bluuebunny/crossref_metadata_2025 huggingface dataset:
On the nature of photo charge carriers in ice
Authors: Petrenko V. F. , Khusnatdinov N. N.
Published: July 2002

A method of photoelectromotive force (PEMF) was developed to find the charge sign, mobility, and lifetime of photo charge carriers in ice generated by photons with energy hν≳6.5 eV. It was determined that the most mobile photo charge carriers are negative ones, with mobility μ increasing from 2×10−3 cm2/V s at T=−10 °C to 4×10−2 cm2/V s at T=−30 °C, and with their lifetime decreasing from 30 to 10 s in the same temperature range. Activation energies of the mobility and the lifetime are Eμ=−0.77 eV and Eτ=0.32 eV, respectively. In addition to the negative photo charge carriers positive ones arise with mobility μ=2.3×10−4 cm2/V s and lifetime τ=26 min at T=−15 °C. We suggest that the negative photo charge carriers in ice are mobile complexes of an electron, vacancy and D-defect (e−+V+D). To take into account a specific mechanism of charge transport in ice, configurational vector Ω, and the generation of complexes (e−+V+D), a reaction of ‘‘autoionization’’ was modified for ice, 2H2O+hν→H3O++OH•int(e−+V+D).
The solution of the time-dependent Schrödinger equation by the (t,t′) method: Complex scaled multiphoton ionization/dissociation resonance wave functions are square integrable
Authors: Moiseyev Nimrod
Published: July 2002

On the basis of the (t,t′) method [J. Chem. Phys. 99, 4590 (1993)], we show that upon complex scaling, the resonances obtained for a general time-dependent (i.e., not necessarily time-periodic) Hamiltonian (in either length, momentum, or acceleration gauge) are associated with square integrable wave functions.
Proton transfer in hydrogen-bonded acid–base complexes in polar solvents
Authors: Staib Arnulf , Borgis Daniel , Hynes James T.
Published: July 2002

A theoretical approach exploiting molecular dynamics simulations to treat adiabatic proton transfer between an acid AH and a base B in a polar, aprotic solvent is presented. The dynamics of the proton transfer, which occurs on the electronic ground state surface of the reactive hydrogen-bonded complex AH...B, is strongly influenced by interaction of the reaction system with the solvent and by the AB stretch vibration. The approach fully incorporates the quantum character of the proton motion as well as that of the AB stretch vibration and yields a mechanistic picture for a thermally activated proton transfer reaction in a polar solvent. Rate constants are computed and solvent frictional effects are analyzed in application of the theory to a model of the system phenol-trimethylamine in methyl chloride solvent. In addition, it is shown how the excitation of the hydrogen bond symmetric stretch mode decelerates the reaction. The simulation results are also compared to a curve-crossing model. The impact of the solvent electronic polarization on the results is discussed.
The metastable quartet state of He+4
Authors: Knowles Peter J. , Murrell John N.
Published: July 2002

Ab initio calculations for the lowest quartet state of He+4 are reported. The resulting delocalized D2d structure, binding energy, and electronic spectrum provide a rationalization for the experimental observation of an especially stable He+4 cluster.
Direct calculation of overtones: Application to the CD3H molecule
Authors: Iung Christophe , Leforestier Claude
Published: July 2002

We report a comparison of two methods designed for directly determining high lying energy levels in a polyatomic molecule. The Bloch operator, in the distorted wave version proposed by Jolicard [Chem. Phys. 115, 57 (1987)], allows one to compute a few specific eigenstates, as specified by some zero-order description. The second method makes use of a spectral transform, in order to open up an energy window in a very dense spectrum. Test calculations have been carried out on the nine-dimensional CD3H molecular system, for different C–H stretch overtone levels. They show that the Bloch formulation rapidly converges the target eigenstate, provided that the intramolecular coupling is not too strong. In the strong coupling regime, the spectral transform has been shown to successfully display all the eigenstates located in the energy window of interest, but requires more computational effort.
Monte Carlo study of the optical properties of the KXeN polyatomic exciplexes
Authors: Fujisaki Akihiro
Published: July 2002

