C. J. , Hatton P. D. , Howard C. J. , Pawley G. S.
Published: July 2002

The crystal structure of deuterated benzene has been investigated over the complete temperature range 4–280 K. Our results are in excellent agreement with previous studies, recorded at a limited number of temperatures, and display a smooth variation of the lattice parameters over the complete temperature range. No evidence of the suggested low-temperature phase transitions, or marked premelting effects have been found. Detailed measurements of the lattice parameters close to the melting point display an anomalous behavior of the b lattice parameter and the orientation of the molecules which is believed to be evidence of a disordering transition at temperatures above the melting point.
Size-dependent photodissociation cross sections for Sr+(NH3)n, n=3–6: Rydberg state formation and electron transfer
Authors: Donnelly S. G. , Farrar J. M.
Published: July 2002

We report photodissociation cross sections for the mass-selected cluster ions Sr+(NH3)n, with n in the range from 3 to 6. The cross sections exhibit large redshifts that increase monotonically with increasing cluster size. For the n=3 cluster, the absorption cross section peaks near 700 nm, while the n=6 cluster spectrum appears to reach a maximum at 1.5 μm, in the near infrared region of the spectrum. A spectral moment analysis of the cross sections shows that <r2≳ for the ground state electronic radial distribution increases by an order of magnitude over the cluster size range from 1 to 6 solvent molecules. In conjunction with path integral Monte Carlo calculations carried out by Martyna and Klein, we argue that the increase in <r2≳ is diagnostic of the increasing Rydberg character of cluster states that correlate with 5s and 5p atomic orbitals on Sr+. An analysis of the charge transfer to solvent process as described by a thermodynamic Born cycle suggests that the rapid stabilization of solvated ion-pair states with increasing cluster size provides a plausible mechanism for introducing Rydberg character into cluster valence states based on 5s/5p atomic ion orbitals.
The general treatment of superexchange versus sequential electron transfer in a three-component system
Authors: Tang J. , Wang Z. , Norris J. R.
Published: July 2002

The mechanisms of superexchange vs sequential electron transfer in a three-component system involving reactant, intermediate, and product are investigated by using the Zusman equation. This approach is nonperturbative with regard to the electronic coupling and thus, in principle, the contributions from all higher order terms are included. This treatment allows examination of a wide range of conditions from nondegenerate to degenerate cases, as well as from nonadiabatic to adiabatic cases for electron transfer processes involving three Marcus parabolas such as believed to occur in photosynthetic reaction centers. New effects of superexchange on electron-transfer rate resulting from degenerate energy-level crossings are predicted.
Unimolecular dissociation in the regular regime
Authors: Yurtsever Ersin , Günay Hülya , Uzer T.
Published: July 2002

The unimolecular dissociation of a model triatomic molecule is analyzed. The model, which is loosely based on the radical HO2, shows mostly regular dynamical behavior although it has enough energy to dissociate. By analyzing the progress towards dissociation, we find that dissociation takes place due to concerted and correlated motions as in the Slater theory of unimolecular reactions rather than the steady accumulation of energy in the dissociative mode.
The vacuum ultraviolet excited electronic states of 1,3-butadiene: Selective enhancement of vibrational modes in resonant Raman transitions
Authors: Strahan Gary D. , Hudson Bruce S.
Published: July 2002

The assignments of many of the vacuum ultraviolet valence and Rydberg states of trans-1,3-butadiene are investigated using resonance Raman spectroscopy which provides data on distortions and vibronic activity. The 1Bg(π-3s) Rydberg state (50 144 cm−1) is confirmed to be vibronically coupled to the 1 1Bu(ππ*) valence state by modes of au symmetry, especially ν12 (CH2 twist). The higher energy Rydberg states were also examined and an analysis was made of both their assignments and their behavior when resonantly enhanced. Selective excitation into the sharp vibrational structure of these Rydberg states provides a striking example of mode specific resonance enhancement. Evidence was sought for the location of the high energy 1Ag (ππ*) state, said to be near 60 000 cm−1, but the results were indefinite.
Determination of hyperfine interaction matrix principal values and principal axis orientations in an orientationally disordered solid: A multifrequency electron spin echo envelope modulation study of nitrogen-15 in a copper(II)–15N–imidazole complex
Authors: Gerfen Gary J. , Singel David J.
Published: July 2002

A comprehensive experimental study of the magnetic field dependences of ESEEM (electron spin echo envelope modulation) for a spin one-half nucleus in an orientationally disordered solid is presented. Modulation effects from the remote (unligated) nitrogen of 15N-labeled imidazole in a Cu(II)–diethylenetriamine–15N-imidazole complex were registered at electron spin excitation frequencies in the 4.3 to 11.5 GHz range, which encompasses the ‘‘match range’’ of the Cu(II)–15N system under study. Field dependent trends in the ESEEM spectra—trends in spectral amplitudes, linewidths, and frequencies—are analyzed to obtain the magnitude and relative signs of the isotropic and axially symmetric hyperfine coupling constants (and to place an upper bound on the rhombic coupling). The relative utility of each of these trends for measuring hyperfine interactions in orientationally disordered solids is discussed. The orientation of the unique axis of the Cu(II)–15N hyperfine interaction matrix within the Cu(II) electron spin magnetic axis system is obtained from the field dependence of the ESEEM frequencies observed with orientationally selective excitation. The results of this study are compared to those previously obtained for relevant Cu(II)–15N systems by a variety of monofrequency ESEEM techniques.
Transient infrared spectroscopy of (η5−C5 H5)Co(CO)2 photoproduct reactions in hydrocarbon solutions
Authors: Dougherty T. P. , Heilweil E. J.
Published: July 2002

