bluuebunny/crossref_metadata_2025 huggingface dataset:
Early state of spinodal decomposition studied with small angle neutron scattering in the blend deuteropolystyrene and polyvinylmethylether: A comparison with the Cahn–Hilliard–Cook theory
Authors: Schwahn D. , Janssen S. , Springer T.
Published: July 2002

The relaxation behavior of a polymer blend, deutero-polystyrene and polyvinylmethylether (d-PS/PVME) was studied by investigating the equilibrium structure factor S(Q) and its time dependence S(Q,t) after the blend had been exposed to a rapid temperature step from T0 to Tf. The structure factor S(Q,t) was determined as a function of scattering vector Q by time-resolved neutron scattering experiments. Two cases were investigated (i) with T0 and Tf both in the homogeneous and (ii) with Tf in the unstable region of the phase diagram. The relaxation was compared with the Cahn–Hilliard–Cook theory. The strongly Q-dependent (nonlocal) Onsager coefficient was determined from the relaxation time and compared with predictions by Pincus et al. The self-diffusion constants of the components were obtained for Q→0. Finally, the experiments at Tf led to the critical behavior of the susceptibility S(Q=0) near the spinodal. An Ising-type critical behavior was observed across the (extrapolated) spinodal.
Energy transfer in highly vibrationally excited acetylene: Relaxation for vibrational energies from 6500 to 13 000 cm−1
Authors: Utz A. L. , Tobiason J. D. , Carrasquillo M. E. , Fritz M. D. , Crim F. F.
Published: July 2002

Vibrational overtone excitation of acetylene molecules to energies between 6500 and 13 000 cm−1 followed by interrogation of the excited states during collisional relaxation determines both the mechanism and rates of energy transfer. A pulsed visible or near-infrared laser excites a single rotational state of C2H2 in the region of the first (2νCH), second (3νCH), or third (4νCH) overtone of the C–H stretching vibration, and an ultraviolet laser probes the excited molecules by laser-induced fluorescence after a variable delay. The self-relaxation rate constant of about 9×10−10 cm3 molecules−1 s−1 is almost twice the Lennard-Jones collision rate constant and is nearly invariant with vibrational level. The energy-transfer rate constants from these population transfer measurements agree with those extracted from pressure-broadening data in both their size and insensitivity to vibrational state. Relaxation by the rare-gas atoms He, Ar, and Xe is nearly half as efficient as self-relaxation, suggesting that the internal structure of the collision partner is not particularly important in determining the relaxation rate. The invariance with vibrational level and the efficiency of rare-gas quenching indicate that rotational energy transfer is the most important relaxation pathway.
Molecular structure and orientational ordering in solid C60
Authors: Michel K. H.
Published: July 2002

A microscopic theory, which describes the orientational dynamics of C60 molecules in the face-centered cubic phase of C60-fullerite, is formulated for the case of a complex molecular structure. Interaction centers which comprise atoms, double bonds, and single bonds as molecular constituents contribute to the intermolecular potential. Orientation dependent physical properties are described in terms of symmetry-adapted rotator functions. It is found that a same set of rotator functions is sufficient even in the case of a complex molecular structure. Phase transition temperatures are discussed for various models of molecular structure.
Chemistry in strong laser fields: An example from methyl iodide photodissociation
Authors: Hammerich Audrey Dell , Kosloff Ronnie , Ratner Mark A.
Published: July 2002

Time-dependent quantum-mechanical theories and simulations provide a clear and intuitive description of molecular processes. Due to ensuing simplification of the theory and the generally employed numerical algorithms, the vast majority of these treatments are based upon perturbation theory. Especially in light of the current level of experimental sophistication, with experiments being realized which are influenced by the spectral, temporal, and spatial shape of the laser pulse, it is important to move beyond treatments limited to weak fields or idealized δ-function wave forms. Various methods to examine the results of high-field simulations are presented. All of the techniques are shown to have the familiar linear response form in the weak-field limit. In a time-dependent framework the difference between the linear and nonlinear response expressions can be seen from expectation values over stationary versus nonstationary states. The high-field photodissociation of methyl iodide illustrates this approach. Methyl iodide represents a physical system well suited for examining the effects of such exciting laser-field characteristics as strength, linewidth, and frequency upon the photodissociation dynamics. Its dissociation occurs upon coupled repulsive excited electronic potential-energy surfaces which have recently been revised to fit the most current experimental data. The effect of the surface intersection has previously been typically studied by examining the branching and the internal state distributions of the products in the two channels as a function of excitation frequency only.