The geometrical structures and optical properties of the K(5S)XeN polyatomic exciplexes are studied by using the Metropolis Monte Carlo method. The many-body potentials between an electronically excited potassium atom and xenon atoms are calculated by using the improved Baylis potential model, in which the Hamiltonian is diagonalized at every Monte Carlo step. At low temperature, the exciplex has a closed structure at the number of xenons N=8. The shifts and the linewidths of the transition energy of the calculated fluorescence spectra of individual exciplexes reflect the structures and thermal properties of each polyatomic exciplex.
An accurate calculation of the three-body potential for the ground state of the helium trimer
Authors: Ro/eggen I. , Almlöf J.
Published: July 2002

An extended group function model has been applied to determine the three-body potential for the ground state of the helium trimer. The basis set for the calculations is an uncontracted (17s,6p,4d,3f) set of Gaussian type functions contracted to [8s,6p,4d,3f]. Three different types of configurations were considered: (i) equilateral triangles of side R, R∈{3.5,4.0,...,8.5,9.0} a.u., (ii) linear configurations with R12=R23, R12∈{3.5,4.0,...,8.5,9.0} a.u., and (iii) a set of pseudo-random configurations. For the equilateral triangular configurations and the linear configurations, and with equal distances larger than 5.5 a.u., the magnitude of the ratio between the three-body potential and the sum of the corresponding two-body potentials, is less than 0.003. The value of the three-body potential for the triangular and the linear configuration with equal distance R=5.669 289 a.u.=3 Å, is respectively 0.040 μH and −0.045 μH. In the long range region the calculated three-body potential is very close to the Axilrod–Teller triple-dipole energy.
A Monte Carlo study of the α–β order–disorder transition in solid nitrogen
Authors: Kuchta Bogdan , Rohleder Krzysztof , Etters Richard D. , Belak James
Published: July 2002

The α–β phase transition in solid N2 has been investigated using the constant pressure Monte Carlo method. This is accomplished by examining both phases in the temperature range 25≤T≤50 K, where they are everywhere at least metastable. It is found that the cubic α phase undergoes an orientational order–disorder transition into a disordered cubic phase as the temperature is increased to T=41 K, and remains in this state until melting. Similarly the orientationally disordered hexagonal phase persists from melting down to 33 K, where it undergoes a transition into a hexagonal structure with short-range orientational order.
Liquid-crystal phase diagram of the Gay–Berne fluid by perturbation theory
Authors: Velasco E. , Somoza A. M. , Mederos L.
Published: July 2002

A thermodynamic linear perturbation theory for the Gay–Berne intermolecular potential has been developed which is able to predict the occurrence of isotropic liquid-vapour coexistence as well as the stability of a nematic phase. The theory can be regarded as a generalisation to molecular fluids of the Weeks, Chandler, Andersen (WCA) pertubative scheme for simple fluids. The reference system, a hard Gaussian overlap model, is analysed within density-functional theory using a standard Onsager-like approach, the ‘‘decoupling approximation,’’ where density correlations are treated to all orders (albeit approximately) whereas orientational correlations are approximated by the low density limit. We implement this idea by using an equivalent system of hard spheres to approximate the density-dependent part of the excess free energy. The structure of the reference system is approximated by the radial distribution function of an equivalent system of hard spheres, scaled with the contact distance of the hard core. The phase diagram for the Gay–Berne fluid obtained from the theory is compared with computer simulation data of the same model potential and found to be in fair agreement. In contrast, a mean-field approximation for the perturbative term gives rather poor results.
The infrared spectrum of the H2–HCO+ complex
Authors: Bieske E. J. , Nizkorodov S. A. , Bennett F. R. , Maier J. P.
Published: July 2002

A combined experimental and theoretical study of the structural properties of the H2–HCO+ ion-neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H2–HCO+ complexes in the 2500–4200 cm−1 range display several vibrational bands, the most intense arising from excitation of the C–H and H2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of Σ–Σ and Π–Π subbands as expected for a parallel transition of complex with a T-shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD(T)/6–311G(2df,2pd) level. The complex is composed of largely undistorted H2 and HCO+ subunits, has a T-shaped minimum energy geometry with an H2...HCO+ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the Π–Π subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H2 internal rotation. The lower J lines in the Σ–Σ and Π–Π subbands have widths of 0.06 cm−1, around three times larger than the laser bandwidth, corresponding to a decay time of ≊90 ps for the upper level. The absence of discernible rotational structure in the ν2 band suggests that it has predissociation lifetime of less than 1 ps.
A high-field magnetic birefringence study of dilute phases of a nonionic surfactant in water
Authors: Sprunt S. , Zhang C. Y. , Litster J. D.
Published: July 2002

