bluuebunny/crossref_metadata_2025 huggingface dataset:
A comprehensive theoretical examination of primary dissociation pathways of formic acid
Authors: Francisco J. S.
Published: July 2002

Primary dissociation pathways have been investigated for formic acid by ab initio molecular orbital methods. Reactant, transition state, and products were fully optimized with unrestricted Hartree–Fock and unrestricted second-order Mo/ller–Plesset wave functions. The activation energy for decarboxylation of formic acid (CO2+H2) is 65.2 kcal mol−1, while that for the dehydration process (CO+H2O) is 63.0 kcal mol−1. These theoretical results suggest that the decarboxylation and dehydration processes are competitive. The activation energy barrier for isomerization of formic acid to yield dihydroxymethylene is 73.7 kcal mol−1 and may be a competitive process. Free radical initiation processes are predicted to be minor.
Photon echoes and related four-wave-mixing spectroscopies using phase-locked pulses
Authors: Cho Minhaeng , Scherer Norbert F. , Fleming Graham R. , Mukamel Shaul
Published: July 2002

The use of phase-locked pulses in various spectroscopic techniques related to the third-order polarization P(3) is analyzed. Using correlation function expressions for the nonlinear response function, we clarify the interrelationship among several photon echo, pump–probe, and spontaneous light emission techniques, without alluding to any specific model for the material system. By combining phase-locked pulses and heterodyne detection it becomes possible to probe separately the real and imaginary parts of the nonlinear response function. Combining two phase-locked pulse excitation with time-resolved detection of the spontaneous light emission allows direct separation of the Raman and fluorescence contributions.
Global phase diagram for reacting systems
Authors: Talanquer Vicente
Published: July 2002

The global phase diagram of a chemically reactive system is described for a model which can be thought of as the ‘‘regular-solution’’ model of a ternary mixture. We consider the particular addition reaction X+Y→Z and analyze the significant features of the phase diagrams which result from the intersection of the chemical equilibrium surface and the phase coexistence manifolds for all possible values of the energy parameters. Based on the nature of molecular interactions, six different general kinds of mixtures can be identified where interesting phenomena such as triple and quadruple points, closed-loop phase coexistence curves, lower and upper areatropy, and critical areatropy appear as a consequence of chemical and phase equilibrium interaction.
Bonded hard-sphere (BHS) theory for the equation of state of fused hard-sphere polyatomic molecules and their mixtures
Authors: Amos Michael D. , Jackson George
Published: July 2002

The bonded hard-sphere (BHS) approach originally developed for diatomic and triatomic molecules is generalized to hard-sphere polyatomic models which are formed by bonding together their constituent hard spheres. The thermodynamic properties of the polyatomic fluid are obtained from the known properties of a corresponding multicomponent mixture of different sized hard spheres with bonding sites. In the limit of complete bonding, hard-sphere polyatomic molecules are formed. As well as the general expressions for polyatomic molecules and their mixtures, the equation of state of hard-sphere chain molecules, which are simple models of homologous series such as the alkanes, perfluoroalkanes, etc., is presented. More specifically, the chain molecules are formed from two different types of hard spheres 1 and 2. Spheres of type 1 make up the backbone of the chain and, in this case, would represent the carbon atoms; spheres of type 2 represent the substituents, i.e., the hydrogen or fluorine atoms. Although the BHS theory is only strictly valid for molecules of tangent spheres, the theory can also be applied to fused hard-sphere (FHS) molecules with overlapping spheres. The latter are more realistic models of actual molecules. In the spirit of the semiempirical scaled particle theories (SPT), the properties of the FHS molecule can be related to those of an equivalent BHS molecule of tangent spheres with the same parameter of nonsphericity, α. When dealing with large polyatomic molecules, α does not need to be calculated numerically as is the case with the usual SPT approach. As an approximation one may use the analytical values of α for the diatomic segments making up the molecule. This simple approach turns out to be quite successful. The BHS equation of state has been compared with ‘‘exact’’ computer simulation data for homonuclear and heteronuclear FHS systems of diatomics, linear and nonlinear triatomics, and tetrahedral pentatomics, as well as mixtures of FHS molecules. Good agreement is found for all of these systems. In general, the BHS approach is more rigorous and easier to extend to larger polyatomic molecules and their mixtures than SPT.
Activated rate processes: Generalization of the Kramers–Grote–Hynes and Langer theories
Authors: Berezhkovskii Alexander M. , Pollak Eli , Zitserman Vladimir Yu.
Published: July 2002