The collinear photodissociation dynamics is examined using a numerically exact time-dependent quantum-mechanical method. The equations of motion for the amplitudes upon the ground and two coupled excited electronic surfaces, explicitly incorporating the laser field, are integrated by a scheme which employs a low-order polynomial approximation to the evolution operator. The effects of the three field characteristics upon the branching ratio and internal state distributions of the products and the spectroscopy of the process are delineated. The course of the photodissociation dynamics is shown to be affected by these characteristics. The results demonstrate the causal connections between the pulse shape and the resulting photoprocesses. Practical manifestations of strong fields (power broadening, sub-threshold absorption, higher harmonic generation, emission shaping of the ground state, temporal development) are stressed.
Collective fluctuations of conserved variables in liquids
Authors: Wan Yi , Stratt Richard M.
Published: July 2002

Prominent among the classes of collective excitations in liquids that one would like to study are those which are compelled to obey some sort of conservation law. The instantaneous normal modes of liquid (which must be translationally invariant or, equivalently, conserve momentum) comprise one such example. The set of relaxation pathways dictated by a master-equation description of energy transfer in a liquid—which must conserve probability—constitutes another. We show that these conservation laws do impose fairly stringent requirements on the nature of the collective behavior, but the resulting excitations can nonetheless be described by liquid-theory methods. Within linear liquid theories, the desired distribution of modes ends up being a combination of a delocalized electronic-band-like portion and a fluctuating local field contribution. We illustrate the results with an explicit calculation (at the master-equation level) of energy-transfer kinetics in a liquid.
Symmetry-adapted perturbation theory calculation of the Ar–H2 intermolecular potential energy surface
Authors: Williams Hayes L. , Szalewicz Krzysztof , Jeziorski Bogumil , Moszynski Robert , Rybak Stanislaw
Published: July 2002

The many-body symmetry adapted perturbation theory has been applied to compute the Ar–H2 potential energy surface. Large basis sets containing spdfgh-symmetry orbitals optimized for intermolecular interactions have been used to achieve converged results. For a broad range of the configuration space the theoretical potential energy surface agrees to almost two significant digits with the empirical potential extracted from scattering and infrared spectroscopy data by Le Roy and Hutson. The minimum of our theoretical potential is εm=−164.7 cal/mol and is reached at the linear geometry for the Ar–H2 distance Rm=6.79 bohr. These values agree very well with corresponding empirical results εm=−161.9 cal/mol and Rm=6.82 bohr. For the first time such a quantitative agreement has been reached between theory and experiment for a van der Waals system that large. Despite such excellent agreement in the overall potential, the exponential and the inverse R components of it agree to only about 20%.
A computer simulation study of fluids in model slit, tubular, and cubic micropores
Authors: Murad S. , Ravi P. , Powles J. G.
Published: July 2002

Computer simulation studies have been carried out, using a novel method, to examine the behavior of fluids in various confined geometries, including, slit pores, square and cylindrical tubular pores, cubic pores, and pores with rough walls. The method used to model these pores allows for the permeability of the pore wall to the confined fluid to be controlled precisely between the impermeable and totally permeable limits, while at the same time maintaining the atomic nature of the pore wall. These systems have been studied with several models for the pore wall for a wide range of state conditions. The results obtained for nonuniform density distributions, wall permeabilities, and diffusion coefficients are examined in detail.
Predissociation effects in the A 2Δ and B 2Σ− states of CD
Authors: Danielsson M. , Erman P. , Hishikawa A. , Larsson M. , Rachlew-Källne E. , Sundström G.
Published: July 2002

Lifetimes have been measured for a large number of rotational levels of the A 2Δ and B 2Σ− states in CD using the high frequency deflection technique. Four B-state levels are found to be influenced by predissociation by rotation which demands a lower value of the CD dissociation energy. The B state predissociations have also been studied using ab initio calculations. The A state levels above the dissociation limit are found to be weakly predissociated through interaction with the ground state continuum. The results are compared with earlier lifetime investigations of CH.
Elastic properties of water under negative pressures
Authors: Alvarenga A. D. , Grimsditch M. , Bodnar R. J.
Published: July 2002

Using Brillouin scattering we have investigated the elastic properties of water under negative pressures. The samples were H2O liquid–vapor inclusions in α-quartz which could be heated to their homogenization temperatures (in the range 120–370 °C); on cooling, negative pressures as high as 1000 bars were reached before a vapor bubble nucleated. The velocity of sound results obtained from our Brillouin experiments indicate that nucleation occurs long before reaching the mechanical instability region where the bulk modulus is zero.
Packing entropy of extended, hard, rigid objects on a lattice
Authors: Li Wenshuo , Freed Karl F. , Nemirovsky Adolfo M.
Published: July 2002

We present a systematic method of evaluating the packing entropy for a set of mutually avoiding extended, hard, rigid objects on a lattice. The method generalizes a simple algebraic representation of the lattice cluster theory developed by Freed and co-workers for systems composed of flexible objects. The theory provides a power series expansion in z−1 for the corrections to the zeroth order mean field approximation partition function, where z is the lattice coordination number. We illustrate the general theory by calculating the packing entropy of four-unit rigid ‘‘square’’ objects on a hypercubic lattice as a function of the volume fraction of the squares. As a particular limiting case, we also evaluate for the packing entropy of two, three, and four squares on a two-dimensional square lattice and find agreement with the cluster expansion.
N4+ photodissociation: Charge exchange of N2+ fragments with Ar
Authors: Bieske Evan
Published: July 2002

