bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of an inverse method to the determination of a two-dimensional intermolecular potential energy surface for the Ar–OH(A 2Σ+, v=0) complex from rovibrational spectra
Authors: Ho Tak-San , Rabitz Herschel , Choi Seung E. , Lester Marsha I.
Published: July 2002

A stable inversion method of determining molecular potentials from a finite number of spectroscopic data is presented. Molecular inverse problems are often underdetermined, unstable, and nonlinear. Specifically, the measured spectra contains only partial information of the sought-after potential and even a small error in the data may cause a large variation in the inverted solution. Moreover, the underlying potential is a complicated nonlinear functional of the spectral data. The inversion algorithm, based on the Tikhonov regularization method, resolves all of the above predicaments and yields accurate sought-after potentials with proper analytic properties. The method is applied to extract two-dimensional Ar–OH(A 2Σ+, v=0) potential energy surfaces from the rotational–vibrational spectral data. Two versions of the recovered potential energy surfaces are obtained using two slightly different sets of rovibrational data. These two potentials are basically equivalent, except in the regions to which the data are insensitive, and possess physically acceptable smooth features with the correct long-range behavior. Both recovered potentials reproduce the observed spectra, the estimated rotational constants, and the binding energy within the experimental accuracy.
Decay of high Rydberg states: A paradigm for intramolecular dynamics in a congested bound level structure coupled to a continuum
Authors: Remacle F. , Levine R. D.
Published: July 2002

The dynamics of very high molecular Rydberg states, with special attention to the extreme long time behavior, is discussed using a quantum mechanical multichannel scattering theoretic formulation. Detailed computational results are provided for an electron revolving about a dipolar core in the presence of an external dc field. Two distinct effects are highlighted, trapping and dilution where the former is an origin of long time stability when there are very many bound states but much fewer states that are directly coupled to the continuum. Both trapping and dilution act to elongate the intermediate time decay of the high Rydberg states. The extent of dilution can be varied by changing the magnitude of the external dc electrical field. The formalism and specific results are discussed also towards the implications to other types of unimolecular processes. In particular it is argued that the study of molecular Rydberg states does suggest a possible route to mode selective chemistry.
Critical clusters in binary mixtures: A density functional approach
Authors: Talanquer V. , Oxtoby David W.
Published: July 2002

We apply a square-gradient density functional theory to study the nucleation of the gas-to-liquid transition for nonideal binary mixtures. For weakly nonideal mixtures, we predict deviations from ideal behavior in the compositions and free energies of critical nuclei. When the nonideality becomes large enough to allow miscibility gaps, some qualitatively new features arise. We find regions in phase space in which more than one critical nucleus can form, as well as cases in which the critical nucleus itself begins to phase separate, forming a structure of cylindrical symmetry in which the two ends are enriched in the two components of the mixture.
Adiabatic path integral molecular dynamics methods. I. Theory
Authors: Martyna G. J.
Published: July 2002

Recently, a finite temperature semiclassical quantum dynamics method based on the use of classical trajectories generated on the centroid potential of mean force, the free energy surface associated with the path integral centroid [Cao and Voth, J. Chem. Phys. 99, 10070 (1993)], was presented. It is natural to consider whether or not a similar theory can be constructed using classical trajectories generated on the true quantum mechanical potential mean force, the free energy surface associated with a single path integral bead (canonical Wigner-type approximations). Here, Wigner schemes are derived that give the exact quantum dynamics of quadratic actions, contain the classical limit and when used as a statistical theories give the correct rate constant for a parabolic barrier. The fundamental difference between the two types theories, Wigner and centroid, is that the Wigner formalism at very least requires a set of instantaneous normal modes while the centroid method is normal mode free.
Constant temperature molecular dynamics simulations by means of a stochastic collision model. II. The harmonic oscillator
Authors: Kast Stefan Michael , Brickmann Jürgen
Published: July 2002

Our recently introduced stochastic method for molecular dynamics simulations at constant temperature [J. Chem. Phys. 100, 566 (1994)] which is based on impulsive collisions between system particles and heat bath particles of finite masses, is extended and analyzed for the case of an ensemble of harmonic oscillators as a simple but theoretically solvable model for interacting systems. This model case can be considered, e.g., as a single normal mode of a polyatomic molecule. Both position space properties and velocity space properties are investigated. Analytical expressions for stationary probability densities and autocorrelation functions are derived. The effect of the truncation of the Taylor series which is the basis of Verlet-type algorithms, on the resulting temperature in position space and velocity space is quantitatively discussed. It is demonstrated that the system temperature in position space and in velocity space is controlled by complex parameter functions.
Differential cross sections for state specific reactive scattering of Na–SF6→NaF–SF5
Authors: Düren R. , Färber M. , Heumann B. , Knepper M. , Mohr S. , Weiss C. , Te Lintel Hekkert S. , Linskens A. F. , Reuss J.
Published: July 2002