We have used high-resolution magnetic birefringence to study very dilute mixtures of the nonionic surfactant pentaethylene glycol mono-dodecyl-ether (C12E5) in water at fields to H=18 T. The induced refractive index anisotropy Δn shows distinct H dependencies in the Lα-lamellar and L3-‘‘sponge’’ phases, and demonstrates that these are separated by a two-phase coexistence region. In the L3 phase and for surfactant concentrations γ≂0.02 (the vicinity of the Lα phase terminus), we observe a sharp change in the magnitude and H dependence of Δn. A quantitative analysis shows this to be consistent with a crossover from a highly-connected ‘‘symmetric sponge’’ to a droplet or ‘‘asymmetric sponge’’ structure as a function of γ, a result previously suggested by neutron scattering and electrical conductivity measurements [R. Strey, R. Schomacker, D. Roux, F. Nallet, and U. Olssen, J. Chem. Soc. Faraday Trans. II 86, 2253 (1990)]. Finally, for γ≳0.02 and at large H, we present evidence of a field-induced transformation of the L3 structure.
A quantum dynamical study of CH overtones in fluoroform. I. A nine-dimensional ab initio surface, vibrational spectra and dynamics
Authors: Maynard Andrew T. , Wyatt Robert E. , Iung Christophe
Published: July 2002

In this series, the characteristics of intramolecular vibrational-energy redistribution (IVR) present in the CH overtones of CHF3 are investigated. Particular attention is given to the multiple time scales and thus mechanisms present in the IVR dynamics. In Part I, a 9-dimensional ab initio potential energy surface is developed to adequately account for the vibrational couplings of all modes. Furthermore, all-mode vibrational state calculations, of large primitive space dimension, are performed using a recently developed wave operator sorting algorithm in tandem with the recursive residue generation method. All fundamentals, first overtones, and bimodal combination states with up to 3 quanta are presented. Also, the A1 and E-symmetry CH polyads are determined through the second overtone. Equilibrium geometry, rotational constants, and vibrational properties agree quantitatively with experiment in most cases. The error is systematic in origin and largely due to the error in the ab initio harmonic frequencies. New vibrational constants and resonance interactions are reported for the background modes. In contrast to the prominent CH stretch–bend Fermi resonance structure, responsible for ultrafast (t<50 fs) energy transfer, the CH polyads also exhibit vibrational fine structure of order 1 to 10 cm−1 due to background-mode coupling. This secondary coupling results in IVR on the picosecond time scale.
Elastic rod model incorporating shear and extension, generalized nonlinear Schrödinger equations, and novel closed-form solutions for supercoiled DNA
Authors: Shi Yaoming , Borovik Andrey E. , Hearst John E.
Published: July 2002

We have derived a generalized one-dimensional time-independent nonlinear Schrödinger equation for the stationary state configurations of supercoiled DNA, based on an elastic rod model which includes deformations of bending, twisting, shear, and extension. Closed-form solutions for the axis of DNA have been obtained in terms of elliptic functions and elliptic integrals. These solutions describe the stationary state configurations of supercoiled DNA.
Liquid–liquid phase equilibria of symmetrical mixtures by simulation in the semigrand canonical ensemble
Authors: de Miguel Enrique , Martín del Río Elvira , Telo da Gama Margarida M.
Published: July 2002

We report a computer simulation study of liquid–liquid phase equilibria of symmetrical fluid mixtures. We note that it is not necessary to conduct a conventional Gibbs ensemble simulation—involving two correlated boxes—when studying this type of phase equilibria. We show that by simulation of a single box in the semigrand canonical ensemble under appropriate conditions, one obtains results which are equivalent to those obtained using the conventional Gibbs ensemble method. The new method is illustrated with an application to liquid–liquid equilibria of a symmetrical square-well binary mixture. The results of our simulations are compared with two sets of recently reported results for the same system, based on the conventional Gibbs ensemble technique. Finite-size effects are investigated in detail, in order to understand the strong system-size dependence reported for this system, over a wide range of temperatures, using the Gibbs ensemble method at constant volume. A finite-size-scaling analysis is also carried out, to check the consistency of our results in the critical region.
Applications of multireference perturbation theory to potential energy surfaces by optimal partitioning of H: Intruder states avoidance and convergence enhancement
Authors: Finley James P. , Chaudhuri Rajat K. , Freed Karl F.
Published: July 2002