The proportion of N4+ photofragmenting to yield N2+ in X 2Σg+v≳0 states has been measured by observing the yield of the fragments’ charge transfer reaction with Ar. Such a determination is possible because N2+ in the v=0 level has a cross section for charge exchange with Ar that is 2 orders of magnitude smaller than in higher vibrational levels. The fraction of N2+(v≳0) fragments increases from 0.30 at 620 nm to 0.37 at 266 nm. Calculations, which treat the N2+ and N2 fragments as quantum harmonic oscillators and the dissociation coordinate classically, predict an N2+ fragment vibrational content that is of the same order as that observed. The substantial population of vibrationally quiescent N2+ fragments, along with previous kinetic energy release studies of N4+ photodissociation, that show a limited fraction of the available energy appears as translational motion [M. F. Jarrold, A. J. Illies, and M. T. Bowers, J. Chem. Phys. 81, 214 (1984)], suggest that the fate of much of the photon’s energy is either in product rotational motion or in the neutral N2 vibration. Mechanisms for the dissociation are discussed.
Quantum mechanical thermal rate constants using flux–flux correlation functions and Padé analytical continuation procedures
Authors: Lefebvre Roland , Ryaboy Victor , Moiseyev Nimrod
Published: July 2002

A new expression for a thermal reaction rate is derived. It is based on the flux–flux correlation function approach and a finite basis set representation but differs from the Miller–Schwartz–Tromp (MST) formulation [J. Chem. Phys. 79, 4889 (1983)] by substitution of time dependent sine functions by parameter dependent Lorentzians. Then we apply Padé extrapolation procedure to eliminate the parameter. This operation replaces the search for stability of the time dependent rate in the MST approach. The convergence of the method is checked on the one dimensional Eckart barrier as an illustrative example.
Mass-selective ionization-detected stimulated Raman spectroscopy of benzene trimer and higher clusters
Authors: Henson B. F. , Venturo V. A. , Hartland G. V. , Felker P. M.
Published: July 2002

We present the results of mass-selective, ionization-detected stimulated Raman spectroscopies on jet-cooled benzene clusters from the trimer to the octamer. The data pertain to the fundamentals of the totally symmetric ring-breathing (ν1) and C–H stretch (ν2) modes of benzene and provide information about geometrical structure and vibrational dynamics. For the trimer, the results indicate a species in which all three benzenes reside in equivalent sites. For the higher clusters the results point to geometries involving two or more inequivalent sites. The ν1-excited trimer is found to decay on a nanosecond, or longer, time scale. The ν2-excited trimer decays on a time scale of greater than 5 ps. Similar timescales characterize the decays of the ν2-excited tetramer and pentamer.
Henry’s law constant for diatomic and polyatomic Lennard-Jones molecules
Authors: Ghonasgi D. , Llano-Restrepo M. , Chapman W. G.
Published: July 2002

Simulation results for the infinite dilution chemical potential of Lennard-Jones dispheres in a Lennard-Jones solvent are reported as a function of bond length at various temperatures and liquid densities. Comparisons are made with hard dispheres at infinite dilution. Predictions from theory of the infinite dilution chemical potential of linear chains of Lennard-Jones spheres in nonspherical and hydrogen bonding solvents are presented. Comparisons of the theory are made with recent simulation results for flexible chains of Lennard-Jones spheres.
Ultrafast photodissociation of I3. Coherent photochemistry in solution
Authors: Banin Uri , Ruhman Sanford
Published: July 2002

We report a comprehensive study of the UV photolysis of I3− in ethanol solution, using femtosecond time resolved transient transmission experiments. We interpret our results to indicate that with high probability, photoexcitation leads to direct formation of di-iodide ions within 300 fs, which are vibrating coherently. Through our experiments we have been able to determine that the time scales for vibrational dephasing, vibrational relaxation, and reorientation of the fragment ions are 400 fs, 4 ps, and 5 ps, respectively. Transmission signals at 620 nm and at 880 nm, which are above and below the λmax of the known absorption of I2−, oscillate at a precisely opposite phase. This and other results presented indicate that through the oscillations we are observing coherent vibration of the I2− photofragment. UV transient transmission experiments have been conducted in order to characterize the time scales for recombination. Preliminary results show that recombination takes place on several time scales. A fast component is assigned to primary recombination, followed by vibrational relaxation on the ground state of I3−. The impulsive nature of the I3− photoexcitation induces coherent symmetric stretching vibration in the ground state tri-iodide population. Finally, the large number of dynamical variables probed directly allows us to record the chronology of this reaction with unprecedented detail. We directly observe a new and potentially informative dynamical variable for this reaction—the absolute phase of fragment vibrations following the photodissociation.
An orientational instability and the liquid–vapor interface of a dipolar hard sphere fluid
Authors: Kasch M. , Forstmann F.
Published: July 2002

Investigating the stability of the dipolar hard sphere fluid by localizing the spinodal we found an additional instability for medium and typical vapor densities, an orientational instability with formation of aligned clusters. For liquid densities we find ordinary vaporization. Then we calculate the structure of the liquid–vapor interface of a dipolar hard sphere fluid in the spirit of the weighted density approximation (WDA) using an equation originally derived by Lovett, Mou, and Buff [J. Chem. Phys. 65, 570 (1976)] and Wertheim [J. Chem. Phys. 65, 2377 (1976)]. The surface tension is evaluated on the same level of approximation.
A theoretical investigation of the geometries, vibrational frequencies, and binding energies of several mixed alkali halide dimers
Authors: Cave Robert J. , Ono Ian
Published: July 2002