Double-differential cross sections for the interaction of Na(3 2S) and Na(3 2P) with SF6 have been measured in crossed beam experiments for center of mass collision energies between 0.25 and 1.75 eV. In comparison with recently reported experiments the reaction with vibrationally excited SF6 is found to be more effective than the one with electronically excited Na. Results from an ab initio CASSCF calculation with Na in the ground state and the 3P state are presented. The experimental findings and the results from the calculation lead us to two different models for the reaction in the ground state and the excited state: While the well known harpooning model is verified for the ground state the reactive collisions with excited Na are mediated by nonadiabatic (nonreactive) transitions to the ground state surface. For these transitions the vibrational motion of SF6 is much more efficient than the relative motion in the collision.
Theory for long time polymer and protein dynamics: Tests for all-atom models of alkane dynamics
Authors: Chang Xiao-yan , Freed Karl F.
Published: July 2002

We extend the matrix expansion method to study the long time dynamics for all-atom models of alkane chain internal dynamics. In order to focus on the influence of the poorly understood memory (often also termed ‘‘internal friction’’) contributions, the theory is compared with Brownian dynamics simulations in which the molecular solvent is replaced by a white noise source of random and frictional forces on the individual carbon and hydrogen atoms of the alkane molecule. The interaction potentials contain torsional potentials and nonbonded interactions, and the same potentials are used for both the theory and the simulations. Hence, the comparisons between theory and simulations involve no adjustable parameters. The first order theory is equivalent to a generalized Rouse model in which harmonic forces exist, in principle, between every pair of atoms in the alkane chain, with the force constants evaluated in terms of static equilibrium correlations. The first order theory provides a decent representation of the long time (t≥100 ps) portions of the C–C motion time correlation functions (dipole and orientational), but the theory is poor for the C–H correlation functions because they have correlation times much shorter than 100 ps. Various higher order mode coupling basis sets are investigated to include the long time influences of the memory terms, and the computations consider the role of the more rapid hydrogen motions in exerting a frictional drag on the slower C–C bond motions. The truncated second order basis provides a rapidly convergent and accurate representation of the long time dynamics.
On the relation between the Wertheim’s two-density integral equation theory for associating fluids and Chandler–Silbey–Ladanyi integral equation theory for site–site molecular fluids
Authors: Kalyuzhnyi Yu. V. , Cummings P. T.
Published: July 2002

It is demonstrated that Chandler–Silbey–Ladanyi integral equation theory for the site–site molecular fluids is the limiting case of complete association of more general two-density integral equation theory for associating fluids developed by Wertheim. The analysis is presented for a site–site molecular system with any number and geometrical arrangement of the sites in the molecule and arbitrary type of the site–site pair interaction.
The lower C2v potential energy surfaces of the singlet states of H2O: A computational study
Authors: Schneider F. , Di Giacomo F. , Gianturco F. A.
Published: July 2002

We present here computational results on 15 C2v potential energy surfaces (PES) of H2O in its singlet state, while further results on the doublet state of the cation of the same system will be reported in a following paper. The PES are shown as 2D contour maps. Reaction pathways for symmetric dissociation of H2O and photoionisation spectra of H2O are discussed and analyzed in some detail. The results were obtained using ab initio multireference configuration interaction (MRDCI) calculations at 184 nuclear arrangements and should provide one of the most extensive analysis of the electronic features of this very important chemical system.
Dynamics of proton transfer in mesoscopic clusters
Authors: Consta Styliani , Kapral Raymond
Published: July 2002

Proton transfer rates and mechanisms are studied in mesoscopic, liquid-state, molecular clusters. The proton transfer occurs in a proton–ion complex solvated by polar molecules comprising the cluster environment. The rates and mechanisms of the reaction are studied using both adiabatic and nonadiabatic molecular dynamics. For large molecular clusters, the proton–ion complex resides primarily on the surface of the cluster or one layer of solvent molecules inside the surface. The proton transfer occurs as the complex undergoes orientational fluctuations on the cluster surface or penetrates one solvent layer into the cluster leading to solvent configurations that favor the transfer. For smaller clusters the complex resides mostly on the surface of the cluster and proton transfer is observed only when the complex penetrates the cluster and solvent configurations that favor the proton transfer are achieved. Quantitative information on the cluster reaction rate constants is also presented.
An interpolated unrestricted Hartree–Fock potential energy surface for the OH+H2→H2O+H reaction
Authors: Jordan Meredith J. T. , Collins Michael A.
Published: July 2002

In this paper we demonstrate, at the UHF/6-311++G(d,p) level of theory, the practical feasibility of using ab initio quantum chemical calculations to generate a molecular potential energy surface (PES) for the OH+H2→H2O+H reaction using our previously suggested interpolation and iteration schemes. The successful, and almost completely automated, merger of the PES algorithm and quantum chemical calculations involves a number of significant practical problems, the solutions of which are presented in detail. The convergence of the interpolated potential surface was monitored in terms of reaction probability and we find that the surface converges once the energy, gradient and Hessian have been calculated at approximately 350 geometries. We also find that, although the initial geometries used consisted only of points along a reaction path for the OH+H2→H2O+H reaction, the potential energy surface iteration process rapidly adds information about other, energetically accessible, reaction channels.
Ab initio study of the ammonia ion–ammonia reaction paths
Authors: Ben Amor Nadia , Maynau Daniel , Spiegelmann Fernand
Published: July 2002