The minimum basis set hydrogen rectangular system (HRS), consisting of four hydrogen atoms arranged in a rectangle, is examined using a variety of partitionings of the Hamiltonian H for high order single and double reference perturbation computations. The potential energy surface is mapped out over a range of geometries in which the length L of one side of the rectangle is varied. Several criteria are derived governing the necessary conditions for perturbative convergence of two-state systems, and these criteria are useful in explaining the behavior of the HRS for the range of geometries and partitioning methods investigated. The divergence caused by intruder states, observed by Zarrabian and Paldus [Int. J Quantum Chem. 38, 761 (1990)] for the nondegenerate, double reference space perturbation expansions at L=3.0 a.u. with traditional partitioning methods, is shown to correspond to avoided crossings with negative real values of the perturbation parameter—backdoor intruder states. These intruder state induced divergences result from too small zeroth order energy differences between the high lying reference space state and an orthogonal space intruder state whose identity depends on the partitioning method. Forcing the valence orbitals to be degenerate enlarges these zeroth order energy differences and, thus, yields a convergent perturbative expansion for L=3.0 a.u.

The convergent or divergent behavior of all the partitioning method computations and the locations of their avoided crossings are accurately predicted by using two-state models composed of the high lying reference space state and the intruder state. A partitioning method is introduced in which the zeroth order state energies are selected to optimize the convergence in low orders of the perturbation expansion. This optimization method yields perturbative convergence which is both rapid and free of intruder state for geometries between L=2.0 and 3.0 a.u. The divergent behavior for various partitioning methods at L=5.0 a.u., also observed by Zarrabian and Paldus, is caused by one or more orthogonal space states and the high lying reference space state that are strongly coupled and have close expectation values of H. The two-state model illustrates why no partitioning choice with a double reference space can yield a satisfactory rate of perturbative convergence for L=5.0. Therefore, the entire potential energy surface is treated using more than one reference space: a double reference space for L≤3.0 a.u. and a single reference space for L≳3.0 a.u. The entire potential surface of interest, which is generated with the optimized partitioning method and the two different reference spaces, is very accurate by third order, with eigenvalues for all geometries considered differing from the FCI by no more than 1 kcal/mol.
Dynamic reaction path analysis based on an intrinsic reaction coordinate
Authors: Taketsugu Tetsuya , Gordon Mark S.
Published: July 2002

We propose two methods that may be used to describe the dynamic reaction path (DRP) based on an intrinsic reaction coordinate (IRC) or minimum energy path, to examine how the actual dynamics proceeds relative to the IRC path. In the first of these, any point on the DRP is expressed in terms of the IRC and the distance from the IRC path. In the second method, any DRP point is expressed in terms of the IRC, the curvature coordinate, and the distance from a two-dimensional ‘‘reaction plane’’ determined by the IRC path tangent and curvature vectors. The latter representation is based on the fact that the 3N−8 dimensional space orthogonal to the reaction plane is independent of an internal centrifugal force caused by the motion along the IRC path. To analyze the relation between geometrical features of the IRC path and the dynamics, we introduce a function that estimates the variation of the reaction plane along the IRC path. As demonstrations, the methods are applied to the dissociation reaction of thiofolmaldehyde (H2CS→H2+CS).
The energy relaxation of a nonlinear oscillator coupled to a linear bath
Authors: Bader Joel S. , Berne B. J. , Pollak Eli , Hänggi Peter
Published: July 2002

We develop an expression for the rate of energy relaxation of a nonlinear oscillator coupled to a linear, dissipative bath. This particular type of model has wide applicability to studies of relaxation rates of vibrational modes in chemical systems. The energy relaxation rate is estimated by relating the anharmonic oscillator to an effective harmonic reference system. The theoretical predictions compare favorably with simulation results for the energy relaxation of a Morse oscillator (i) coupled to an Ohmic bath and (ii) coupled to a bath with exponentially decaying friction. The dependence of the initial relaxation rate on the excitation energy of a Morse oscillator is qualitatively different for the two cases. When the oscillator is coupled to an Ohmic bath, the initial relaxation rate decreases as a function of the excitation energy. When exponentially decaying friction is employed, however, the initial relaxation rate is an increasing function of the excitation energy.
Local structure and mobility of ions in polymer electrolytes: A molecular dynamics simulation study of the amorphous PEOxNaI system
Authors: Neyertz Sylvie , Brown David
Published: July 2002