Ultrafast photochemistry of CpCo(CO)2 in n-hexane, 1-hexene, and solvent mixtures (298 K) are reported. Multichannel infrared spectra (0.4 ps, 4 cm−1 resolution) identify the vibrational cooling of CpCo(CO)(n-hexane) and CpCo(CO)(1-hexene). The conversion rate of transient n-hexane species to stable 1-hexene product for 0.2–2.0 mol/liter 1-hexene/n-hexane solutions (k=1.4±0.2×1010 M−1 s−1) was determined.
Response to ‘‘Comment on ‘Molecular dynamics of the surface tension of a drop’ ’’ [J. Chem. Phys. 1

, 7844 (1994)]
Authors: Nijmeijer M. J. P. , Bruin C. , van Leeuwen J. M. J.
Published: July 2002

Our method to obtain the Tolman length from simulations of liquid drops has been criticized for its numerical inaccurateness and an alternative has been proposed. We argue that the alternative is incorrect on theoretical grounds while furthermore the resulting Tolman length conflicts with our results. Finally, we take this opportunity to reanalyze our data in view of recent, related simulations of planar liquid–vapor interfaces.
Effective core potential-configuration interaction study of electronic structure and geometry of small anionic Agn clusters: Predictions and interpretation of photodetachment spectra
Authors: Bonačić-Koutecký V. , Češpiva L. , Fantucci P. , Pittner J. , Koutecký J.
Published: July 2002

The ground state geometries of small anionic Ag−n(n=2–9) clusters were determined in the framework of the Hartree–Fock procedure employing a relativistic effective core potential (RECP) accounting for core–valence correlation (CVC) effects. Large scale configuration interaction (CI) calculations for 5s electrons only were carried out in order to determine the ground state energies of anionic and neutral species as well as of excited states of the latter in the geometries of the former. The calculated vertical detachment and excitation energies account for the observed photodetachment spectroscopic patterns and permit an assignment of the cluster geometries. Structural and electronic properties of small Ia and Ib anionic clusters are compared.
Raman scattering by high-density dispersions. III. Calculation of Raman intensities for molecules physisorbed on linear chains of spherical particles
Authors: Xu Muliang , Dignam Michael J.
Published: July 2002

In parts I and II of this series, the dominant role of ‘‘primary’’ Raman moments of degree greater than one, to Raman intensities for molecules forming a uniform coating on pairs of spherical particles in close proximity, is demonstrated. In this paper, the effect of chain length, coverage, particle size, and separation, and the location of the particle in the chain on the intensity are explored for linear chains of coated particles. Intensity enhancements calculated for visible excitation are up to 30 times those for two particles, being ∼4×106 for infinite chains of silver particles of radius 25–50 Å separated by the thickness of a layer of CO ∼105 for platinum and ∼104 for germanium chains. Furthermore, the enhancement for chains falls off appreciably more slowly with particle separation. Graphs of intensity vs coverage vary greatly in shape depending on particle size, molecular size, and polarizability.
Two-step melting of 2-propanol adsorbed on graphite
Authors: Morishige K. , Kobayashi N.
Published: July 2002

The x-ray diffraction measurements of 2-propanol adsorbed on graphite have been taken over the temperature range 150–240 K and the coverage range 2.28–4.23 molecules/nm2. The analysis of the data by means of the least-squares fitting of the complete pattern suggests that 2-propanol dimers bound by hydrogen bonding are arranged to give a lamellar structure of the molecules on the surface. The melting of the solid monolayers occurs by a two-step process involving an intermediate phase, possibly a defective two-dimensional smectic liquid crystal. The results are compared with the melting behavior observed for the monolayers of n-propanol.
New measurement method for the quasielastic light scattering with the sinusoidal electric field by use of an extended Wiener–Khinchin theorem
Authors: Ito Kohzo , Ooi Shuichi , Kimura Yasuyuki , Hayakawa Reinosuke
Published: July 2002

A new method for measuring the power spectrum of the quasielastic light scattering with the sinusoidal electric field (QELS-SEF) has been developed on the basis of a Wiener–Khinchin theorem extended to the periodically stationary systems such as QELS-SEF. The extended theorem enables us to obtain the power spectrum directly from the scattered light intensity without calculating the autocorrelation function.
Time-resolved electron paramagnetic resonance spectra of photoexcited triplet states of electron–donor–acceptor complexes in frozen solution: Methylated benzenes and chlorinated phthalic anhydrides
Authors: Murai Hisao , Minami Masashi , I’Haya Yasumasa J.
Published: July 2002