The three reactions NH+3+NH3→NH2+NH+4 (proton transfer), NH+3+NH3→NH+4+NH2 (atom transfer) and NH+3+NH3→NH3+NH+3 (charge transfer) are studied in an ab initio framework. All geometry optimizations are carried out at the MP2 level, and a SDCI(TQ) calculation is performed at the optimized geometry. For the charge transfer reaction, the energy is calculated as a function of the N–N internuclear distance. The intermediate complex is found to have D3d symmetry. The geometry of the NH3+NH+3 system is optimized for each value of the N–N distance. For the proton transfer reaction, the energy is calculated as a function of two variables which are the two N–H internuclear distances of the central part N–H–N of the complex. For each N–H–N configuration, other coordinates of the system are completely optimized. This approach shows that the atom transfer reaction can be interpreted as a charge transfer process followed by a proton transfer. The influence of the vibrational excitation of the NH+3 reagent on the reaction is discussed.
Phase stability of binary non-additive hard-sphere mixtures: A self-consistent integral equation study
Authors: Lomba E. , Alvarez M. , Lee L. L. , Almarza N. G.
Published: July 2002

We have tested the capabilities of a new self-consistent integral equation, closely connected with Verlet’s modified closure, for the study of fluid-fluid phase separation in symmetric non-additive hard-sphere mixtures. New expressions to evaluate the chemical potential of mixtures are presented and play a key role in the construction of the phase diagram. The new integral equation, which implements consistency between virial and fluctuation theorem routes to the isothermal compressibility, together with chemical potential and virial pressure consistency via the Gibbs-Duhem relation, yields a phase diagram which especially at high densities agrees remarkably well with the new semi-Grand Ensemble Monte Carlo simulation data also presented in this work. Deviations close to the critical point can be understood as a consequence of the inability to enforce virial-fluctuation consistency in the neighborhood of the spinodal decomposition curve.
Monte Carlo simulation of n-member associating fluids: Application to antigen–antibody systems
Authors: Busch N. A. , Wertheim M. S. , Yarmush M. L.
Published: July 2002

An association biased Monte Carlo (ABMC) method of simulating associating systems with two bonding sites on each particle is described. The method includes a strategy for frequently forming two simultaneous bonds by a single particle during a Monte Carlo move. This strategy is employed to obtain adequate configuration statistics at each state point and is particularly important when ring formation is possible. A variety of thermodynamic and physicochemical parameters of the associating system were monitored including the compressibility factor, internal potential energy, isothermal compressibility, chain and ring number, and shape were monitored. Our analysis indicated that there is a strong dependence of these monitored quantities upon the angle between vectors representing the bonding sites on each particle. Also presented are results which suggest the existence of a two phase region, which we believe is a gas–liquid coexistence, which is dependent upon density, bonding energy, and the relative angle between the bonding sites.
Photodissociation of alkyl iodides and CF3I at 304 nm: Relative populations of I(2P1/2) and I(2P3/2) and dynamics of curve crossing
Authors: Kang Wee Kyung , Jung Kwang Woo , Kim Dong-Chan , Jung Kyung-Hoon
Published: July 2002

Photodissociation of n-alkyl iodides and CF3I has been studied using state-selective ionization and pulsed-field time-of-flight (TOF) mass spectrometry. The (2+1) resonance-enhanced multiphoton ionization cross-section ratio of I(2P1/2) and I(2P3/2) is measured from I2 photodissociation in the 304 nm region. Using this ratio, the relative populations of I(2P1/2) and I(2P3/2) by a pure parallel excitation to the 3Q0 state of n-alkyl iodides and CF3I are obtained. This method can exclude the effects of clusters and spectrum overlap in different dissociation paths from branching ratio calculations. The product quantum yield of I(2P3/2) obtained from branching ratio reveals the 3Q0–1Q1 curve crossing probability—which increases from 0.70 for CH3I to 0.86 for n-C4H9I (with recoil velocity decrease of fragments at the curve crossing point). The potential energies at the crossing point and the Landau–Zener parameters ζ, calculated from our modified model, are 385 kJ/mol and 1100 m/s for n-alkyl iodides, and 391 kJ/mol and 177 m/s for CF3I, respectively.
Photodissociation spectroscopy of MgCH+4
Authors: Cheng Y. C. , Chen J. , Ding L. N. , Wong T. H. , Kleiber P. D. , Liu Dean-Kuo
Published: July 2002