Solid polymer electrolytes are ionically conducting phases formed by dissolving salts in an amorphous polymer matrix. In this study, the local structure and dynamics of Na+ and I− ions in molecular dynamics (MD) simulations of the amorphous poly(ethylene oxide)-based electrolyte PEOxNaI (x=48,20,3) are analyzed at both 400 and 500 K. The fully atomistic model reproduces many phenomena seen experimentally and provides a picture of the complex correlations between cation, anion, and polymer host in these systems. The composition of the first coordination shell around the cations illustrates the concentration-dependent competition between iodines and PEO backbone oxygen atoms to coordinate the positively charged cations. Contiguous polymer segments tend to form near-planar polydentate loops around the sodiums while the anions are usually placed above and/or below the PEO...Na+ quasiplane. This geometry results in optimal coordination of both types of ligands to the cation in a similar pattern to that found in crystalline PEO-based systems and crown ethers. Although the observation of large-scale diffusion is still effectively precluded by the currently accessible MD time scale, discrete ligand exchanges do occur in the coordination shell of the cations. Examples of these dynamical events, including various jumps between local potential coordination sites, are shown and their net result on ionic motion is discussed.
Diffuse interface theory for homogeneous vapor condensation
Authors: Gránásy László
Published: July 2002

The excess free energy of nuclei is evaluated in terms of a characteristic interface thickness related to bulk physical properties. A curvature correction to the surface tension of nuclei up to second order in terms of the characteristic thickness is obtained. A nucleation theory free of adjustable parameters is presented. The predictions are compared with experiments on homogeneous condensation of nonpolar (nonane, toluene, Ar), polar (water, alcohols), and metal (Fe, Pb, Hg, Cs) vapors. For molecular liquids the applicability of the theory correlates with the surface entropy. In case of nonpolar, weakly polar, and metallic substances the experiments are described better than by the classical theory.
The correlated product state distribution of ketene photodissociation at 308 nm
Authors: Morgan Christopher G. , Drabbels Marcel , Wodtke Alec M.
Published: July 2002

The correlated product state distribution for ketene photodissociation (CH2CO→CH2+CO) at 308 nm has been measured by using quantum-state-specific metastable time-of-flight (TOF) spectroscopy. This distribution is a matrix whose elements are the probability that if CO is produced in the dissociation with quantum-state ‖nCO〉, CH2 will be produced with quantum-state ‖nCH2〉. It was found that ketene photodissociation yields CH2 in three resolved states; the 1A1(000), and 1A1(010) states of CH2 are the major channels, while the 3B1 state is a minor channel. In addition to this scalar distribution, the vector correlations between the recoil velocity and the angular momentum of the CO fragment (v⋅j correlation), expressed by the β00(22) bipolar moment, have also been obtained as a function of the kinetic energy release of the photoreaction. The correlated product state distribution was found not to follow the predictions of phase space theory, suggesting that dynamic hindrances exist in the photoreaction that have not been previously observed. A phase space theory calculation with restricted impact parameter values was also performed and compared to experiment. The impact parameter restricted phase space theory more accurately reproduced all of the correlated product state information obtained in this work as well as previous uncorrelated product state distributions for CH2 and CO. Both the ranges and the values of the allowed impact parameters obtained from these restricted calculations increase as the rotational energy of CO increases. Also, the values of the allowed impact parameters for 1A1(010) CH2 are larger than for 1A1(000) CH2. This strongly suggests that C–C–O bending modes are hindered at the transition state and therefore play an important role in the photodissociation.
Ab initio electronic structure of PtH+, PtH, Pt2, and Pt2H from a one-electron pseudopotential approach
Authors: Zurita Silvia , Rubio Jaime , Illas Francesc , Barthelat Jean Claude
Published: July 2002

A one-electron pseudopotential having an averaged d9 open shell in the core has been obtained for the ground state of Pt atom. With this pseudopotential we have studied the electronic structure of PtH+, PtH, Pt2, and Pt2H using large basis sets and an exact full configuration interaction wave function. In order to obtain reliable results, core–core repulsion, core–polarization and core–valence correlation effects have to be taken into account. These effects have been included through suitable core–core and core–polarization potentials. Comparison between existent and present calculations show that the one electron pseudopotential for Pt can be regarded as an average of the different real states arising from coupling of s valence shells with the open 5d9 levels of the d shell. Provided the extreme simplicity of the model, results obtained for small platinum containing systems are in fair agreement with previous highly sophisticated calculations and indicate that this one-electron pseudopotential for Pt can be effectively used to model surroundings of large surface cluster models or Pt complexes where d–d bonding is not dominant.
Monte Carlo simulation of polymer chain collapse in athermal solvents
Authors: Luna-Bárcenas Gabriel , Bennett Gerald E. , Sanchez Isaac C. , Johnston Keith P.
Published: July 2002