The excited triplet states of phthalic anhydride and its chlorinated derivatives in methyl substituted benzene derivatives were studied at 77 K by a time-resolved EPR technique. The increase of the number of the methyl group on the benzene decreased the zero-field splitting parameters of their triplet states. This was explained by the contribution of the electron–donor–acceptor (EDA) complex. Especially, in the case of tetrachlorophthalic anhydride in hexamethylbenzene, the apparent inversion of the spin-polarization was observed. These data evidenced the negative sign of the D value in highly charge-transferred (CT) complex. This experiment reveals that the employment of D value instead of D* provides much better estimation of the CT character of the EDA complex.
The Zimm model applied to extended single polymers
Authors: Quake Stephen R.
Published: July 2002

A mean field theory is developed for the Zimm model and compared to the exact solution, computed numerically, for the case of a single extended polymer. An analytic calculation in the dumbbell approximation extends the preaveraging treatment. We find that the dynamic scaling of the preaveraged Zimm model is preserved for short polymers, but that the relaxation function is inhomogeneous in length for long polymers. These calculations can be applied to recent experimental results.
Phonon relaxation processes in crystals (NaNO3) at high pressure and low temperature
Authors: Jordan M. , Schuch A. , Righini R. , Signorini G. F. , Jodl H.-J.
Published: July 2002

NaNO3 is investigated in a diamond anvil cell in the pressure range of 0–9 GPa at 21 and 142 K by means of high-resolution Raman spectroscopy (HRRS). The pressure dependent linewidth of ν1 (symmetric stretch) is determined and discussed in the framework of anharmonic lattice dynamics. The main relaxation pathways are depopulation processes which are influenced by anharmonic terms in the expansion of the crystal potential and by multiphonon densities of states. The interpretation is supported by numerical calculation of multiphonon densities of states.
Spectral quantization of transition state dynamics for the three-dimensional H+H2 reaction
Authors: Skodje Rex T. , Sadeghi R. , Köppel Horst , Krause Jeffrey L.
Published: July 2002

Applying a spectral quantization method, we find the positions and widths of 32 transition state resonances in the three-dimensional reaction H+H2 with J=0. The assignment of many of the resonances appears to follow asymmetric stretch and bend progressions for a linear triatomic molecule.
Excited states and photochemical reactivity of fulvene. A theoretical study
Authors: Dreyer Jens , Klessinger Martin
Published: July 2002

Semiempirical and ab initio calculations including electron correlation have been carried out for the ground and excited states of fulvene. Vertical singlet and triplet excitation energies including several Rydberg transitions, which have been calculated using an extended basis set augmented with even tempered diffuse functions, are in good agreement with experiment. Spectroscopic minima were obtained for the 1A1(S2) and 3B2(T1) states of planar fulvene, and on the semiempirical level also for the 1A″(S1) state, which is only of Cs symmetry. Twisting of the exocyclic double bond was studied by means of correlation diagrams as well as by calculating potential energy hypersurfaces. At 90° twist all stationary points are identified as transition states at the ab initio level, while two minima (S0 and S2) and one transition state (T1) result from the semiempirical calculations. Taking into account pyramidalization of the exocyclic methylene group yields a minimum for the T1 state and a conical intersection of the S1 and S0 states, which is not identical with the one that is to be expected for a critically heterosymmetric biradicaloid. The photochemistry of fulvene is being discussed in the light of the present results.
Vibrational product state dynamics in solution phase bimolecular reactions: Transient infrared study of CN radical reactions
Authors: Raftery Daniel , Gooding Edward , Romanovsky Alexander , Hochstrasser Robin M.
Published: July 2002

Solution phase bimolecular reactions of the CN radical are observed using transient infrared methods. Photolysis of ICN in chloroform produces CN radicals that react after thermalization to form HCN and ClCN as products. In the deuterated solvent, some of the nascent DCN molecules are formed in a vibrationally excited level. This constitutes the first observation of vibrationally excited products subsequent to a bimolecular reaction in solution. Temperature-dependent measurements indicate that the barrier to reaction is low but that solvent friction is important in the reaction. The observed product state energy partitioning and slow reaction rates indicate that the dynamics of reactive barrier crossings are significantly altered upon solvation.
Conformational dynamics in bulk polyethylene: A molecular dynamics simulation study
Authors: Boyd Richard H. , Gee Richard H. , Han Jie , Jin Yong
Published: July 2002