Results are presented from ab initio calculations on several mixed alkali halide dimers made up of Li, Na, F, and Cl. All of the dimers are investigated at the restricted Hartree–Fock level to provide ab initio estimates of geometries, vibrational frequencies, and binding energies. The dimer LiNaF2 is then treated using a variety of basis sets at the Hartree–Fock level, as well as at correlated levels (second-order Mo/ller–Plesset and coupled-cluster singles and doubles) to examine the sensitivity of the calculations to use of higher levels of theory. The results are then compared to the experimental data available for the LiNaF2 molecule, previous theoretical results for the monomers, and recent semiempirical calculations on the mixed dimers.
Classical trajectory simulation of the cluster–atom association reaction I–Arn+I→I2+nAr. II. Diffusion of captured iodine and evaporative cooling of I2
Authors: Hu Xiche , Martens Craig C.
Published: July 2002

This is Part II of a series of papers in which we address the role of microscopic solvation in the association reaction between a free iodine atom and an iodine doped van der Waals cluster: I+I(Ar)n→I2+nAr. The influence of microscopic solvation on the I+I to I2 reactivity, reaction mechanism, energetics, and product energy partitioning is the major focus of our study. The overall reaction for I+I(Ar)12→I2+12Ar can be characterized by three fundamental processes: (1) capture of the incident iodine atom by the I(Ar)12 cluster; (2) diffusive migration of the captured I atom on the surface or in the interior of the cluster, leading ultimately to an encounter with the other I atom to form a highly excited I*2 molecule; (3) vibrational relaxation of the nascent I*2 product, leading to evaporative cooling and decomposition of the cluster. Part I [J. Chem. Phys. 98, 8551 (1993)] dealt with the capture process. This article focuses on the chemical dynamics of the subsequent processes of diffusion, vibrational energy transfer, and evaporative cooling. The stabilization of the chemically activated I*2 molecule through evaporative cooling eliminate the need of a third body collision as required in isolation gas phase recombination. The overall distribution of final energies is nonstatistical for the chemically activated I*2Arn. The final vibrational energy of I2 exhibits a nonthermal structure even after all the argon atoms are evaporated. In addition to monoatomic sequential evaporation, a ‘‘fissioning’’ mechanism, leading to the formation of at least one multiatom fragment, is observed. The relationship between structure and dynamics is explored. The dynamics of vibrational relaxation, diffusion of the captured iodine, evaporation, and fragmentation pattern, final I2 energy partitioning are found to be strongly dependent upon structure and temperature of the doped cluster. A spectroscopic experimental verification of the above observations is also proposed.
Gas-phase reactivity of sulphur ions with ammonia
Authors: Niedner-Schatteburg Gereon , Schindler Thomas , Berg Christian , Wössner Dirk , Bondybey Vladimir E.
Published: July 2002

The gas-phase reactions of sulphur cluster ions, cations S+n (n=1,...,8) and anions S−n (n=2,...,6) with NH3 have been studied by means of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Relative and absolute reaction rate constants as well as reaction efficiencies have been determined. The S+4 cation has by the far highest reactivity with all other sulphur cations being significantly less reactive. Besides some fragmentation, no reactivity of the anions was found. Structural conclusions are made based on the present experiments, on previous extensive ab initio calculations, and on complementary experiments.
Pressure induced vibrational frequency shifts of ethane and methyliodide. Evidence for the formation of C–H hydrogen bonds in high density fluids
Authors: Lee Meng-Rong , Ben-Amotz Dor
Published: July 2002

High pressure Raman scattering vibrational frequency shift studies of C2H6, C2H6/CH2Cl2, and CH3I are compared with perturbed hard sphere theory predictions. Good agreement is found for the C–C and C–I modes, while the C–H modes are found to soften anomalously at high density, suggesting the formation of C–H hydrogen bonds.
Rotational analysis of n=4–7 Rydberg states of CO observed by ion-dip spectroscopy
Authors: Komatsu Masaaki , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Rotationally resolved spectra of the 5s–7s, 5p–7p, 5d, 6d, and 4f–6f Rydberg states (v=1) of jet cooled CO have been measured by ion-dip spectroscopy with triple resonance excitation via the 3sσ B 1Σ+(v=1) state. The dip spectra due to the high Rydberg (v=1)←B 1Σ+(v=1) transition revealed the states most of which have not been observed by other spectroscopy. By the rotational analysis of the dip spectra, electronic term values, quantum defects, and rotational constants were obtained. For the np Rydberg states, the change of the angular momentum coupling between the p Rydberg electron and the CO+ core was clearly observed as a function of the principal quantum number, n, indicating a transition from Hund’s case (b) to Hund’s case (d). It was found that the rotational constant of the nsσ state increases with n, while that of the ndσ state decreases. The changes in the rotational constants were interpreted by the mixing between the nsσ state and the (n−1)dσ state and the mixing coefficients for those states were determined. For the 4f, 5f, and 6f Rydberg states, only the rotational levels belonging to e-symmetry were observed in the dip spectrum. It indicates the selective predissociation through the low lying D′ 1Σ+ state.
Competition effects in surface diffusion controlled reactions: Theory and Brownian dynamics simulations
Authors: Khakhar D. V. , Agarwal U. S.
Published: July 2002