The variational transition state theory approach for dissipative systems is extended in a new direction. An explicit solution is provided for the optimal planar dividing surface for multidimensional dissipative systems whose equations of motion are given in terms of coupled generalized Langevin equations. In addition to the usual dependence on friction, the optimal planar dividing surface is temperature dependent. This temperature dependence leads to a temperature dependent barrier frequency whose zero temperature limit in the one dimensional case is just the usual Kramers–Grote–Hynes reactive frequency. In this way, the Kramers–Grote–Hynes equation for the barrier frequency is generalized to include the effect of nonlinearities in the system potential. Consideration of the optimal planar dividing surface leads to a unified treatment of a variety of problems. These are (a) extension of the Kramers–Grote–Hynes theory for the transmission coefficient to include finite barrier heights, (b) generalization of Langer’s theory for multidimensional systems to include both memory friction and finite barrier height corrections, (c) Langer’s equation for the reactive frequency in the multidimensional case is generalized to include the dependence on friction and the nonlinearity of the multidimensional potential, (d) derivation of the non-Kramers limit for the transmission coefficient in the case of anisotropic friction, (e) the generalized theory allows for the possibility of a shift of the optimal planar dividing surface away from the saddle point, this shift is friction and temperature dependent, (f) a perturbative solution of the generalized equations is presented for the one and two dimensional cases and applied to cubic and quartic potentials.
First-order nonadiabatic coupling matrix elements from multiconfigurational self-consistent-field response theory
Authors: Bak Keld Lars , Jo/rgensen Poul , Jensen Hans Jo/rgen Aa. , Olsen Jeppe , Helgaker Trygve
Published: July 2002

A new scheme for obtaining first-order nonadiabatic coupling matrix elements (FO-NACME) for multiconfigurational self-consistent-field (MCSCF) wave functions is presented. The FO-NACME are evaluated from residues of linear response functions. The residues involve the geometrical response of a reference MCSCF wave function and the excitation vectors of response theory. Advantages of the method are that the reference state is fully optimized and that the excited states, represented by the excitation vectors, are strictly orthogonal to each other and to the reference state. In a single calculation the FO-NACME between the reference state and several excited states may be obtained simultaneously. The method is most well suited to describe situations where the dominant configurations for the two states differ mainly by a single electron replacement. When the dominant configurations differ by two electrons many correlating orbitals are required in the MCSCF reference state calculation to accurately describe the FO-NACME. FO-NACME between various states of H2, MgH2, and BH are presented. These calculations show that the method is capable of giving quantitatively correct results that converge to the full configuration interaction limit. Comparisons are made with state-averaged MCSCF results for MgH2 and finite-difference configuration interaction by perturbation with multi-configurational zeroth-order wave function reflected by interactive process (CIPSI) results for BH.
Improved electron propagator methods: An investigation of C4, C−4, and C+4
Authors: Ortiz J. V.
Published: July 2002

Several new electron propagator methods for the calculation and interpretation of vertical and adiabatic electron binding energies are applied to C4, C4−, and C4+. Among these methods are geometry optimizations of doublet species using neutral and dianionic reference states, renormalized calculations of vertical electron binding energies using unrestricted Hartree–Fock reference states, and analysis of correlated energy differences and corresponding orbitals in terms of one-electron expectation values. The adiabatic electron affinity of linear C4 is calculated to be 3.69 eV, in excellent agreement with photoelectron spectra. The corresponding result for the cyclic isomer, 2.17 eV, is very close to estimates from Coulomb explosion imaging. Ionization energies for both isomers also are calculated. Geometrical changes accompanying addition or removal of electrons are interpreted in terms of electrostatic interactions between nuclei and electrons assigned to Feynman–Dyson amplitudes.
The molecular Stark effect in regions of high state density: Overall simplicity and underlying complexity in the response to a static electric field
Authors: Fraser Gerald T. , Pate Brooks H.
Published: July 2002

We present the high-resolution (11 MHz) infrared measurement of the molecular Stark effect for the R(0) transition of the acetylenic C–H stretch in 2-propyn-1-ol. The field-free spectrum is fragmented into three eigenstate components due to the effects of intramolecular vibrational energy redistribution (IVR). As the field strength increases from 0 to 25 kV/cm, the number of eigenstates increases linearly. The center-of-gravity of the fragmented R(0) transition follows the simple, second-order Stark shift (Δν∝E2) expected for the bright state. However, when viewed at the eigenstate level, the mechanism of the Stark shift is rather complex. At lower field strengths, the eigenstates shift in energy, as occurs for Stark effects in lower state density regimes. As the number of coupled states increases, energy shifting of the eigenvalues is quenched. To preserve the second-order Stark shift of the center-of-gravity, the intensity ‘‘rolls over’’ the largely rigid eigenvalue structure. For molecules in regions of high state density, the reduced energy shifting of the eigenvalues as the electric field is increased means that lack of deflection by inhomogeneous electric fields is not necessarily a consequence of the molecule being nonpolar.
Monte Carlo simulations of temperature programmed desorption spectra
Authors: Meng B. , Weinberg W. H.
Published: July 2002

We introduce a Monte Carlo algorithm which simulates first-order temperature programmed desorption spectra using the correct connection between Monte Carlo simulation time and real time. The TPD spectra obtained from the Monte Carlo simulations are exactly the same as those calculated from the governing continuum differential equation for a noninteracting system. We discuss in some detail the effects of lateral interactions between adsorbates on the temperature programmed desorption spectra. We find that peak splitting of the temperature programmed desorption spectra characterizes an adsorbate configuration transition on the surface. Furthermore, we analyze in detail the behavior of the coverage-dependent activation energy and preexponential factor for the case of a repulsive nearest-neighbor interaction and find that kinetic compensation occurs in this case.
Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: The archetypical case of carbon monoxide/water coadsorption on Pt(111)
Authors: Kizhakevariam Naushad , Jiang Xudong , Weaver Michael J.
Published: July 2002