Molecular dynamics (MD) simulation has been used to study chain dynamics in bulk polyethylene over a wide range of temperature. This range includes the volumetric glass transition. The latter was determined from the volume temperature relation established via constant pressure MD runs. Conformational transition rates were monitored over the temperature range and found to remain Arrhenius in nature even through the glass transition. The activation energy was found to correspond to a single torsional barrier. Correlation of transitions was monitored as well and, in common with Brownian dynamics simulations of solutions and previous bulk simulations, ±2 neighbor correlations were found to be common. Several new types however were identified. In addition, it was found that ±0 self correlation becomes increasingly important as temperature is lowered. In contrast to the conformational transition rates, the relaxation times for decay of the torsional angle autocorrelation function were found to display non-Arrhenius Williams–Landel–Ferry (WLF) Vogel–Fulcher temperature dependence. The disparity between the relaxation time and conformational transition behavior was traced to the spatial distribution over the bonds of the conformational jumps becoming increasingly heterogeneous as temperature decreases. This in turn is attributed to the bulk packing causing bonds to be trapped at and oscillate about torsional angles away from the torsional energy minima.
Temperature dependence of the 14N quadrupole coupling constant of isocyanomethane
Authors: Stringfellow Thomas C. , Farrar Thomas C.
Published: July 2002

We report NMR relaxation time measurements of the 14N quadrupole coupling parameter χN of isocyanomethane as a function of temperature in the solution-state mixture of 6.4 mol %  H3CN ≡ C, 33.5 mol% d6-ethylene glycol, and 60.1 mol % d6-ethanol. We obtain values for the molecular correlation times using the T2/T1 method, which is a function only of the spectral density functions. The results demonstrate a significant, and approximately quadratic, temperature dependence of χN, which shows a minimum value of about 55 kHz at 222 K; the largest value measured was 125 kHz at 161 K. The results are compared with, and discussed within the context of, more traditional methods. The value of χN for neat isocyanomethane in the solid phase was measured to be 26.3 kHz.
Effects of velocity changing collisions on line shapes of HF in Ar
Authors: Demeio Lucio , Green Sheldon , Monchick Louis
Published: July 2002

The generalized Hess method (GHM) gives a line shape expression which is formally equivalent to the Rautian–Sobel’man hard collision model of Dicke narrowing, but differs radically in the definition of one of the relaxation terms. The relaxation term leading to pressure broadening is the same, but the term leading to Dicke narrowing and ultimately to Doppler line shapes at zero density differs in certain important respects: (1) in GHM it is a weighted sum of the pressure broadening coefficient and an optical diffusion coefficient and (2) there is no sharp distinction between ‘‘velocity changing’’ and ‘‘phase changing’’ collisions. The Dicke narrowing term should thus be understood as including both collision types irretrievably intermixed, with GHM providing a prescription for both relaxation terms. Applied to HF v=0→1, j→j±1 absorption spectra in a bath of Ar and using an accurate interaction potential obtained from spectra of the van der Waals complex and essentially exact close coupling scattering S matrices, GHM provides a rather good description of recently measured line shapes.
Control of long time dependence of CO binding to heme systems by inhomogeneous spread in electronic prefactor
Authors: Sungar N. , Gerstman B. S.
Published: July 2002

Recent work has emphasized that the rebinding kinetics of ligands to heme systems can exhibit nonexponential kinetics below the glass transition due to inhomogeneous spreads in factors other than enthalpy. We show that an expected distribution in the nonadiabatic electronic prefactor will lead to a t−1/2 dependence for the rebinding rate at long times. This will occur at temperatures below the glass transition but for T≳1/2αR, where α characterizes the enthalpy spread and R is the gas constant. We show that this spread in prefactor explains the long time behavior in the appropriate temperature range for Mb–CO and Mb3–CO.
Theoretical study of the valence-shell photoelectron spectra of cis-, trans-, and iso-butene
Authors: Galasso V.
Published: July 2002

The valence-shell ionization potentials of cis, trans, and iso-butene have been calculated using ab initio Green function and cluster-expansion based formalisms. The theoretical results yield a very satisfactory description for the general features observed in both the outer- and inner-valence regions of the photoelectron spectra. In the inner-valence region, the computed spectra consist of a large number of peaks due to mixing between single ionizations and simultaneous shake-up configurations, in agreement with experimental evidence gained from synchrotron radiation and (e,2e) spectra.
Microwave spectrum of alkali metal tetrahydroborate. III. Rotational transitions of LiBD4, NaBD4, and KBH4 in the ground vibrational states and molecular structures of MBH4 (M=Li, Na, and K)
Authors: Kawashima Yoshiyuki , Hirota Eizi
Published: July 2002

In order to determine the molecular structure of alkali metal tetrahydroborates in a systematic way, we have extended the measurements of rotational transitions to LiBD4, NaBD4, and KBH4 in the ground vibrational states. The observed spectra, which all conformed well to the pattern expected for a symmetric top molecule, yielded rotational and centrifugal distortion constants for 7Li11BD4, 7Li10BD4, 6Li11BD4, 6Li10BD4, Na11BD4, Na10BD4, 39K11BH4, 39K10BH4, and 41K11BH4. The four observed rotational constants of LiBD4 gave rs(Li–B) to be 1.931 09(14) Å, which, when combined with an assumption that θ(Db–B–Dt)=113.0°, led to r(B–Db) and r(B–Dt) to be 1.250±0.025 Å and 1.212∓0.032 Å, respectively, where the uncertainties of the B–D distances are primarily due to that of θ(Db–B–Dt) estimated to be ∓1.0°. [The suffixes attached to the deuterium atom, b and t, denote bridge and terminal, respectively.] The rs(Li–B) distance in LiBD4 is significantly shorter than that in LiBH4, 1.939 38(10) Å.