Surface diffusion controlled reactions on a heterogeneous catalyst surface comprising randomly placed circular reactive sites are considered. The diffusing species adsorbs onto the surface following Langmuir–Hinshelwood kinetics and reacts instantaneously on contact with a reactive site. Approximate theories are formulated to describe the process for high concentrations of the reactive sites, when competition between the sites is significant, following three different approaches: (i) modification of the single sink theory; (ii) using a cell model; and (iii) using an effective medium theory. The predictions of the theories are compared with the results of multiparticle Brownian dynamics simulations for the overall reaction rate, the bulk concentration of the reactive species, and the ensemble averaged concentration profile around a reactive site. The effective medium theory is found to give the best results among the theories considered, and the predictions are in good agreement with the computational results.
Conformations and diffusion of n-pentane adsorbed on a metallic surface
Authors: Silverberg Marvin
Published: July 2002

The diffusion of n-pentane molecules on metallic surfaces is modeled by molecular dynamics simulations. On atomically smooth, face-centered-cubic (111) crystal surfaces, adsorbed n-pentanes have several conformational adsorption states, each of which diffuses at a different rate. Diffusion barriers are distinct from intramolecular conformational isomerization barriers. Weak friction is observed. Hopping diffusion is observed near the diffusion threshold. As temperature is raised, correlated barrier crossing and recrossing increases. Adsorbed n-pentanes slide, transverse to their axes, without changing their orientation with respect to the surface. Conformational changes occur on a longer time scale than diffusional barrier crossing and are restricted by the surface. At temperatures far above the diffusion threshold, partial molecular desorption occurs and conformational isomerization is less restricted. The molecules slide, roll, and tumble continuously across the surface.
Many-body perturbation theory of electrostatic interactions between molecules: Comparison with full configuration interaction for four-electron dimers
Authors: Moszynski Robert , Jeziorski Bogumil/ , Ratkiewicz Artur , Rybak Stanisl/aw
Published: July 2002

Many-body perturbation theory for a direct calculation of the electrostatic interaction energy is developed. Since no multipole expansion is used, the obtained electrostatic energy includes the short-range contributions resulting from the overlap (penetration) of monomers’ charge distributions. The influence of intramonomer electronic correlation is accounted for by the perturbation expansion in terms of the Mo/ller–Plesset type fluctuation potentials for the interacting molecules. Two types of expansions are introduced: one based on the standard Mo/ller–Plesset expansion of the electron density, and the second accounting for the perturbation induced modifications of the monomer’s Fock operators, i.e., for the so-called response or orbital relaxation effects. Explicit orbital expressions for the terms through the fourth order in the intramonomer fluctuation potentials are derived. In this way the leading three-particle correlation contribution to the electrostatic energy is taken into account. Numerical results of perturbative calculations through the fourth order for the He2, (H2)2, and He–H2 systems are presented and compared with the complete configuration interaction results obtained using the same basis sets. It is found that the convergence of the many-body perturbation expansion of the electrostatic energy is fast. The sum of corrections through the fourth order reproduces to within few percent the correlation part of the electrostatic energy computed with the full configuration interaction wave function.
Kinetics of atomic hydrogen+adsorbed Br reactions on Si(100) and Si(111) surfaces
Authors: Koleske D. D. , Gates S. M.
Published: July 2002

The kinetics of adsorbed Br removal on Si(100) and Si(111) using atomic hydrogen are presented for surface temperatures from 50–560 °C. Both the surface H and Br coverages are measured in real time as the reaction proceeds using time-of-flight scattering and recoiling spectroscopy (TOF-SARS). For both surfaces below TS=400 °C, the Br removal rate has a first-order dependence on Br coverage (θBr) and first-order dependence on atomic hydrogen flux (FH). On Si(111), the Br removal rate also has a first-order dependence on the surface hydrogen coverage (θH). The data for TS≳400 °C are explained by a thermal HBr desorption process, with a measured activation energy of 45 kcal/mol on Si(100) and 43 kcal/mol on Si(111). At low TS (<400 °C), Br is removed via an apparently activationless mechanism with activation energies of 0.7 kcal/mol on Si(100) and 1.4 kcal/mol on Si(111). Although the experimental observations on Si(100) are consistent with an Eley–Rideal (ER) reaction mechanism, other mechanisms should be considered. We suggest that Hat may be partially accommodated at the surface in a mobile precursor state before reaction with adsorbed Br. During accommodation of Hat, chemical potential energy is delivered to the surface and may ‘‘chemically activate’’ a local area causing HBr desorption to occur. The degree of ‘‘chemical activation’’ will depend on the rate and degree of Hat accommodation, with the maximum delivered in direct Si–H bond formation.
The balance between theoretical method and basis set quality: A systematic study of equilibrium geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities
Authors: Thomas J. Russell , DeLeeuw Bradley J. , Vacek George , Crawford T. Daniel , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Analytic gradient methods have been used to predict the equilibrium geometries, dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities of HCN, HNC, CO2, CH4, NH4+, HCCH, H2O, H2CO, NH3, and FCCH at the self-consistent-field (SCF), the single and double excitations configuration interaction (CISD), the single and double excitations coupled-cluster (CCSD), and the single, double, and perturbative triple excitations coupled-cluster [CCSD(T)] levels of theory. All studies were performed using a triple zeta plus double polarization (TZ2P) basis set and a TZ2P basis set augmented with one set of higher angular momentum functions [TZ (2df,2pd)]. The predicted equilibrium geometries, dipole moments, harmonic vibrational frequencies, and IR intensities were compared to available experimental values. The geometries were predicted accurately at the highest levels of theory. Most of the dipole moments were found to agree favorably with experiment. With the TZ2P basis set, the average absolute errors in harmonic vibrational frequencies with respect to experiment were 9.9%, 3.8%, 1.5%, and 2.3% for the SCF, CISD, CCSD, and CCSD(T) methods, respectively. With the TZ(2df,2pd) basis set, the four methodologies yielded average absolute errors of 10.3%, 6.3%, 3.7%, and 2.2%, respectively. When the absolute errors for bending modes of triply bonded molecules and the a1 umbrella mode of NH3 were excluded from the previous two sets of averages, the TZ2P average errors became 7.3% (SCF), 3.0% (CISD), 1.1% (CCSD), and 1.1% [CCSD(T)], and the TZ(2df,2pd) average errors became 7.4% (SCF), 3.5% (CISD), 1.5% (CCSD), and 0.6% [CCSD(T)]. Theoretical IR intensities were generally close to given experimental values. Among the eight methodologies investigated in this research, the TZ2P CCSD and the TZ(2df,2pd) CCSD(T) methods exhibited the best balance between theoretical method and basis set quality. This ‘‘balance’’ was evident in the simultaneous prediction of the most accurate values overall for the molecular properties compared.
Identities, perturbative expansions, and recursion relations for mapping operators, effective Hamiltonians, and effective operators
Authors: Hurtubise Vincent
Published: July 2002