Infrared reflection-absorption spectroscopic (IRAS) along with work-function measurements are presented for carbon monoxide and deuterated water co-dosed onto Pt(111) at 110 K in ultrahigh vacuum (uhv) in comparison with coverage- and potential-dependent infrared spectra for the analogous in situ Pt(111)-aqueous electrochemical interface. The twin objectives are to assess the manner and degree to which the former ‘‘uhv electrochemical modeling’’ tactic mimics the vibrational spectral properties of the latter interface, and to exploit the additional molecular structural information obtainable for the former surface configuration in elucidating solvation effects for the in situ electrochemical system.

Postdosing water onto CO adlayers in uhv yields progressive attenuation of the C–O stretching (νCO) IRAS bands associated with atop and twofold bridging coordination on the clean Pt(111) surface, being replaced by bands at lower frequencies that are more characteristic of the CO coverage (θCO)-dependent νCO features observed for the electrochemical interface. This ‘‘titration-like’’ spectral response, indicating that the added water assembles into hydrogen-bound islands which are nonetheless microscopically intermixed with the contiguous CO adlayer regions, is typically complete by water coverages corresponding to about 2–3 ‘‘equivalent bilayers.’’ Water dosage-dependent spectra in the O–D stretching (νOD) region for zero or low predosed CO coverages display the hallmarks of markedly increased hydrogen bonding beyond the bilayer point, whereas such hydrogen bonding is prevalent even at submonolayer water dosages for intermediate or near-saturated θCO values.

The work-function decreases, −ΔΦ, induced by water dosing also are indicative of synergistic water/CO interactions, larger −ΔΦ values being obtained in the presence of low or intermediate predosed CO coverages than in its absence. A distinction is drawn between ‘‘specific’’ and ‘‘nonspecific’’ water co-dosing effects upon the CO adlayer structure; the former involves adjacent CO/D2O coadsorbate interactions, while the latter is describable most simply by the changes in the surface potential. The θCO-dependent CO site occupancies and νCO frequencies for the hydrated uhv and electrochemical Pt(111) systems are found to be in close concordance, the latter when the interfaces are compared at equivalent values of the surface potential. The findings thereby support a close structural connection between analogous low-temperature hydrated uhv and ambient-temperature electrochemical interfaces.
Quantum mechanical calculations of the rate constant for the H2+OH→H+H2O reaction: Full-dimensional results and comparison to reduced dimensionality models
Authors: Manthe Uwe , Seideman Tamar , Miller William H.
Published: July 2002

The cumulative reaction probability is calculated for the H2+OH→H+H2O reaction in its full (six) dimensionality for total angular momentum J=0. The calculation, which should give the (numerically) exact result for the assumed potential energy surface, yields the cumulative reaction probability directly, without having to solve the complete state-to-state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300°<T<700°. The result deviates significantly from the experimental rate constant, suggesting that the potential energy surface needs to be improved. A systematic series of reduced dimensionality calculations is carried out in order to characterize the behavior and reliability of these more approximate treatments; a comparison of the full dimensional results with previous reduced dimensionality calculations is also made.
Theoretical studies of the interaction between aluminum and poly(p-phenylenevinylene) and derivatives
Authors: Lögdlund M. , Brédas J. L.
Published: July 2002

We report the results of a theoretical study of the interaction between aluminum and poly(p-phenylenevinylene) and derivatives of poly(p-phenylenevinylene). This allows us to investigate the initial stages of formation of the metal/conjugated polymer interface. Applying the semiempirical Austin model 1 method, we find that aluminum reacts preferentially by forming covalent bonds with the vinylene linkages in both poly(p-phenylenevinylene) and poly(2,5-dimethoxy-p-phenylenevinylene). When carbonyl groups replace the methoxy groups, i.e., to form poly(2,5-dialdehyde-p-phenylenevinylene), new reactive sites are induced, leading to structures involving aluminum–oxygen bonds, with stabilities comparable to those involving reaction with the vinylene groups. In all of the three systems investigated, the interaction with aluminum induces major modifications of the polymer chains with interruptions of the π system caused by formation of sp3 sites. Charge distribution analysis indicates that electron charge transfer takes place from the aluminum atoms to the polymer chains.
Extrapolated Born–Oppenheimer energy for the ground state of the hydrogen molecule
Authors: Kol/os Wl/odzimierz
Published: July 2002

Ground state energy of the hydrogen molecule, at the equilibrium internuclear separation, has been computed in the Born–Oppenheimer approximation. The generalized James–Coolidge expansion has been used for the variational wave function. The expansion length has been systematically extended, and from the results a limiting value of the energy has been obtained which is by 0.009 cm−1 lower than the best variational value. Improved agreement is obtained with experimental values of the ionization potential for H2 and HD. For D2 the disagreement between theoretical and experimental values of the ionization potential seems to be significant.
A combined use of perturbation theory and diagonalization: Application to bound energy levels and semiclassical rate theory
Authors: Hernandez Rigoberto
Published: July 2002