This large secondary isotope effect is ascribed partly to the large amplitude rocking (or internal rotation) motion of the BH4 group. By fixing the Li–B distance to the respective rs values, all of the eight rotational constants of LiBH4 and LiBD4 were simultaneously analyzed to determine the structure of the BH4 group, where the isotope effect was taken into account only for the B–H distances in a form δ=r(B–H)–r(B–D). Again θ(Hb–B–Ht)=θ(Db–B–Dt) had to be fixed to 113.0∓1.0°. The isotope shift δ was found to be not very dependent on the value of θ and was determined to be 0.006 26(6) Å. The two B–H distances were obtained to be r(B–Hb)=1.257±0.025 Å and r(B–Ht)=1.218∓0.032 Å. The data on NaBD4 were analyzed by assuming θ(Db–B–Dt)=111.0∓1.0° and r(B–Db)−r(B–Dt)=0.04(I) or 0.03(II) Å, yielding r(Na–B)=2.2978(I) or 2.2987(II)±0.0055 Å and r(B–Db)=1.269(I) or 1.258(II)∓0.040 Å. An alternative way of the analysis is to combine the data on the H and D isotopic species; the assumptions on θ(Hb–B–Ht)=θ(Db–B–Dt)=111.0∓1.0° and on the difference r(B–Hb or Db)–r(B–Ht or Dt)=0.04(I) or 0.03(II) Å lead to the following results: r(Na–B)=2.3075(I) or 2.3080(II)±0.0028 Å, r(Na–B) (in NaBH4)–r(Na–B) (in NaBD4)=0.0097(I) or 0.0092(II)∓0.0028 Å, r(B–Hb)=1.278(I) or 1.267(II)∓0.040 Å, and δ=0.0086∓0.0001 Å for both I and II. The rs(K–B) distance was obtained from the three observed rotational constants to be 2.656 41(20) Å, which, combined with the assumption of θ(Hb–B–Ht)=110.8∓1.0°, led to r(B–Hb)=1.272±0.030 Å and r(B–Ht)=1.233∓0.030 Å.
Structures, energetics, and spectra of aqua-sodium(I): Thermodynamic effects and nonadditive interactions
Authors: Kim Jongseob , Lee Sik , Cho Seung Joo , Mhin Byung Jin , Kim Kwang S.
Published: July 2002

Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)]. Various configurations were investigated to find the stable structures of the clusters. The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer-shell water molecules. The thermodynamic quantities of the stable structures were compared with experimental data available. Entropy-driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies. On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution. The nonadditive interactions in the clusters are found to be large. To have reliable binding energies, up to four-body interactions need to be included. Nevertheless, as a good approximation to calculate the total electron correlation energy, only water–water two-body interactions can be taken into account because other many-body interactions are partially offset.
Transport properties of polymer melts from nonequilibrium molecular dynamics
Authors: Xu Z. , de Pablo Juan J. , Kim S.
Published: July 2002

We report new results for polymer melts of bead-spring chain molecules obtained by nonequilibrium molecular dynamics simulations of a Couette flow. The beads of these molecules are connected by finitely extendable nonlinear springs. A multiple time scales algorithm has been extended to nonequilibrium situations for integration of the equations of motion. Our model fluids exhibit shear thinning and nonzero normal-stress differences. We have performed simulations both at constant volume and at constant pressure for melts of linear chains of up to 50 beads. We find that, at constant pressure, the density of long chain melts increases significantly with shear rate. We also report results of simulations for branched chain models. We find that the length of the side groups largely enhances the viscosity at low shear rates.
The aluminum/polyethylene terephthalate interface: A joint theoretical and experimental study
Authors: Calderone A. , Lazzaroni R. , Brédas J. L. , Le Quoc Toan , Pireaux J. J.
Published: July 2002

The aluminum/polyethylene terephthalate interface is investigated with a combined theoretical and experimental approach, in order to understand the interactions occurring at the molecular level when the metal is deposited onto the polymer surface. The theoretical approach consists in performing quantum-chemical calculations on short molecular model systems interacting with a few Al atoms. The geometric structure of the organometallic complex is optimized and its stability as well as the changes in charge density due to Al bonding are evaluated. The theoretical results are compared to experimental x-ray photoelectron spectroscopy data collected during the early stages of interface formation. In particular, the evolution of the polymer core level peaks is paralleled to the Al-induced modification of the charge density on the model molecules. Al is found to react preferentially with the ester group by forming covalent bonds with the oxygen and/or the carbon atom of that group.
A comparison of integral equations and density functional theory versus Monte Carlo for hard dumbbells near a hard wall
Authors: Henderson Douglas , Quintana Jacqueline , Sokol/owski Stefan
Published: July 2002