We derive perturbation expansions for the mapping operators (k,l) that transform a full Hilbert space time-independent Hamiltonian H and operator A into, respectively, a finite (multidimensional) space effective Hamiltonian h and effective operator a. The eigenvalues of h are identical to some of those of H, and a produces exact matrix elements of A for the corresponding states. Our derivations are substantially both more general and simpler than most literature ones and yield simple linear recursive expressions for k and l. Both these mapping solutions and new identities involving h, a, k, and l straightforwardly produce new recursive relations for h and the first known recursive a expressions. We apply our results to the Bloch, Kato, and all norm-preserving formalisms, including the canonical one. The new h and a identities are also shown to be suitable for iterative and multireference coupled cluster approaches.
Static structure of the two-dimensional hard-disk plus Yukawa fluid
Authors: Aranda-Espinoza H. , Medina-Noyola M. , Arauz-Lara J. L.
Published: July 2002

The static structure of the two-dimensional hard-disk plus Yukawa fluid is studied on the basis of the hypernetted chain approximation. We find that the most relevant features exhibited by the three-dimensional hard-sphere plus Yukawa system are also exhibited by its two-dimensional counterpart.
The role of added polymer in dilute lamellar surfactant phases
Authors: Brooks J. T. , Cates M. E.
Published: July 2002

We study theoretically the effect of adding polymer to the dilute lamellar phase of surfactant solutions. The polymer, assumed semidilute and in a good solvent, is taken to have a repulsive or attractive interaction with surfactant bilayers, but has no other effect on their properties. Such polymers can mediate an attractive or repulsive interaction between bilayers which we combine with pre-existing interactions (the Helfrich repulsion, van der Waals, and hydration) using a simple additive model. The polymeric contribution to the free energy is estimated using a blob model (for nonadsorbing chains) and the Cahn–de Gennes scaling functional (for adsorbing chains). Phase diagrams for the system are obtained under various conditions. For a lamellar phase which is bound (i.e., which cannot be indefinitely diluted) as a result of van der Waals forces, nonadsorbing polymer is completely expelled as a semidilute solution, leading to a compression of the lamellar stack. Adsorbing polymer can enter the phase, also tending to cause a slight reduction in the maximum layer spacing. For an unbound lamellar phase controlled by Helfrich forces, nonadsorbing polymer is expelled if the bilayers are stiff, but for flexible enough layers, a significant amount of polymer can be solubilized. At too high a volume fraction of polymer, part of the polymer is expelled as a solution, whereas at high surfactant fractions, a phase separation arises between two lamellar phases (one containing polymer, the other not). For the case of adsorbing chains, small amounts of added polymer cause the system to become bound with expulsion of excess solvent. For large amounts of added polymer, the unbound behavior is recovered, whereas for intermediate polymer content, phase equilibria involve either two lamellar phases (each containing polymer), or one such phase coexisting with a polymer solution.
Accurate coupled cluster reaction enthalpies and activation energies for X+H2→XH+H (X=F, OH, NH2, and CH3)
Authors: Kraka Elfi , Gauss Jürgen , Cremer Dieter
Published: July 2002