A new method, mixed diagonalization, is introduced in which an effective Hamiltonian operator acting on a reduced dimensional space is constructed using the similarity transformations of canonical Van Vleck perturbation theory (CVPT). This construction requires the characterization of modes into two categories, global and local, which in the bound vibrational problem are tantamount to the large and small amplitude vibrations, respectively. The local modes in the Hamiltonian are projected out by CVPT, and the resulting Hamiltonian operator acts only on the space of global modes. The method affords the treatment of energy levels of bound systems in which some vibrational assignments are possible. In addition, it systematically provides a reduced dimensional Hamiltonian which is more amenable to exact numerical solution than the original full-dimensional Hamiltonian. In recent work, a semiclassical transition state theory (SCTST) rate expression has been written in terms of a Hamiltonian operator parameterized by the imaginary action along the local reaction path in the transition state region [Chem. Phys. Lett. 214, 129 (1993)]. We show that the Hamiltonian constructed by mixed diagonalization has this form, and can be used to obtain more accurate semiclassical rate expressions.
Structure and relaxation of end-linked polymer networks
Authors: Duering Edgardo R. , Kremer Kurt , Grest Gary S.
Published: July 2002

The kinetics of end crosslinking a linear polymer melt and the dynamics of the resulting network is studied using molecular dynamics (MD) simulations. Starting from equilibrated melts of linear chains of length ranging from 1/3Ne to 3Ne, where Ne is the entanglement length, tetrafunctional crosslinkers are attached to a fraction x of the chain ends. When a free end comes within a short capture distance rx from an unsaturated crosslinker, the chain ends are attached. With a stoichmetric number x of crosslinkers, the long time kinetics for the number of free ends and the number of unsaturated crosslinkers decays as a power law in time t−a, with a≊0.5 for the present range of chain lengths. The resulting networks are then used to study the effect of entanglements on the motion of the crosslinks and the modulus of the network. Using a cluster search algorithm, the microscopic characteristics of the networks are determined. This allows us to compare our simulated networks to theoretical models of rubber elasticity without any adjustable parameters. While our results for the modulus are close to the phantom network model for the shortest chains, for the longer chains our results clearly support the Edwards tube model. Already for the presently employed largest chains the modulus is about 1.9 times larger than predicted from the affine model. Asymptotically, within an affine network model, about 2.2 entanglement lengths are needed to contribute to the modulus in the same way as one elastically active chemical crosslink. The data agree very well with recent experiments.
Fast energy delocalization upon vibrational relaxation of a deuterated zeolite surface hydroxyl
Authors: Bonn Mischa , Brugmans Marco J. P. , Kleyn Aart W. , van Santen Rutger A.
Published: July 2002

In this time-resolved study of vibrational dynamics of deuterated surface hydroxyls at acid sites in the zeolite Mordenite, we investigate the O–D T1 vibrational lifetime and transient band shifts. It is shown that after infrared excitation of the stretching mode of a surface hydroxyl, the excess energy is rapidly distributed over delocalized low-energy lattice modes upon de-excitation. This is asserted from the observation that nonexcited hydroxyls are perturbed by the relaxation of their excited counterparts immediately after this relaxation. This observation can be made owing to better resolution in transient transmission spectroscopy obtained by deuteration of the surface hydroxyls. This assignment allows for accurate estimates of lattice temperatures after relaxation of the vibration. Further, from the observation that the vibrational lifetime is dependent on frequency (increasing from 25 to 70 ps with increasing frequency), it is concluded that the deuterated acidic protons are hydrogen bonded to lattice oxygen atoms in the zeolite.
The role of adsorbate structure in the photodissociation dynamics of adsorbed species: Methyl iodide/MgO(100)
Authors: Fairbrother D. Howard , Briggman K. A. , Stair P. C. , Weitz Eric
Published: July 2002

The 257 nm photodissociation dynamics of methyl iodide multilayers adsorbed on MgO(100) have been studied using resonantly enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) to detect both methyl and iodine photofragments. The photofragments result from direct photolysis of the methyl iodide adsorbate. Methyl fragments with a translational and internal energy content comparable to that found in the gas phase dissociation of the isolated molecule are produced. However, the measured I/I* branching ratio is modified in favor of ground state (I) iodine production. Collisionally slowed methyl fragments, characterized by a translational temperature close to that of the surface, are also observed. Iodine fragments with velocities in excess of the gas phase limit are produced as a result of collisional energy transfer between iodine and faster moving methyl photofragments. A small amount of laser induced photodesorption, leading to the production of molecular methyl iodide, was also detected. The observed photofragmentation dynamics can be accounted for based on the ordered antiparallel structure adopted by the adsorbate molecules that are preferentially aligned along the surface normal within the physisorbed layer.
Energy and angular distributions in dissociative photodetachment of O−4
Authors: Sherwood C. R. , Garner M. C. , Hanold K. A. , Strong K. M. , Continetti R. E.
Published: July 2002