A comparison of Percus–Yevick–Pynn–Lado model theory and a density functional (DF) theory of nonuniform fluids of nonspherical particles is performed. The DF used is a new generalization of Tarazona’s theory. The conclusion is that DF theory provides a preferable route to describe the system under consideration. Its accuracy can be improved with better approximation for the direct correlation function (DCF) for bulk system.
The T1 resonance Raman spectra of 1,3,5-hexatriene and its deuterated isotopomers: An ab initio re-investigation
Authors: Negri Fabrizia , Orlandi Giorgio
Published: July 2002

We report a quantum-chemical study of the resonance Raman spectra of the lowest triplet state of 1,3,5-hexatriene. Optimized structures in T1 and in the state (Tn) in resonance with the excitation wavelength are obtained at the ab initio CASSCF level of theory. Vibrational force fields of the E-, Z- and P-rotamers in the lowest triplet state are evaluated and the displacement parameters that govern the activity of totally symmetric modes are derived. The vibrational structure of the RR spectra is modelled for d0-hexatriene and four deuterated isotopomers. Comparison of simulated and observed spectra shows that the activity of the E-isomer of hexatriene accounts very satisfactorily for the observed bands. The contribution of the Z-form is shown to be negligible in agreement with its population in T1.
Entrainment, phase resetting, and quenching of chemical oscillations
Authors: Vance William , Ross John
Published: July 2002

We examine the effect of periodic and discrete perturbations on the phase of an oscillatory chemical reaction system near a Hopf bifurcation. Discrete perturbations reset the phase of the oscillation and periodic perturbations entrain the frequency of the oscillation for perturbation frequencies in a small range about each rational multiple of the natural frequency. These phase responses may be determined from time series of a single essential species. The new phase resulting from discrete perturbations and the relative phase between the oscillation and the forcing of an entrained oscillation are described by the same response function, which is a simple sinusoid. We show that for single species perturbations, the amplitude and phase offset of this response function equal the magnitude and the argument, respectively, of the corresponding component of the adjoint eigenvector of the Jacobi matrix (that corresponds to a pure imaginary eigenvalue). These phase response methods are simpler than quenching studies for determining the adjoint eigenvectors, and in addition yield the local isochrons of the periodic orbit.
Calculation of shapes of dipolar domains in two-dimensional films: Effect of dipole tilt
Authors: Mayer M. A. , Vanderlick T. K.
Published: July 2002

Dispersed domains in two-dimensional two-phase systems often exhibit complex and intriguing morphologies. For many of these systems, it is possible to predict the shape of such a domain through an evaluation of the free energy. In this paper, we extend our previous numerical technique for calculating domain shapes by allowing for titled dipoles; the original technique assumed dipole moments oriented normal to the system plane. The solution diagram is presented in the vicinity of the primary shape transition for varying tilt angles. We find that circular domains are only stable in the absence of dipole tilt. Moreover, we find that the discontinuous transition found when the dipoles are vertical gives way to continuous elongation with increasing dipole tilt. Interestingly, it appears that the critical angle is independent of domain size.
Bistability in an uncatalyzed bromate oscillator in a continuously fed stirred tank reactor
Authors: Dutt Arun K. , Müller S. C.
Published: July 2002

Uncatalyzed gallic acid oscillating system has been investigated in a continuously fed stirred tank reactor (CSTR). In the [Bromate]0–[Bromide]0 concentration space, a region has been located where a bistability is observed between an oscillatory branch and a flow branch. To our knowledge this is the first evidence of bistability in an uncatalyzed bromate oscillator. Some observations have been explained in terms of the skeleton mechanism proposed in the past.
Monte Carlo simulations of the counter ion effect on the conformational equilibrium of the N, N′-diphenyl-guanidinium ion in aqueous solution
Authors: Nagy Peter I. , Durant Graham J.
Published: July 2002

Results of calculations for the equilibrium of the syn–syn, anti–syn, and anti–anti conformers of the N, N′-diphenyl-guanidinium ion in aqueous solution are sensitive to whether a counter ion is considered. Relative internal free energies were calculated upon MP2/6-31G*//HF/4-31G energies (second order Mo/ller–Plesset energies obtained when using the 6-31G* basis set at geometries optimized at the Hartree–Fock level and using the 4-31G basis set) and relative solvation free energy terms were obtained by Monte Carlo simulations. Without considering a counter ion only a small fraction of the solute has been predicted to adopt the anti–anti conformation in the solution. When considering acetate and chloride counter ions with salt concentration of 0.11 mol/ℓ at 310 K, mimicking physiological conditions, the counter ion close to the cation stabilizes the anti–anti form significantly. Though there are not local free energy minima for the present systems with close counter ions because of the relatively weak ion–ion interaction due to the largely delocalized total charge and atomic charge alternation in the cation, the constraint for the C(guanidinium)...C(carboxylate) separation of 4.6 Å allows an insight into the arginine...aspartate or glutamate interactions commonly found in peptides. The N–H(guanidinium)...O(carboxylate) hydrogen bonds are not stable due to thermal motion in aqueous solution. The neighboring water molecules, however, move into the space in-between the charged groups and comprise a hydrogen bonded network. Interactions with a chloride counter ion may be significant for the drug delivery process to the receptor site. Close contact between the N, N′-diphenyl guanidinium and a chloride ion is also not favored, though it may occur temporarily and then would favor the anti–anti conformer. Deviation from the relative solvation free energy obtained for the conformational change of the single cation is still some tenths of a kcal/mol with ions separated as much as 12.4 Å. While solvation energetics is affected even at such a separation, solution structure around the ions can be basically characterized without considering the effect of a remote counterpart.
Dynamics of liquid acetone: Computer simulation
Authors: Bródka A. , Zerda T. W.
Published: July 2002