Coupled cluster calculations at the CCSD(T)/[5s4p3d/4s3p] and CCSD(T)/[5s4p3d2 f1g/4s3p2d] level of theory are reported for reactions X+H2→XH+H [X=F (1a), OH (1b), NH2 (1c), and CH3 (1d)] utilizing analytical energy gradients for geometry, frequency, charge distribution, and dipole moment calculations of reactants, transition states, and products. A careful analysis of vibrational corrections leads to reaction enthalpies at 300 K, which are within 0.04, 0.15, 0.62, and 0.89 kcal/mol of experimental values. For reaction (1a) a bent transition state and for reactions (1b) and (1c) transition states with a cis arrangement of the reactants are calculated. The cis forms of transition states (1b) and (1c) are energetically favored because of electrostatic interactions, in particular dipole–dipole attraction as is revealed by calculated charge distributions. For reactions (1a)–(1d), the CCSD(T)/[5s4p3d2 f1g/4s3p2d] activation energies at 300 K are 1.1, 5.4, 10.8, and 12.7 kcal/mol which differ by just 0.1, 1.4, 2.3, and 1.8 kcal/mol, respectively, from the corresponding experimental values of 1±0.1, 4±0.5, 8.5±0.5, and 10.9±0.5 kcal/mol. For reactions (1), this is the best agreement between experiment and theory that has been obtained from ab initio calculations not including any empirically based corrections. Agreement is achieved after considering basis set effects, basis set superposition errors, spin contamination, tunneling effect and, in particular, zero-point energies as well as temperature corrections. Net corrections for the four activation energies are −1.05, −0.2, 1.25, and 0.89 kcal/mol, which shows that for high accuracy calculations a direct comparison of classical barriers and activation energies is misleading.
Long-time dynamics in poly(dimethylsiloxane) networks studied by deuterium nuclear magnetic resonance
Authors: Sotta P. , Deloche B.
Published: July 2002

A deuterium NMR (nuclear magnetic resonance) method is proposed to investigate long-time correlation of chain orientation in poly(dimethylsiloxane) networks. It is based on the observation of a quadrupolar echo after a Jeener pulse sequence, which is possible owing to the presence of nonzero residual interactions in the system related to local chain orientations. The time evolution of the quadrupolar echo amplitude is calculated in the framework of a slow chain reorientational motion. An experimental result is presented which shows that local orientation is frozen within the time scale investigated, up to 0.3 s.
Molecular dynamics of water in oriented DPPC multilayers studied by quasielastic neutron scattering and deuterium-nuclear magnetic resonance relaxation
Authors: König S. , Sackmann E. , Richter D. , Zorn R. , Carlile C. , Bayerl T. M.
Published: July 2002

The dynamics of water between highly oriented multilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium-nuclear magnetic resonance (NMR) longitudinal (T1) relaxation were employed to investigate both the high-frequency motions of water (10−9–10−11 s time scale) and their anisotropy, while 2H-NMR transverse (T2) relaxation was used for obtaining information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond-time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules tightly bound to DPPC head groups with a correlation time τrot≊62 ps at 55 °C and a rotational radius of 1±0.1 Å, but with no detectable translational degrees of freedom. The 2H-NMR T1 data (nanosecond-time scale) can be explained satisfactorily on the basis of fast rotations with the correlation time above and a slower reorientation of the rotational axis (correlation time τ1≊6 ns).

Both QENS and 2H-NMR T1 measurements provide an apparent activation energy of Ea=32±1.0 kJ/mol for this process. Increasing the hydration level of the multilayers leaves the rotational motion essentially unchanged, but enables additional translational motion which can be considered as a jump diffusion process (diffusion coefficient D=16±1×10−10 m2/s at 44 °C and a mean residence time of τ0=2.0±0.5 ps) of nonbound water. It is interesting to note that this diffusion is completely isotropic on the characteristic length scale of this QENS experiment (≤10 Å). Temperature variation shows that the phase state of the lipids has no significant effect on the high frequency dynamics of the water molecules. Measurements of the 2H-NMR quadrupolar splitting of water (D2O) at temperatures around the phase transition temperature Tm of the oriented DPPC multilayers clearly show a coexistence of the crystalline Lβ′ phase and of the fluid Lα phase over a range of up to 4 °C at both sides of Tm. The intermediate Pβ′ (‘‘ripple’’) phase is suppressed as we worked at hydration levels below saturation. In the coexistence range, exchange of water takes place between crystalline and fluid lipid domains due to water diffusion. This exchange causes a pronounced minimum of the 2H-NMR transverse relaxation time T2 at Tm since this low frequency process satisfies approximately a critical damping condition for a two-site chemical exchange process.
Predictions of free energy differences from a single simulation of the initial state
Authors: Smith Paul E. , Gunsteren Wilfred F. van
Published: July 2002

We have investigated the feasibility of using higher derivatives of the free energy, with respect to a coupling parameter λ, for predicting free energy changes between different states from a single simulation corresponding to the initial state, i.e., λ=0 in the normal thermodynamic integration approach. Terms up to and including the fifth derivative have been used. For a diatomic with point charges (dipole moment=2.4 D) in a box of water, the free energy corresponding to charge rearrangements of the order of ±0.25e could be predicted accurately after 750 ps of simulation and inclusion of the third derivative. Predictions were best for small perturbations. After 1000 ps of simulation, the first, second, third, and probably the fourth derivatives had converged, while the fifth was still oscillating. The possibility of predicting a multitude of free energy changes from a single simulation is discussed.
Nonequilibrium mass and heat transport in polymers: Spectroscopic approach
Authors: Huang Jianbing , Urban Marek W.
Published: July 2002