An anisotropic product angular distribution has been observed in the dissociative photodetachment of O−4 at 523 nm. Energy and angular distributions of coincident O2 products from the process O−4+hν→O2+O2+e− were measured using translational energy spectroscopy in a fast ion beam. The angular distribution peaks perpendicular to the electric vector of the laser beam.
Direction of the nucleation current through the saddle point in the binary nucleation theory and the saddle point avoidance
Authors: Berezhkovskii L. M. , Zitserman V. Yu.
Published: July 2002

Careful analysis and reexamination of Stauffer’s theory of homogeneous binary nucleation is carried out. The possibility of deviation from Stauffer’s formula for the direction of nucleation current through the saddle point is indicated. We obtained the improved expression for the angle of the flow through the saddle point using a simple method based on the treatment of stochastic equations for the number of particles in the embryo. The limiting behavior of a binary homogeneous nucleation when concentration of one of the components in a vapor is much less than the other is studied. For this case we considered the possibility of a saddle point avoidance and transition to uniary nucleation. It is shown that the multidimensional theory of anomalous growth of the embryos, proposed by us recently, gives a good description of the limiting situations including the transition to a one-component nucleation. The result for binary nucleation rate obtained from our theory agrees well with that obtained earlier from the direct numerical solution of the Fokker–Planck equation.
Correlation states of ethylene
Authors: Desjardins S. J. , Bawagan A. D. O. , Liu Z. F. , Tan K. H. , Wang Y. , Davidson E. R.
Published: July 2002

High resolution synchrotron photoelectron spectra (PES) of ethylene have been obtained at several photon energies in the range 30 to 220 eV. Further evidence is presented that the correlation (satellite) peak at 27.4 eV binding energy is ‘‘intrinsic’’ in nature. A new correlation peak at 21.4 eV binding energy, however, is found to be a ‘‘dynamic’’ correlation. Several PES of 1-13C-ethylene have also been obtained and have been found to be identical to those of normal ethylene. Both of the correlation peaks are also present in the labeled species with similar photon energy behaviors. Sophisticated theoretical calculations are found to agree quantitatively with the experimental PES spectra.
Photodesorption of CO and CO+ from Pt(111): Mechanism and site specificity
Authors: Fukutani Katsuyuki , Song Moon-Bong , Murata Yoshitada
Published: July 2002

Ultraviolet photodesorption of CO and CO+ from Pt(111) at 80 K is investigated by (2+1) resonance-enhanced multiphoton ionization and reflection absorption infrared spectroscopy. Desorption of CO and CO+ occurs at the on-top site as single-photon and three-photon processes, respectively. The rotational, vibrational, and translational temperatures of desorbed CO are approximately 130, 3700, and 2000 K, which are considerably higher than the sample temperature. The threshold energy of neutral CO desorption lies between 2.3 and 3.5 eV suggesting that an unoccupied 2π state is responsible for the desorption.
Structural characterization of an electrolytic aqueous solution, LiCl⋅6H2O, in the glass, supercooled liquid, and liquid states
Authors: Prével B. , Jal J. F. , Dupuy-Philon J. , Soper A. K.
Published: July 2002

Using the technique of hydrogen and deuterium substitution, the structure of water in concentrated lithium chloride aqueous solutions (LiCl⋅6H2O) is explored for the liquid, supercooled liquid, and glass states. It is found that changes in structure between the glass and supercooled states are minor, but that a major change in water structure occurs as the supercooled liquid is heated above the temperature of peritexy of the penta-hydrate (Tp=207 K). In particular the 4.4 Å peak in the OO pair correlation function of pure water, which is normally viewed as indicating tetrahedral short range coordination in water, is absent in the LiCl solution at the same temperature, but reappears strongly in the glass and supercooled states. Corresponding changes occur in the HH and OH correlation functions. In addition, correlations appear to extend nearly twice as far in the glass and supercooled liquid, compared to the room temperature liquid.
Monte Carlo simulation of branched and crosslinked polymers
Authors: Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

Novel Monte Carlo simulation techniques are presented for efficient isobaric–isothermal simulations of branched chains and polymer networks with tri- and tetra-functional sites. Molecular rearrangements are performed by means of extended continuum configurational bias moves applied to single-path polymer portions. Volume fluctuations are performed via slab moves, which are extended in this work to effectively handle networks of arbitrary complexity. These methods are applied to determine the volumetric properties of linear and branched chains (with athermal and square-well interaction sites). Novel results are also presented for the compressibility of athermal and thermal polymer networks having a perfect, diamondlike connectivity.
Molecular dynamics simulation study of the anomalous thermal conductivity of clathrate hydrates
Authors: Inoue Ryoji , Tanaka Hideki , Nakanishi Koichiro
Published: July 2002

Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in order to account for anomalous thermal conductivity of clathrate hydrates. Evaluated thermal conductivities of clathrate hydrates are lower than those of ice from both types of MD simulations. The evaluated values agree qualitatively with experimental observation. Its anomalous temperature dependence is also reproduced. It is found that those unique properties of clathrate hydrates arise mainly from encaging of guest species and that guest Xe atom even in large cage couples strongly with the host lattice and vibrations relevant to Xe have a large anharmonic contribution. Those modes with large anharmonicity interact with other modes and scatter phonons. This leads to a small thermal conductivity. Increasing temperature changes the ‘‘effective’’ potential energy curves of guests in a different way from a the normal crystal. Softening of the modes relevant to guests is suppressed, which is unique in clathrate hydrates reducing anharmonicity of the low frequency modes coupled with guest motions. It is concluded that the most likely mechanism for the anomalies is the resonance scattering model among three accounts proposed so far.
A quasiclassical trajectory calculation for the Penning ionization H2O+He*(23S)→H2O++He+e−: Rotational cooling effects
Authors: Ishida Toshimasa
Published: July 2002