The photodissociation spectroscopy of MgCH+4 has been studied in a reflectron time-of-flight mass spectrometer. MgCH+4 molecular absorption bands are observed to the red of the Mg+(3 2PJ←3 2S1/2) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive (Mg+), and two reactive fragmentation products (MgH+ and MgCH+3) are observed, all with similar action spectra. The product branching is independent of wavelength, Mg+:MgCH+3:MgH+∼60:33:7. The absorption is assigned to the transition (1 2E←1 2A1) in C3v symmetry (with η3 coordination), followed by a geometrical relaxation of the complex toward states of 2B1 and 2B2 symmetry in C2v geometry (with η2 coordination). Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg+ product angular distribution to be isotropic, with an average translational energy release which increases slightly from Et∼370±150 cm−1 at 332.5 nm to Et∼520±180 cm−1 at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD+4 show the kinetic energy release in the nonreactive channel to be significantly larger for the CD4 case, ranging from Et∼540±180 cm−1 at 332.5 nm to Et∼830±200 cm−1. These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initio potential surface calculations suggest a possible dynamical mechanism to explain these unusual results.
Isotropic second-order dipolar shifts in the rotating frame
Authors: Ernst Matthias , Kolbert Andrew C. , Schmidt-Rohr Klaus , Pines Alexander
Published: July 2002

An experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio-frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation-dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field-dependent chemical shift in solids due to the second-order dipolar perturbation observed in magic-angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one-bond C–H coupling compared to a shift of a few Hertz in the laboratory-frame experiment. In addition to the isotropic shift, a line broadening due to the P4(cos β) terms is observed when the experiment is carried out under magic-angle sample spinning (MAS) conditions, leading to the requirement of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation of CH, CH2, and CH3 groups in a 2D spectrum under MAS is demonstrated. Implemented under DOR it could be used as a technique to select carbon atoms according to the number of directly attached protons.
On the effects of truncating the electrostatic interactions: Free energies of ion hydration
Authors: Kalko S. G. , Sesé G. , Padró J. A.
Published: July 2002

Free energies of ion hydration have been evaluated by using the thermodynamic integration technique (the slow growth method) in molecular dynamics simulations. Ionization processes of Na, Ca, and Cl have been considered. The cutoff method and the Ewald summation approach have been used alternatively in truncating the long-range electrostatic interactions. The extended simple point charge (SPC/E) model [J. Phys. Chem. 91, 6269 (1987)] and the ion–water potentials of Straatsma and Berendsen [J. Chem. Phys. 89, 5876 (1988)] have been used in our simulations. The results have been tested against experimental measurements and the Ewald method has led to the best performances. A good agreement with the Born model was found in the charging process of a calcium ion. Changes in the organization of water molecules around the ion have been evaluated from equilibrium molecular dynamics simulations performed at different stages of this process. Ion–water radial distribution functions, and the orientation of water molecules as well as residence times have been analyzed.
Trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110)
Authors: Kelly D. , Weinberg W. H.
Published: July 2002

We have employed molecular beam techniques to investigate the molecular trapping and trapping-mediated dissociative chemisorption of C3H8 and C3D8 on Ir(110) at low beam translational energies, Ei≤5 kcal/mol, and surface temperatures, Ts, from 85 to 1200 K. For Ts=85 K, C3H8 is molecularly adsorbed on Ir(110) with a trapping probability, ξ, equal to 0.94 at Ei=1.6 kcal/mol and ξ=0.86 at Ei=5 kcal/mol. At Ei=1.9 kcal/mol and Ts=85 K, ξ of C3D8 is equal to 0.93. From 150 K to approximately 700 K, the initial probabilities of dissociative chemisorption of propane decrease with increasing Ts. For Ts from 700 to 1200 K, however, the initial probability of dissociative chemisorption maintains the essentially constant value of 0.16. These observations are explained within the context of a kinetic model which includes both C–H (C–D) and C–C bond cleavage. Below 450 K propane chemisorption on Ir(110) arises essentially solely from C–H (C–D) bond cleavage, an unactivated mechanism (with respect to a gas-phase energy zero) for this system, which accounts for the decrease in initial probabilities of chemisorption with increasing Ts. With increasing Ts, however, C–C bond cleavage, the activation energy of which is greater than the desorption energy of physically adsorbed propane, increasingly contributes to the measured probability of dissociative chemisorption. The activation energies, referenced to the bottom of the physically adsorbed molecular well, for C–H and C–C bond cleavage for C3H8 on Ir(110) are found to be Er,CH=5.3±0.3 kcal/mol and Er,CC=9.9±0.6 kcal/mol, respectively. The activation energies for C–D and C–C bond cleavage for C3D8 on Ir(110) are 6.3±0.3 kcal/mol and 10.5±0.6 kcal/mol, respectively. The desorption activation energy of propane from Ir(110) is approximately 9.5 kcal/mol. These activation energies are compared to activation energies determined recently for ethane and propane adsorption on Ir(111), Ru(001), and Pt(110)–(1×2), and ethane activation on Ir(110).
State-selective photofragment imaging of iodine atoms via photodissociation of CF3I at 277 nm
Authors: Kim Yong Shin , Kang Wee Kyung , Jung Kyung-Hoon
Published: July 2002