Molecular dynamics simulations of liquid acetone are performed for the temperature range from 248 K to 323 K and pressures up to 2 kbar. The acetone molecule is modeled by four sites and intermolecular interactions are described by the optimized potential for liquid simulation (OPLS). The Ewald method and spherical truncation of the dipole–dipole interactions are used, and it is shown that both techniques give almost the same description of molecular motion. The calculated rotational relaxation times as well as translational diffusion coefficients satisfactorily agree with the experimental data. Rotational diffusion coefficients obtained from angular velocity correlation functions and rotational correlation times show anisotropy of reorientational motion of the acetone molecule.
Microstructure and rheology of polydisperse, charged suspensions
Authors: Rastogi Sanjeev R. , Wagner Norman J. , Lustig Steven R.
Published: July 2002

Nonequilibrium Brownian dynamics simulations are used to study the effect of polydispersity on the thermodynamics, rheology, microstructure, and shear-induced disorder–order transition in suspensions of charged colloids. Approximately 43 000 particles with 2000 different components of a discretized Schulz distribution at polydispersities from 0% to 30% are simulated on a massively parallel computer. Recent advances in the integral equation theory for polydisperse suspensions are tested and verified with respect to both structure and equilibrium mechanical properties. The low shear rate rheology for both monodisperse and polydisperse suspensions is found to be well represented by the Ree–Eyring model. At higher shear rates an ordered ‘‘string’’ phase is shear induced for low polydispersities (<10%). Increasing the polydispersity further (≳20%) inhibits the ordering, suggesting the existence of a critical polydispersity beyond which a colloidal suspension cannot be induced into an ordered state by shearing. The validity of a generalized, nonequilibrium Stokes–Einstein relationship for polydisperse colloids is also investigated.
A generalized approach to the control of the evolution of a molecular system
Authors: Tang Hui , Kosloff Ronnie , Rice Stuart A.
Published: July 2002

The theory of active control of molecular motion by use of shaped laser pulses is developed emphasizing the role of interference and using thermodynamic analogies. Attention is focused on the control of the dynamics in a system with n states coupled by radiation, and the phase relations which generate particular control schemes are derived. Among the new results reported is an optimal control scheme which constrains the value of the phase. The n-state model can be considered to represent a molecule with n electronic potential energy surfaces and an arbitrary number of degrees of freedom or as the skeleton spectrum of system where each level in the spectrum can be associated with a specific set of quantum numbers for all of the degrees of freedom. We show how the control of the dynamics of an n-state molecule can be represented in terms of the control of the dynamics of a precisely defined surrogate fewer state system. This reduction is illustrated by use of a surrogate two state system to describe the dynamics of population transfer in a three state system.
Laser-rf double resonance spectroscopy of 174YbF in theX 2Σ+ state: Spin-rotation, hyperfine interactions, and the electric dipole moment
Authors: Sauer B. E. , Wang Jun , Hinds E. A.
Published: July 2002

We report the first laser-radio-frequency double resonance spectrum of a simple lanthanide compound, the paramagnetic radical 174YbF. Measurements of the rf intervals as a function of the rotational quantum number N allow us to determine precise spin-rotation and fluorine hyperfine interaction coupling constants for the X 2Σ+(v=0 and v=1) ground states of 174YbF. The results for v=0 are γ0=−13.424 00(16) MHz, γ1=3.982 3(11) kHz, γ2=−25(1) mHz, b0=141.795 6(5) MHz, b1=−0.510(11) kHz, c=85.402 6(14) MHz, C=20.38(13) kHz. For v=1 they are γ0=−33.811 8(7) MHz, γ1=4.323(6) kHz, γ2=−28(9) mHz, b0=139.89(4) MHz, b1=−0.7(4) kHz, c=86.75(5) MHz, C=18.3(1) kHz. A direct microwave measurement of the first rotational interval in X 2Σ+(v=0) gives the rotational constant B0=0.241 292 7(7) cm−1. Finally, the Stark shift of hyperfine transitions in the first two rotational states of X 2Σ+(v=0) are analyzed to determine the electric dipole moment μe=3.91(4) D. We find that although the gross structure of YbF in its ground state resembles that of an alkaline earth monofluoride, the 4f shell is evidently important even in the low-lying excited states and leads to unusual behavior in the ground-state magnetic structure.
Infrared laser-induced post-pulse dissociation of CF2HCl and CF2Cl2 under high pressure and fluence conditions
Authors: Strube W. , Wollbrandt J. , Rossberg M. , Linke E.
Published: July 2002