A quasiclassical trajectory calculation is performed for the Penning ionization system H2O+He*(23S)→H2O+(2B2,2A1,2B2)+He+e− at H2O rotational temperatures of 300, 200, 100, and 25 K. The resonance potential and the widths for the three ionized states are fitted to analytical functions on the previous ab initio points [J. Chem. Phys. 102, 4169 (1995)]. The calculational results are compared with experimental measurements. The total and partial ionization cross sections are calculated in the energy range 0.05–1.0 eV. As the rotational temperature is lowered, the following results are predicted: the total cross section decreases with collision energy, and the dominant ionization into the 2A1 state is more enhanced. These results are due to the increasing drawing of trajectories into the attractive H2O lone pair region with decreasing rotation frequency. Opacity functions and total and partial ionization probabilities for each trajectory are analyzed to interpret the results obtained for the cross sections.
Comparison of mobility equivalent diameter with Kelvin-Thomson diameter using ion mobility data
Authors: Mäkelä Jyrki M. , Riihelä Marko , Ukkonen Ari , Jokinen Vilho , Keskinen Jorma
Published: July 2002

Mobility distributions of both positive and negative cluster ions were studied in nitrogen gas enriched by ethanol, isopropanol, n-butanol and acetone vapours, respectively. The ions were generated into the system by a radioactive source normally used as a bipolar aerosol charger. The ion mobility distributions were measured with a time-of-flight mobility spectrometer (TOF). The measured ion mobility can be used to estimate the mobility equivalent diameter via conventional Millikan–Fuchs formula based on Stokes law corrected by molecule slip effects. When the vapour concentration is increased the cluster ions will get larger due to enhanced molecule attachment onto ions. Here the vapour concentration is assumed to be high enough so that the chemical tracer effects would become of minor importance. Therefore the Kelvin–Thomson equation for ion induced nucleation is first assumed to be valid. The Kelvin–Thomson diameter of the ions in all cases was found to be systematically smaller than the corresponding mobility equivalent diameter according to Millikan–Fuchs, indicating of inconsistencies between the formulas based on macroscopic quantities at 1 nm size range. Thereafter, other available mobility vs diameter relationships were surveyed. The closest agreement between mobility equivalent and Kelvin–Thomson diameters at this size was obtained using the relationship given by Tammet [J. Aerosol Sci. 26, 459 (1995)].
Electron transfer in the inverted region: Adiabatic suppression and relaxation hindrance of the reaction rate
Authors: Georgievskii Yuri , Burshtein Anatoly I. , Chernobrod Boris M.
Published: July 2002

Kramers’s model is applied to an electron transfer reaction in the inverted region. The reaction rate is considered at different values of the coupling matrix element and the damping, which is a measure of the interaction of the reaction coordinate with the other, nonreactive degrees of freedom of the system. The coupling characterizes overlap of the electron orbits of the donor and acceptor. It is shown that at the low/high damping the reaction rate is controlled by the slowest of the two rates. One is the thermally averaged transition rate (the reaction rate in the intermediate damping regime) and another one is the rate controlled by the energy/spatial diffusion. We demonstrate that the reaction rate as a function of the electron coupling shows a striking difference from the normal region case. The rate passes via maximum and decreases exponentially with the increase of the coupling.
Calculation of contributions of one- and two-electron spin-orbit coupling terms to the parity-violating energy shifts for amino acids and helical alkanes
Authors: Kiyonaga Hiroshi , Morihashi Kenji , Kikuchi Osamu
Published: July 2002

Rigorous calculations were performed for spin-orbit coupling terms of the Breit–Pauli Hamiltonian in the calculation of the parity-violating energy shifts Epν using the sum-over-state perturbation theory, and the role of one-electron and two-electron spin-orbit coupling terms was examined. Calculations on amino acids and helical n-alkanes have suggested that inclusion of one-center terms both of one- and two-electron spin couplings is sufficient to estimate the Epν values quantitatively.
Six-dimensional quantum calculations of vibration-rotation-tunneling levels of ν1 and ν2 HCl-stretching excited (HCl)2
Authors: Qiu Yanhui , Zhang John Z. H. , Bačić Zlatko
Published: July 2002