The photodissociation of CF3I cooled in a supersonic molecular beam has been investigated at 277 nm by state-selective photofragment imaging. Fragmented iodine atoms of two spin–orbit states are state-selectively ionized and projected onto a two-dimensional position-sensitive detector, to obtain their speed and angular distribution. The anisotropy parameter for an excited iodine atom I*(2P1/2), β(I*), is found to be 1.83 and is consistent with a dissociation lifetime in the order of 150–350 fs from rotational correlation function. Contrary to earlier reports, the parallel-like distribution for the ground state iodine atom I(2P3/2) at this wavelength, shows a more favorable curve-crossing dissociation path (68%) from 3Q0 to 1Q1 and a less favorable direct dissociation path (32%) from 3Q1. The recoil energy distribution of I is found to be broader than that of I* and is correlated with a variety of energy disposal channels by an e symmetry vibration at the crossing point. The results are compared with previous works, and the strong photon energy dependence of the energy partitioning in CF3+I* channel and curve crossing are interpreted in terms of the final state interaction and curve crossing probability, respectively.
Finite-difference Heisenberg approach: The treatment of unbound states and the elimination of artificial boundary reflections
Authors: Sharafeddin Omar A.
Published: July 2002

We apply our recently proposed finite-difference Heisenberg (FDH) approach to the treatment of unbound states and show that, by using this approach, the problem of artificial reflections of the scattered wave packet from the boundaries is totally eliminated. This is because the basis and coordinate frame are dynamic and thus adjust themselves, as dictated by the potential function, to the evolving wave function. The disadvantage of this approach is that it may scale as M3 for M basis and thus become expensive if M is large. However, accurate results can be obtained efficiently for a limited range of energies in the scattered wave packet using a small basis. These results are demonstrated by one-dimensional (1D) scattering from an Eckart barrier.
Photofragment imaging by sections for measuring state-resolved angle-velocity differential cross sections
Authors: Syage Jack A.
Published: July 2002

We describe a two-dimensional (2D) imaging technique for recording state-specific photofragment angle-velocity (θ,v) distributions. In these experiments the photofragment images are recorded as 2D sections of the 3D angular distributions using state-specific ionization in a time-of-flight mass spectrometer. We compare this method to previous methods that record 2D projections of the 3D distribution. The 2D sections represent cartesian flux-velocity maps in the center of mass and are related to angle-velocity differential cross sections by a simple geometric factor. Two studies are highlighted. In the first, new results are presented for the A state photodissociation of CH3I to CH3+I. (θ,v) images are presented for I atom in the 2P3/2 and 2P1/2 spin–orbit states following photodissociation at 266 and 304 nm. The principal result is detection of the weak perpendicular transitions to the 3Q1 state (at 304 nm) and the 1Q state (at 266 nm) that underlie the strong parallel transition to the 3Q0 state. We also report the ratio of cross sections σ⊥/σ∥, the anisotropy and branching ratio for I(2P3/2) and I(2P1/2), and the 3Q0–1Q surface crossing probability. In a second study the photodissociation of O3 to O2(v)+O(3Pj=2,1,0) was measured. A bimodal anisotropic velocity distribution was measured for O(3P) corresponding to maximum in the O2(v) vibrational distribution of v=15 and 27, in general agreement with a previous measurement. The anisotropies of the high- and low-velocity components were measured to be β≊1.1 and 0.4, respectively.
Time and temperature dependence of optical linewidths in glasses at low temperature: Spectral diffusion
Authors: Silbey R. J. , Koedijk J. M. A. , Völker S.
Published: July 2002

The standard theoretical model of two-level systems in low-temperature glasses is modified so that the temperature dependence of the effective homogeneous optical linewidth is in agreement with experiment. This alters the time dependence of the width due to spectral diffusion. The new results fit recent experiments without the need for gaps in the distribution function of flip rates of the two-level-systems or the addition of extra distribution functions.
Contributions of rotation and translation to polarizability anisotropy and solvation dynamics in acetonitrile
Authors: Ladanyi Branka M. , Klein Shannon
Published: July 2002

In liquids, a substantial portion of the polarizability anisotropy is interaction induced and its time correlation function (TCF), observable in depolarized light scattering, optical Kerr effect (OKE) and other experiments, decays through translational as well as rotational mechanisms. In this work we develop methods, based on instantaneous normal mode (INM) analysis and on a theory due to W. A. Steele [Mol. Phys. 61, 1031 (1987)], to identify the contributions of rotation, translation, and their cross correlations to polarizability anisotropy dynamics. The Steele theory is also used to extend the analysis of the dynamical origin of solvation response beyond the short-time regime. We use this methodology to investigate the contributions of rotation and translation to the OKE response of acetonitrile at room temperature and to compare it to electrostatic solvation dynamics in this liquid. We find that rotation is the largest contributor to polarizability anisotropy dynamics, but that the translational and translation–rotation cross correlation components are important at all times relevant to the rise and decay of the OKE signal. The polarizability anisotropy INM spectrum of acetonitrile is quite similar to the corresponding electrostatic solvation spectrum determined earlier [B. M. Ladanyi and R. M. Stratt, J. Phys. Chem. 99, 2502 (1995)]. This similarity extends to the polarizability anisotropy and solvation energy velocity TCFs and their decomposition into rotational, translational, and their cross-correlation components. The implication of these results to the use of the OKE spectral density to predict the solvation response in acetonitrile and in other liquids is discussed.
Molecular dynamics study of the thermodynamic and structural properties of methanol and polarizable/non-polarizable carbon tetrachloride mixtures
Authors: Veldhuizen R. , de Leeuw S. W.
Published: July 2002