By computer simulation, Dijkstra et al. [M. Dijkstra, D. Frenkel, and J. P. Hansen, J. Chem. Phys. 101, 3179 (1994)] reported the first entropy-driven polymer chain collapse in an athermal solvent. To gain a better understanding of chain collapse physics in the absence of attractive interactions, we performed on-lattice NVT Monte Carlo simulations on a single polymer chain immersed in a hard-core solvent of variable size, shape, and density. In general, solvent quality decreases with increasing solvent density and incipient chain collapse occurs at a unique critical density for a given solvent size and shape. The critical density is smaller for large solvent molecules, but solvent shape also plays a role. Unfavorable solvent-chain excluded volume (EV) interactions drive the collapse transition. The EV interaction is reduced and the solvent entropy increases when the chain collapses, but there is an accompanying and unfavorable loss of chain conformational entropy. At the transition density these opposing entropy changes are comparable in magnitude. In the special and important case where the EV interaction is chain conformation independent, no collapse is observed at any solvent density.
Electron transfer of betaine-30 in the inverted region
Authors: Fuchs Christofer , Schreiber Michael
Published: July 2002

We use a density matrix theory to describe the photoinduced electron transfer dynamics of betaine-30 in solution, including environmental effects which may lead to relaxation and dephasing. We restrict ourselves to only one reaction coordinate. The remaining degrees of freedom of the molecule and the solvent form the environment which is bilinearly coupled to the relevant system. We investigate the S1→S0 reverse electron transfer of betaine-30 in solution, which occurs in the Marcus inverted region. We compare the temperature dependence of our theoretical results with experimental data and other theoretical predictions.
Multiconfigurational molecular dynamics with quantum transitions: Multiple proton transfer reactions
Authors: Hammes-Schiffer Sharon
Published: July 2002

We present the new method ‘‘multiconfigurational molecular dynamics with quantum transitions’’ (MC-MDQT) for the simulation of processes involving multiple proton transfer reactions. MC-MDQT is a mixed quantum/classical molecular dynamics method that allows the quantum mechanical treatment of the nuclear motion of multiple hydrogen atoms and accurately describes branching processes (i.e., processes involving multiple channels or pathways). MC-MDQT is based on the surface hopping method MDQT, which has already been applied to single proton transfer reactions in solution, where the nuclear motion of only the hydrogen atom being transferred is treated quantum mechanically. The direct extension of MDQT to multiple proton transfer reactions, where many hydrogen atoms must be treated quantum mechanically, is not computationally practical. In MC-MDQT a multiconfigurational self-consistent-field method is combined with MDQT to allow the quantum mechanical treatment of multiple hydrogen atoms while still including the significant correlation. The adiabatic states are expanded in a basis set of single configurations, which are products of one-particle states calculated using effective Hamiltonians derived from the occupied adiabatic state. Thus the one-particle states and the multiconfigurational adiabatic states must be calculated self-consistently. Both the MC-MDQT and the full basis set expansion MDQT methods are applied to a model system comprised of two quantum protons moving in double well potentials and one classical harmonic solvent degree of freedom. The results show that MC-MDQT incorporates the significant correlation and accurately describes branching processes. The MC-MDQT method is also used to study model systems comprised of three quantum protons and one classical solvent degree of freedom.
Ewald artifacts in liquid state molecular dynamics simulations
Authors: Smith Paul E. , Pettitt B. Montgomery
Published: July 2002

An investigation into the effects of the anisotropic nature of the Ewald potential for the treatment of long range electrostatic interactions in liquid solutions has been performed. The rotational potential energy surface for two simple charge distributions, and a small protein, have been studied under conditions typically implemented in current biomolecular simulations. A transition between hindered and free rotation is observed which can be modeled quantitatively for simple charge distributions. For most systems in aqueous solution, the transition involves an energy change well below kBT. It is argued that, for solvents with a reasonably high relative permittivity, Ewald artifacts will be small and in many cases may be safely ignored.
Short-time photodissociation dynamics of A-band and B-band bromoiodomethane in solution: An examination of bond selective electronic excitation
Authors: Man Shi-Qing , Kwok Wai Ming , Phillips David Lee , Johnson Alan E.
Published: July 2002