Results of the first full-dimensional (6D) quantum calculations of the vibrational levels of the ν1 and ν2 HCl-stretch excited (HCl)2, for total angular momentum J=0, are presented. Three 6D potential energy surfaces (PESs) were employed. Two widely used PESs, the ab initio PES of Bunker and co-workers and the semiempirical PES by Elrod and Saykally, are found to give negligible tunneling splittings (⩽5×10−2 cm−1) for the vibrational eigenstates of the ν1/ν2 excited (HCl)2, in sharp disagreement with the experimental tunneling splittings in the ν1 and ν2 fundamentals, −3.32 and 3.18 cm−1. In an effort to overcome this problem, a 6D electrostatic interaction potential is constructed and added to the ES1 PES; the resulting 6D PES is denoted ES1-EL. Quantum 6D calculations on the ES1-EL PES yield greatly improved tunneling splittings for ν1 (−2.31 cm−1) and ν2 (2.45 cm−1), which are 70% and 77%, respectively, of the corresponding experimental values. The ν1 and ν2 fundamental HCl-stretching frequencies calculated on the ES1-EL PES are only 5.9 cm−1 lower and 2.9 cm−1 higher, respectively, than their experimental counterparts. In addition, the quantum 6D calculations on the ES1-EL PES provide a comprehensive characterization of the ν1/ν2 supported vibrational eigenstates of (HCl)2, including their energies, assignments, and tunneling splittings. The vibration-rotation-tunneling dynamics of (HCl)2 in the ν1 and ν2 excited states which emerged from our calculations differs substantially from that observed for the HF-stretch excited (HF)2.
Spectroscopic and dielectric properties of liquid water: A molecular dynamics simulation study
Authors: Bursulaya Badry D. , Kim Hyung J.
Published: July 2002

The spectroscopic and dielectric properties of liquid water under an ambient condition are studied via a molecular dynamics (MD) computer simulation method. By employing the recent TAB/10D potential model [B. D. Bursulaya, J. Jeon, D. A. Zichi, and H. J. Kim, J. Chem. Phys. 108, 3286 (1997)], the evolving solvent electronic structure is incorporated into the simulation. Thus both the induced dipole and polarizability variations of individual water molecules with the fluctuating nuclear configuration are accounted for. The MD results on far-IR absorption, depolarized Raman scattering (DRS) and optical Kerr effect (OKE) spectroscopy are in reasonable agreement with experiments. It is found that the nonlinear electronic response of water to its fluctuating environment plays an important role in the DRS and OKE; it significantly enhances the contribution of the water librational motions to the spectra, compared with that of hindered translations. This indicates that not only molecular dynamics but also accompanying electronic structure modulations are essential to quantitative understanding of various electronic spectroscopy. The effects of H/D isotope substitution are briefly discussed.
Density functional study of first row transition metal dihalides
Authors: Wang S. G. , Schwarz W. H. E.
Published: July 2002

In contrast to former ligand field predictions, the standard energetic order of the metal 3d orbitals, δ<π<σ, is not reproduced here. Throughout, the 3dσ molecular orbital (MO) level is found rather low lying because of σ-donating ligand induced 3d-4s hybridization in these linear dicoordinated compounds, while the 3dπ is rather high due to the pronounced π-donor character of the halogen ligands at the short distances of the digonal halides. This results in an unexpected electron distribution for the ground state and in an unusual order of electronic states. The calculations are in general consistent with recent experimental findings. The approach yields semiquantitatively correct geometric parameters and vibrational frequencies, and qualitatively correct trends for the dissociation energies. Those properties show a secondary periodicity, similar to the one known for octahedral high-spin complexes, though for different electronic reasons. Unknown structures, vibrational frequencies, and dissociation energies are predicted.
The bifurcation rearrangement in cyclic water clusters: Breaking and making hydrogen bonds
Authors: Brown M. G. , Keutsch F. N. , Saykally R. J.
Published: July 2002

Tunneling patterns observed in the vibration–rotation–tunnelling spectrum of (H2O)5 measured near 2.7 THz established the time scale for bifurcation rearrangements to be approximately 40 ns. This relatively local process is likely to be relevant in the dynamics of liquid water and ice.
High density adsorbed oxygen on Rh(111) and enhanced routes to metallic oxidation using atomic oxygen
Authors: Gibson K. D. , Viste Mark , Sanchez Errol C. , Sibener S. J.
Published: July 2002

Exposure of Rh(111) to atomic oxygen leads to the facile formation of a full-coverage and ordered (1×1)-O monolayer which is stable at room temperature. This result differs markedly from the half-coverage (2×1)-O overlayer which forms at saturation when using molecular oxygen. This demonstrates that kinetic rather than thermodynamic constraints inhibit the formation of dense oxygen overlayers when O2 is the oxidant. We also report that O absorption into the bulk proceeds much more readily when using O rather than O2, a finding with direct implications for enhanced methods of low-temperature metallic oxidation. These results demonstrate that there are important fundamental differences in the way in which low-energy beams of atomic and molecular oxygen interact with metals.
Two-color three-pulse photon echoes as a probe of electronic coupling in molecular complexes
Authors: Yang Mino , Fleming Graham R.
Published: July 2002

A two-color photon echo peak shift measurement to probe the electronic coupling strength in molecular complexes is proposed. Exciton transfer between the electronic eigenstates is neglected and the baths associated with each monomer are assumed to be independent of each other. Within this simplified model, we derive a useful relation which can be used to estimate the electronic coupling strength via a combination of a normal one-color and the present two-color peak shift measurements. A simulation based on the cumulant expansion technique illustrates the validity of our suggestion.
Monte-Carlo simulations of chemical reactions in molecular crystals
Authors: Even J. , Bertault M.
Published: July 2002