Molecular dynamics simulations have been performed for methanol/carbon tetrachloride mixtures over the whole composition range at 323 K and zero pressure. The OPLS (optimized potentials for liquid simulation) potential energy parameters by Jorgensen were used to model the methanol potential. Both a non-polarizable carbon tetrachloride model taken from McDonald, Bounds, and Klein [Mol. Phys. 45, 521 (1982)] as well as a polarizable model were used. The latter model was devised by combining the model of McDonald, Bounds, and Klein with the atomic polarizabilities proposed by Applequist, Carl, and Fung [J. Am. Chem. Soc. 94, 2952 (1972)]. We show that the role of the methanol–carbon tetrachloride interactions are very important in discussing the thermodynamic mixing properties. In order to reproduce the asymmetric behavior of the excess enthalpies with respect to composition it is necessary to include the non-additive polarization interaction. The structure and especially hydrogen bonding properties are discussed. Radial distribution functions show a strong tendency of methanol to preserve the local order similar to the one in the pure fluid. The deviations from random mixing are more pronounced at the lower mole fractions. This is explained by a frustration model. At low methanol concentrations the molecules get more freedom to align themselves in energetically favorable (hydrogen bonded) configurations. Throughout the composition range, the majority of the methanol molecules is found to be engaged in two hydrogen bond, As in the pure fluid, this leads to the pattern of hydrogen bonded winding chains. Upon dilution the degree of cross-linking between the chains diminishes whereas the free monomer fraction rises. Furthermore a significant number of the remaining chains close to form cyclic polymers.
The random parking of spheres on spheres
Authors: Mansfield Marc L. , Rakesh Leela , Tomalia Donald A.
Published: July 2002

Given a ‘‘target’’ sphere of radius r1 and ‘‘probe’’ spheres of radius r2, we consider, as a function of r2/r1, how many probe spheres, on average, can be attached to the target sphere if (1) the attachment sites are chosen at random, (2) the probe spheres are not permitted to overlap, and (3) each attachment is irreversible. We also consider two separate extremes for selecting new attachment sites: Each probe sphere is either permitted to diffuse into place from a large distance, or the attachment site is chosen completely at random. Diffusion-controlled attachment produces a slightly higher packing density than completely random attachment.
Kinetics of diffusion-influenced reversible reaction A+B ⇌
C in solutions
Authors: Gopich Irina V. , Doktorov Alexander B.
Published: July 2002

Reversible diffusion-influenced pseudo first order reaction A+B ⇌C with static particles in excess is rigorously studied. Under most general assumptions, the problem of the reversible reaction kinetics is reduced to the consideration of the effective irreversible reaction studied by conventional methods. In the framework of the average t-matrix approximation (ATA) we reproduce some results derived earlier and establish their applicability limits. Rigorous investigation of the kinetics behavior at long times shows that the t−3/2 law predicted earlier and reproduced by ATA has a different concentration-dependent amplitude. On the basis of diagrammatic summation, providing correct long-time asymptotics, a modified theory has been developed. The range of validity of the modified theory is much wider than that of ATA and similar theories.
Continuum Monte Carlo simulation at constant pressure of stiff chain molecules at surfaces
Authors: Haas F. M. , Hilfer R.
Published: July 2002

Continuum Monte Carlo simulations at constant pressure are performed on short chain molecules at surfaces. The rodlike chains, consisting of seven effective monomers, are attached at one end to a flat two dimensional substrate. It is found that the model exhibits phases similar to the liquid condensed and liquid expanded phases of Langmuir monolayers. The model is investigated here for a wide range of pressures and temperatures using a special form of constant pressure simulation compatible with the symmetry breaking during tilting transitions in the liquid condensed phases. At low pressures, the chains undergo a tilting transition exhibiting tilt directions towards nearest and also next nearest neighbors depending on temperature. At elevated temperatures and low pressure the film enters a fluidlike phase similar to the liquid expanded phase observed in experiment.
Rotationally inelastic scattering in CH4+He, Ne, and Ar: State-to-state cross sections via direct infrared laser absorption in crossed supersonic jets
Authors: Chapman William B. , Schiffman Aram , Hutson Jeremy M. , Nesbitt David J.
Published: July 2002