We have obtained resonance Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A-band and B-band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode vibrational descriptions were used with the results of the calculations to find the short-time photodissociation dynamics in terms of internal coordinates. The A-band short-time photodissociation dynamics indicate that the C–I bond becomes much longer, the C–Br bond becomes smaller, the I–C–Br angle becomes smaller, the H–C–Br angles become larger, the H–C–I angles become smaller, and the H–C–H angle becomes a bit smaller. The B-band short-time photodissociation dynamics indicate the C–Br bond becomes much longer, the C–I bond becomes slightly longer, the I–C–Br angle becomes smaller, the H–C–I angles become larger, the H–C–Br angles become smaller, and the H–C–H angle becomes slightly smaller. Both the A-band and B-band short-time photodissociation dynamics appear to be most consistent with an impulsive ‘‘semi-rigid’’ radical model qualitative description of the photodissociation with the CH2Br radical changing to a more planar structure in the A-band and the CH2I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A-band and B-band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short-time photodissociation dynamics suggest a moderate amount of coupling of the C–I and C–Br chromophores.
Evidence for kinetic effects in the folding of large RNA molecules
Authors: Morgan Steven R. , Higgs Paul G.
Published: July 2002

When RNA secondary structures predicted by free energy minimization programs are compared with the structures obtained by phylogenetic comparison it is found that there are often substantial differences. Here we consider a sample of large RNA molecules including 16S and 23S ribosomal RNA, and RNaseP RNA, and compare the minimum free energy and phylogenetic structures for domains of different size within the molecule. The pattern of differences between the structures is consistent with the idea that the native structures are influenced by the kinetics of the folding process, rather than solely by free energy minimization. It is found that the free energy of small domains in the phylogenetic structure of size 100 bases or less is usually much lower (i.e., more stable) than the average value of the minimum free energy of typical domains of corresponding size, whereas the reverse is true for large domains. We interpret these results using a picture of the folding process where short range secondary structure elements form first, which gradually rearrange to form domains of larger and larger size. Once low free energy domains of a moderate size are formed, these structures will remain trapped, because free energy barriers associated with structural rearrangement will be too large. The result is a structure containing small low free energy domains linked by a few longer range helices which fit in as best they can at a later stage of folding. We also discuss other possible causes of error in the predicted minimum free energy structures, and conclude that the difference between the minimum free energy and the phylogenetic structures cannot simply be put down to errors in the free energy parameters used in the model.
Experimental and theoretical characterization of the S1–S transition of benzo[a]pyrene
Authors: Gittins Christopher M. , Rohlfing Eric A. , Rohlfing Celeste M.
Published: July 2002

We use fluorescence excitation and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy to characterize the S1–S0 transition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a tunable ionization laser provides an improved measurement of the molecular ionization potential, 7.103±0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrational modes in the S1 and S0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of theory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between ∼380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational frequencies. In total, 27 of the 61 a′ symmetry vibrational modes and 5 of the 29 a″ vibrational modes in the S1 state have now been assigned.
Photodissociation dynamics of OClO
Authors: Davis H. Floyd , Lee Yuan T.
Published: July 2002

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A 2A2) vibrational states. For all levels studied, corresponding to OClO(A 2A2←X 2B1) excitation wavelengths between 350 and 475 nm, the dominant product (≳96%) was ClO(2Π)+O(3P). We also observed production of Cl+O2 with a quantum yield of up to 3.9±0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A 2A2) excited state vibrational mode. The Cl+O2 yield was enhanced slightly by exciting A 2A2 levels having symmetric stretching+bending, but diminished by as much as a factor of 10 for neighboring peaks associated with symmetric stretching+asymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO(2Π)+O(3P) channel. The photochemical dynamics of OClO possesses two energy regimes with distinctly different dynamics observed for excitation energies above and below ∼3.1 eV (λ∼400 nm). At excitation energies below 3.1 eV (λ≳400 nm), nearly all energetically accessible ClO vibrational energy levels were populated, and the minor Cl+O2 channel was observed. Although at least 20% of the O2 product is formed in the ground (X 3Σ−g) state, most O2 is electronically excited (a 1Δg). At E<3.1 eV, both dissociation channels occur by an indirect mechanism involving two nearby excited states, 2A1 and 2B2. Long dissociation time scales and significant parent bending before dissociation led to nearly isotropic polarization angular distributions (β∼0). At excitation energies above 3.1 eV (λ<400 nm), the Cl+O2 yield began to decrease sharply, with this channel becoming negligible at λ<370 nm. At these higher excitation energies, the ClO product was formed with relatively little vibrational energy and a large fraction of the excess energy was channeled into ClO+O translational energy. The photofragment anisotropy parameter (β) also increased, implying shorter dissociation time scales. The sharp change in the disposal of excess energy into the ClO products, the decrease of Cl+O2 production, and more anisotropic product angular distributions at E≳3.1 eV signify the opening of a new ClO+O channel. From our experimental results and recent ab initio calculations, dissociation at wavelengths shorter than 380 nm to ClO+O proceeds via a direct mechanism on the optically prepared A 2A2 surface over a large potential energy barrier. From the ClO(2Π)+O(3P) translational energy distributions, D0(O–ClO) was found to be less than or equal to 59.0±0.2 kcal/mol.
Engineering a simple polarizable model for the molecular simulation of water applicable over wide ranges of state conditions
Authors: Chialvo Ariel A. , Cummings Peter T.
Published: July 2002