Chemical reactions in molecular crystals, yielding new entities (dimers, trimers,…, polymers) in the original structure, are simulated for the first time by stochastic Monte Carlo methods. The results are compared with those obtained by deterministic methods. They show that numerical simulation is a tool for understanding the evolution of these mixed systems. They are in kinetic and not in thermodynamic control. Reactive site distributions, x-ray diffuse scattering, and chain length distributions can be simulated. Comparisons are made with deterministic models and experimental results obtained in the case of the solid state dimerization of cinnamic acid in the beta phase and in the case of the solid state polymerization of diacetylenes.
Spinodal decomposition of symmetric diblock copolymer/homopolymer blends at the Lifshitz point
Authors: Kielhorn L. , Muthukumar M.
Published: July 2002

We have derived a mean field Landau–Ginzburg free energy expression for a ternary symmetric homopolymer/diblock copolymer system from a microscopic Edwards Hamiltonian. We then used the time-dependent Landau–Ginzburg equation to model the time evolution of the order parameters after a critical quench into the region of thermodynamic instability in three dimensions. We observe the accumulation of the diblock copolymer at the interface of the domains through expulsion from the interior of the domains. We also see saturation of the diblock copolymers during domain coarsening and consequently the formation of separate, copolymer-rich, domains. We believe these interfacial saturation effects to be a major reason for the slow down of domain growth as the effect is found to be more pronounced at higher diblock copolymer concentrations.
On the photoelectron spectrum of the NO dimer, and the ground state of (NO)2+
Authors: East Allan L. L. , Watson James K. G.
Published: July 2002

The vibrational structure of the photoelectron spectrum of the NO dimer is calculated by ab initio methods, and compared with the observed ZEKE (zero electron kinetic energy) spectra published by I. Fischer et al. [J. Chem. Phys. 96, 7171 (1992)] and A. Strobel et al. [J. Phys. Chem. 99, 872 (1995)]. The ground state potential of (NO)2+ is calculated to have almost iso-energetic minima at planar cis (C2v) and trans (C2h) geometries, but the Franck–Condon factors from the ground state of (NO)2 strongly favor transitions to the cis isomer. The good agreement for the vibrational frequencies and intensities shows that the (NO)2+ isomer reached in the photoelectron spectrum has a cis-planar C2v structure, rather than the twisted C2 structure proposed by Fischer et al., or the trans-planar C2h structure proposed by A. Strobel et al.
Nematic–isotropic transition in polydisperse systems of infinitely thin hard platelets
Authors: Bates Martin A. , Frenkel Daan
Published: July 2002

We study the phase behavior of model colloidal systems composed of infinitely thin hard platelets, with polydispersity in the size of the particles. Semi-grand Gibbs ensemble simulations are used to study the coexisting nematic and isotropic phases for a range of systems with varying polydispersity. Particle size segregation is observed in the two coexisting phases, with the larger particles tending to be found in the nematic phase. This fractionation becomes more evident with increasing polydispersity. We examine the relationship between the size of a particle and its orientation in the nematic phase and find that the larger particles tend to be more orientationally ordered than the smaller ones. The coexistence densities determined from the simulations are compared to those obtained from recent experiments on colloidal platelets.
Shear influences on the phase separation of blends made of homopolymer A and random copolymer A-B
Authors: Wolf B. A.
Published: July 2002

Shear induced changes in the demixing conditions are discussed for the copolymer blends of the present type in terms of a distortion of quasichemical equilibria established in the stagnant systems between the two different kinds of polymer segments. Calculations were performed in an attempt to rationalize experimental results obtained with blends exhibiting upper critical solution temperatures. The proposed model accounts for the experience that the first observable effect of shear consists of an enlargement of the two-phase regime (a behavior which differs fundamentally from that of homopolymer blends or homopolymer solutions) and yields the correct order of magnitude for the effects. The influences of copolymer composition, number of segments of the components, and temperature dependence of the Flory-Huggins interaction parameter are discussed in detail. Furthermore, it is demonstrated how the present considerations relate to an earlier approach based on the energy a system can store while it flows.
Theory for determination of the low-frequency time-dependent response function in liquids using time-resolved terahertz pulse spectroscopy
Authors: Kindt James T. , Schmuttenmaer Charles A.
Published: July 2002

With the current rapid improvement in techniques for generating and detecting femtosecond terahertz (fs-THz) pulses, it is now possible to detect time-dependent perturbations to low-frequency intermolecular far-infrared modes during solvation events. The interpretation of these time-resolved terahertz spectroscopy experiments has generally relied on a collapse of the information available in the fs-THz pulse to give a one-dimensional time-dependent function, whose physical significance is not well defined. We describe a method to exploit the full electric field profile of the pulse as a function of pump/probe delay time to obtain a two-dimensional function that describes the system’s time-dependent dielectric response. We discuss the physical significance of this response function in terms of the time evolution of solute/solvent modes following photoexcitation, and give examples to show how its qualitative features relate to the measurable signal.