Absolute integral state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of CH4 with the rare gases He, Ne, Ar, at center of mass collision energies of 460±90, 350±70, and 300±60 cm−1, respectively. CH4 seeded in Ar buffer gas is cooled in a pulsed supersonic expansion into the three lowest rotational levels allowed by nuclear spin statistics corresponding to A(J=0), F(J=1), and E(J=2) symmetry. Rotational excitation occurs in single collisions with rare gas atoms from a second pulsed supersonic jet. The column integrated densities of CH4 in both initial and final scattering states are subsequently probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), single mode color center laser. Total inelastic cross sections for collisional loss out of the J=0, 1, and 2 methane states are determined in absolute units from the linear decrease of infrared absorption signals as a function of collider gas concentration. Tuning of the ir laser source also permits probing of the collisionally excited rotational states with quantum state and velocity resolution; column integrated scattering densities are measured for all energetically accessible final states and used to infer absolute inelastic cross sections for state-to-state energy transfer. The observed trends are in good qualitative agreement with quantum state resolved pressure broadening studies; however, the dependences of the rotationally inelastic cross sections on nuclear spin modification (i.e., J) and rotational inelasticity (i.e., ΔJ) is not well predicted by conventional angular momentum or energy gap models. More rigorous comparison with the quantum state-resolved scattering data is obtained from full close coupled scattering calculations on trial potential energy surfaces by Buck and co-workers [Chem. Phys. Lett. 98, 199 (1983); Mol. Phys. 55, 1233, 1255 (1985)] for each of the three CH4+rare gas systems. Agreement between theory and experiment for He+CH4 is nearly quantitative, but some discrepancies are noted for the heavier rare gases. Finally, the possible influence of sequential collision channels on the measurement of the smallest observable cross sections is investigated via a master equation analysis.
Infrared laser spectroscopy of the A 2Πu←X 2Σ+g system of the Si−2 anion
Authors: Liu Zhuan , Davies Paul B.
Published: July 2002

Two bands of the A 2Πu←X 2Σ+g transition of Si−2 lying in the mid-infrared region have been detected using diode laser velocity modulation spectroscopy. The anion was generated in an air cooled low pressure ac discharge through mixtures of silane and hydrogen, and 130 lines of the (1,0) and (2,0) bands lying between 670 and 810 cm−1 and 1200 to 1340 cm−1 respectively were measured and assigned. A fit to standard Hamiltonians for 2Σ and 2Π states yielded the most precise spectroscopic constants so far obtained for the anion. These have been used to derive several equilibrium parameters for the A state. The equilibrium internuclear distance of the A state is re=2.182986(16) Å. The fine structure and equilibrium parameters are compared with earlier low resolution results and with ab initio calculations on Si−2.
Microwave measurements of the gas-phase molecular structure of cobalt tetracarbonyl hydride
Authors: Kukolich S. G. , Sickafoose S. M.
Published: July 2002

Microwave rotational spectra for six isotopomers of HCo(CO)4 were measured using a Flygare–Balle type spectrometer. Transitions were measured in the 4–10 GHz range. The data were analyzed to obtain rotational constants, 59Co quadrupole coupling strengths, distortion constants and spin–rotation interaction strengths. The common isotopomer is clearly a symmetric top, with C3v symmetry. For the common isotopomer B=1131.7212(9) MHz and eQq(59Co)=116.62(3) MHz. The rotational constants were used to determine the complete, three-dimensional structure of this complex. The measured H–Co bond length of 1.52(2) Å is significantly shorter than the previous value from electron diffraction work. Other structural parameters for this complex are reported and discussed.
The electric dipole moment and hyperfine interactions of KOH
Authors: Cederberg J. , Olson D. , Rioux D. , Dillemuth T. , Borovsky B. , Larson J. , Cheah S. , Carlson M. , Stohler M.
Published: July 2002

A molecular beam spectrometer has been used to observe the hyperfine spectrum and determine the electric dipole moment of the KOH molecule. This EDM had never been measured previously, although theoretical calculations had been made. A refined line shape fitting procedure has helped make possible this determination from the molecular beam hyperfine spectrum. The value of the EDM, which we found for the ground vibrational state, is 7.415±0.002 debye. The hyperfine constants, including the potassium nuclear quadrupole, both spin–rotation interactions, and the tensor and scalar spin–spin interactions have been determined for several vibrational states.
Fourier transform spectroscopy of torsionally excited C-13 methanol in the CO-stretching band
Authors: Lees R. M. , Mukhopadhyay I. , Predoi Adriana , Lewis-Bevan W. , Johns J. W. C.
Published: July 2002

Torsionally excited n=1 and n=2 subbands of the CO-stretching fundamental have been assigned in the Fourier transform infrared spectrum of 13CH3OH recorded at a resolution of 0.002 cm−1. For the n=1 torsional state, subbands with K values up to 7 have been identified for A torsional symmetry and up to 5 for E symmetry. For n=2, four subbands have so far been assigned. Subband origins have been obtained by fitting the observed wave numbers to J(J+1) power-series expansions. The expansion parameters represent the data compactly to close to the experimental uncertainty in the absence of line blending or perturbations. Significant downshifting of the n=1 origins is observed, correlated closely with proximity to the n=0 OH-bending state. J-localized perturbations have been observed and characterized for several subbands. Two of the far-infrared laser lines optically pumped by the 10R(14) CO2 laser line have been assigned.
Calculation of the rotational Raman spectrum of H2 in ice
Authors: Mei H. S. , Xiao L. , Coker D. F.
Published: July 2002