This study attempts to combine the principles of nonequilibrium thermodynamics and photoacoustic Fourier transform infrared (PAFT-IR) spectroscopic detection as utilized in the analysis of nonequilibrium processes in matter. Such processes as diffusion and glass transition temperatures in polymers are analyzed. It is shown that the nonequilibrium mass balance equation, which governs diffusion processes, and the nonequilibrium internal energy balance responsible for the photoacoustic detection can be correlated. As a result, the relationship between the concentration of diffusant molecules in time and the photoacoustic intensity as a function of concentration can be established. Analysis of a nonequilibrium nature of the thermal processes near glass transmission temperatures shows that thermal nonequilibrium relaxations can be described in terms of rate constants for which the weight fractions with various energies can be correlated to the temperature fluctuations in a photoacoustic experiment. In this case, the internal energy balance equation can be represented by a regular heat expression.
Inner-shell excitation and site specific fragmentation of poly(methylmethacrylate) thin film
Authors: Tinone Marcia C. K. , Tanaka Kenichiro , Maruyama Junya , Ueno Nobuo , Imamura Motoyasu , Matsubayashi Nobuyuki
Published: July 2002

Soft x-ray excitations in the 250–600 eV photon energy range on poly(methylmethacrylate) (PMMA) result in ionic fragmentation of the original polymer with the most intense ions corresponding to CH+3, H+, CH+2, CH+, CHO+, and COOCH+3. The photon energy dependence of ion desorption from thin films of PMMA was measured to investigate the primary steps in radiation induced decomposition following carbon and oxygen 1s electron excitations using monochromatic pulsed-synchrotron radiation. It was clearly found that the decomposition depends on the nature of the electronic states created in the excited species. The fragmentation pattern changes depending on the transitions of the 1s electron to a Rydberg orbital, an unoccupied molecular orbital or the ionization continuum. Moreover, the fragmentation occurs specifically around the site of the atom where the optical excitation takes place. Excitations from carbon and oxygen 1s to σ* states seem to be specially efficient for ion production as observed in the case of CH+3, CH+2, and CH+ at 288.7 and 535.6 eV, and in the case of CHO+ at 539.3 eV.
Growth of hexagonal string phases in sheared colloid simulation
Authors: Visscher P. B. , Heyes D. M.
Published: July 2002

Strongly sheared colloids often form strings oriented along the shear direction, which in turn often order into a quasi-2D hexagonal lattice. We examine this ordering process in detail to elucidate its strong system-size dependence, which is a major obstacle to establishing a connection between simulation and experiment. We catalog the possible lattices and use an energy criterion to predict which one a particular system is likely to crystallize in. The process is similar to nucleation and growth in a true 2D system except that strings crystallize without point defects (vacancies and interstitials), unlike 2D disks, because particle (string) number is not conserved.
Computational studies of halogen chemistry on rare-gas surfaces. III. Photodissociation in submonolayer chlorine films on Ar(111) and Xe(111)
Authors: Trayanov A. L. , Prisant M. G.
Published: July 2002

The photodissociation reaction of submonolayer molecular chlorine films on rare-gas surfaces has been studied using molecular dynamics computer simulation. In particular, we have considered the coverage and temperature dependence of escaping photofragment yield, angular distribution, and translational energy distribution of the reaction h(ν=29 661 cm−1) +Cl2(1Σ)−Cl2[Θ]ads−Rg(111)[T]→2Cl⋅(1Π)[E=9699 cm−1]+Cl2[Θ]ads−Rg(111)[T′], where E is the initial potential energy of the dissociating fragments, Θ indicates the film coverage, T is the substrate temperature, and Rg indicates the rare-gas substrate which is either argon or xenon. Even at the submonolayer coverages studied here (X2[adsorbed]/Rg[surface]<1), fewer than half of the photodissociation fragments escape the surface. The mean translational energy of the escaping fragments as a function of coverage indicates that, on average, escaping fragments suffer collisions and lose energy before leaving the surface. At all coverages, the translational energy distribution is roughly Gaussian and peaked at approximately one-third of the total photodissociation energy. However, at higher coverages, the escaping fragment may actually carry more than half of the photodissociation translational energy. The average direction of escaping photofragment velocities as a function of coverage generally reflects the original diatom orientation in the adlayer but is shifted toward the surface normal. With increasing coverage, the angular distribution of fragment velocities goes from roughly Gaussian in the range between 0° and 90° to highly peaked about the surface normal.
Lattice theory of solvation and dissociation in macromolecular fluids. II. Quasichemical approximation
Authors: Olender Roberto , Nitzan Abraham
Published: July 2002

Solvation and association of ions in polymer solvents used for polymer electrolytes is studied in the framework of the Guggenheim quasichemical approximation using a lattice model. The pure solvent is described using the mean-field approximation of Sanchez and Lacombe. The model accounts for the short range part of the ion–polymer interaction, in particular at specific interaction sites (such as the cation–oxygen attraction in polymer hosts), while long range dieletric effects are added as in the Born theory of solvation. We obtain expressions which relate thermodynamic quantities of solvation (free energy, entropy, enthalpy, and volume) to properties of the pure solvent. From these, the equilibrium constant for the ion pairing can be obtained as a function of temperature, pressure, and solvent properties. A consistent fit to the experimental data in poly(propylene oxide) (PPO) can be obtained if specific interaction sites for the cations are assumed. Dependence of the solvation and the equilibrium constant on the polymer chain length which is seen in the short chain limit saturates and disappears beyond a few monomer sizes. The relative roles of short range and of dielectric interactions is discussed.