The unimolecular decomposition of the halogenated methanes CF2HCl (one main channel) and CF2Cl2 (two main channels) in the focused beam of a pulsed CO2 laser under high pressure and fluence conditions (p=100 Pa–2 kPa; Φ=5–200 J/cm2) was studied by a special laser-induced fluorescence (LIF) technique, permitting spatially resolved fragment concentration measurements in the focal region. Considerable amounts of CF2 product were formed after the end of the laser pulse. In the one-channel-dissociation case of CF2HCl LIF measurements of the CF2 yield distribution Y(z,r) can be related to the spatial distribution of the average absorbed energy in the parent molecules. Only part of the absorbed energy is consumed by multiphoton dissociation, while most reactant molecules remain highly vibrationally excited in the focus volume far into the double cone. Using the long-lived CF2 also as a probe for measuring the rotational, translational, and vibrational temperatures, the redistribution of the internal energy in the molecules and fragments involved is monitored. The post-pulse production of CF2 is shown to be caused by the energy pooling v–v transfer mechanism, while contributions of pyrolytic and gas dynamic processes are of little importance.
Influence of hydrodynamic flow on nucleated wetting
Authors: Law Bruce M. , Pak Hyuk K.
Published: July 2002

In this paper we demonstrate that a small transverse temperature gradient of a few mK/cm induces the nucleation of a wetting layer at the liquid–vapor surface of the critical binary liquid mixture hexadecane+acetone. The nucleation phenomenon is well described by a classical surface nucleation theory [B. M. Law, Phys. Rev. Lett. 72, 1698 (1994)] where far from the critical temperature the height of the energy barrier is independent of the magnitude of the temperature gradient while the characteristic nucleation time A=1/(attempt frequency) is approximately inversely proportional to the transverse temperature gradient which induces laminar fluid flow.
Magneto-infrared spectra of matrix-isolated NiH and NiH2 molecules and theoretical calculations of the lowest electronic states of NiH2
Authors: Li S. , Van Zee R. J. , Weltner, Jr. W. , Cory M. G. , Zerner M. C.
Published: July 2002

Two vibronic transitions, 2Δ3/2(v=0)←2Δ5/2(v″=0) and 2Π3/2(v=0)←2Δ5/2(v″=0) at 923 and 2560 cm−1, respectively, were observed for NiH in solid argon (and krypton) at 4 K. These Ω=3/2←Ω=5/2 transitions were shifted and broadened by magnetic fields of up to 4 T. Also, its ground state vibrational frequency has been observed in neon, argon, and krypton matrices. Bands in the 600–2000 cm−1 region were assigned as frequencies of the NiH2 (NiD2, NiHD) molecule. Contrary to earlier ab initio calculations, this assignment implies that the molecule is strongly bent in its ground electronic state. This discrepancy was explored theoretically by considering the lowest triplet (linear) and singlet (bent) states of NiH2. The relative energies of the two states have not been satisfactorily resolved, but the calculated structural and vibrational properties of the singlet state are in good agreement with the observations.
Pressure effects on the HS→LS relaxation in [Zn1−xFex(6-mepy)3tren](PF6)2
Authors: Wang Wei , Chan I. Y. , Schenker Sabine , Hauser Andreas
Published: July 2002

Laser flash photolysis experiments were performed on the mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x=0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude.
Molecular thermodynamics of binary mixture adsorption: A scaled particle theory approach
Authors: Talbot Julian
Published: July 2002

We examine the thermodynamic properties of two-dimensional fluid mixtures of hard convex particles using scaled particle theory (SPT). Analytic expressions are obtained for the excess area, Gibbs free energy, and excess entropy of a binary mixture. For typical fluid densities and for a range of area and perimeter ratios of the two species the fluid mixtures exhibit small negative deviations from ideality. The excess quantities are smaller than the corresponding bulk (three dimensional) mixtures which offers some explanation for the success of the ideal adsorbed solution (IAS) theory. According to the SPT, binary mixtures of hard particles are stable for all compositions and no fluid-fluid demixing transition is possible. The SPT equations are used to examine the adsorption equilibrium between an ideal bulk phase and an adsorbed phase. Adsorption isotherms and selectivities are computed for a range of area and perimeter ratios, equilibrium constant ratio, and bulk mole fraction. Unlike the widely used multicomponent Langmuir equations, the selectivity computed from the SPT isotherms exhibits strong sensitivity to these parameters. The selectivity of the smaller species always increases with increasing bulk pressure which may lead to a selectivity reversal. Finally, we discuss systems where the adsorbed molecules can adopt various orientations with respect to the surface normal.
Separation of time scales in species conversion by diffusion-controlled reactions in a binary suspension of spheres
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

Transient diffusion-controlled reactions in a binary suspension of equal-sized spheres are studied in a region of the equilibrium phase diagram, where a separation of time scales occurs. In Ising model language, the suspension consists of hard spheres with an internal classical spin taking two values, with a square well Ising interaction and with a magnetic field present. Transitions can occur whenever a pair of spheres approaches within the range of the Ising interaction. A separation of time scales is observed near the ferromagnetic phase transition. This allows a simplified phenomenological description of the relaxation of magnetization.