Calculation of the rotational Raman spectrum of H2 in ice Ih is reported using previously developed nonadiabatic correlation function methods and accurate potentials. We explore the importance of the quantal treatment of the H2 center-of-mass translational motion. Calculated rotational Raman linewidths quantitatively reproduce experimental results. Observed differences in trends in bandwidth for liquid and solid phases are understood in terms of time scale and strength of dynamical mixing of rotational states.
Fast evaluation of electron intracule and extracule densities on large grids of points
Authors: Cioslowski Jerzy , Liu Guanghua
Published: July 2002

A new approach to fast evaluation of the electron intracule and extracule densities on large grids of points is described. Substantial (50- to 100-fold) speed ups over the conventional algorithms are attained through the use of precomputed intermediates in the grid-dependent phase of calculations. These intermediates are evaluated only once in a grid-invariant procedure that employs efficient two-stage integral screening to reduce computational effort. In addition to delivering high performance, the new algorithm facilitates calculations of analytical gradients and Hessians of the intracule and extracule electron densities. For regular grids with shared components of Cartesian coordinates, the present method allows the factorization of the primitive quartet contributions that makes the cost of calculations proportional to the cubic root of the number of grid points.
A spectral filter approach to the wave operator treatment of large matrix eigenvalue problems
Authors: Jolicard G. , Grosjean A. , Killingbeck J. P.
Published: July 2002

A Green function filter operator (H−E.1)−1 is used within a wave operator iterative method for the computation of interior eigenvalues of large Hamiltonian matrices. Test calculations on a coupled oscillator system show a significant improvement in the convergence of the wave operator iterative method when the filter is used with a one-dimensional active space. Less convincing results are obtained with multidimensional active spaces involving several states in near resonance; other filter functions such as (H−E.1)−2 seem to be more suitable for this case.
Rogers–Young approximation for the concentration profile of a colloidal suspension in front of a highly repulsive wall
Authors: González-Mozuelos P. , Alejandre J.
Published: July 2002

In this work we present an extension of the Rogers–Young approximation to the determination of the concentration profile of a colloidal suspension of charged particles in front of a flat highly repulsive wall. The thermodynamic consistency condition used here is the matching of the force per unit area exerted by the macroparticles over the wall, with the corresponding osmotic pressure in the bulk determined from the virial equation. The suspension is described as a one-component fluid, constituted by the macroparticles, with all the relevant interactions modeled by the Derjaguin–Landau–Verwey–Overbeek effective potentials. The predictions of this new scheme are compared with Monte Carlo computer simulation data. The main conclusion is that the Rogers–Young approximation is in closer agreement with the simulation data than the hypernetted chain and rescaled mean approximations presented in previous works.
Optical characterization of thin films: Beyond the uniform layer model
Authors: Mann E. K. , Heinrich L. , Voegel J. C. , Schaaf P.
Published: July 2002

Scanning angle reflectometry (SAR) is used to investigate the structure of antigen/antibody layers deposited on a silica surface. The reflectivity curves are analyzed by the means of the optical invariants, which permits the determination of three structural characteristic parameters for the layer. A new invariant is constructed that yields information about the repartition of mass in the layer and thus goes beyond the usual uniform layer model. In our case, this model does not hold: the layer appears to be denser away from the adsorbing silica surface than near that surface.
Classical trajectory and adiabatic channel study of the transition from adiabatic to sudden capture dynamics. II. Ion–quadrupole capture
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

Classical trajectory calculations of ion–permanent quadrupole+induced dipole capture processes are performed over wide ranges of conditions. The results are expressed in two-parametric analytical form, and analyzed with respect to the transition from sudden to adiabatic dynamics. The results are compared with the statistical adiabatic channel model (SACM). For this purpose, adiabatic channel potential curves are expressed in approximate analytical form. The agreement between classical trajectory and SACM results is excellent in the range of adiabatic quasiclassical motion of the collision partners. Analytical expressions can well describe the temperature variation of the capture rate constant in the transition range between the quantum and classical limits.
Electron correlation effects on the static longitudinal second hyperpolarizability of polymeric chains. Mo/ller–Plesset perturbation theory investigation of hydrogen model chains
Authors: Champagne Benoı̂t , Mosley David H.
Published: July 2002

Ab initio calculations of the static longitudinal second hyperpolarizability of molecular hydrogen model chains have been performed at different levels of approximation to investigate the effects of including electron correlation, as well as the variation of these effects as a function of the atomic basis set. Inclusion of electron correlation within the Mo/ller–Plesset schemes limited to second (MP2), third (MP3), and fourth (MP4) order in electron–electron interactions leads to an increase of the longitudinal second hyperpolarizability per unit cell which is mainly due to the second-order correction provided that a sufficiently extended basis set is used. Indeed, whereas the basis set does not influence significantly the UCHF and CHF values, a too small basis set underestimates the positive second-order contribution, overestimates the negative third-order contribution and leads to a fourth-order contributions having an incorrect sign. This positive electron correlation correction for the second hyperpolarizability is opposite to the negative correction brought to the static longitudinal polarizability per unit cell for which the CHF level with a split-valence basis set provides reliable polarizability estimates. The present investigation points out that good estimates of the second hyperpolarizability are already obtained by using the MP2 procedure with a sufficiently extended basis set.