We perform a systematic analysis of the relationship between the molecular geometry, the force-field parameters, the magnitude of the induced dipoles, and the resulting site–site microstructure of a model for water consisting of simple point charges plus a self-consistent point dipole polarizability. We constrain the model to represent the experimental values of the pressure and the configurational internal energy of water at ambient conditions, while keeping a permanent dipole moment of 1.85 D. The resulting force fields are then used to perform additional simulations at high temperature to determine the effect of polarizabilities on the site–site structure, and to make contact with neutron scattering experiments as well as ab initio simulation results. We show that the parameterization of the model is possible for 0≤ROM≤0.25 Å, where ROM is the oxygen-to-negative charge distance along the bisectrix of the H–O–H angle, resulting in total dipole moments from 2.88 to 3.03 D, with polarization energies accounting for 40%–57% of the total configuration internal energy of water. These results, in conjunction with the behavior of the short range site–site correlation functions, highlight the shortcomings of the simple point charge approximation for the polarization behavior at short intermolecular distances, and give a meaningful reference from which we can attempt to overcome these defects.
Nonlinear electro-optical response. I. Steady state Kerr effect relaxation arising from a weak ac electric field superimposed on a strong dc bias field
Authors: Déjardin J. L. , Déjardin P. M. , Kalmykov Yu. P.
Published: July 2002

The dynamic Kerr effect (excluding inertial effects) of an assembly of both polar and anisotropically polarizable molecules acted on by a strong external dc electric field superimposed on a weak ac electric field is evaluated by starting from the Smoluchowski equation valid in the noninertial limit. The calculation proceeds by expanding the expectation value of the second Legendre polynomial which describes the Kerr effect relaxation, as a power series up to the second order in the small ac field strength, so that frequency components in ω and 2ω exist. This is accomplished using the matrix continued fraction method which allows one to express exactly the solution of the infinite hierarchy of differential-recurrence relations for the first and second order ac responses of ensemble averages of the Legendre polynomials (relaxation functions). In order to illustrate these results, diagrams showing the behavior of the real and imaginary parts of the complex birefringence functions are presented. In particular, the relaxation spectra clearly indicate the differences between the first order response (equivalent to that obtained using linear response theory) and the second order one. We present also the solutions for the particular cases where only either permanent or induced dipole moments are taken into account. The calculation then radically simplifies as the matrix continued fractions now reduce to scalar ones.
The splitting of excited electronic states in optically inactive molecules due to the parity-violating electron-nuclear interaction
Authors: Ivanov L. N. , Letokhov V. S.
Published: July 2002

This paper considers optically inactive molecules possessing a symmetry plane. Degenerated excited electronic states in such molecules may, in principle, differ in symmetry with respect to mapping onto the symmetry plane. Should this prove the case, the parity-nonconserving electron-nuclear interaction (PNI) causes the degenerated electronic level to suffer a splitting linear in the Weinberg constant. The paper analyzes from this standpoint the lowest-lying excited states in the ten-electron HF, NH3, B2, and H2O molecules. Two of them, namely HF and NH3, possess the necessary and sufficient symmetry properties for such a linear splitting to occur. Factors are discussed that augment the PNI-induced splitting of the excited states under consideration in comparison with the splitting of the ground state in left- and right-handed modifications of optically active molecules. Computations confirm the occurrence of a great (≈10−13 eV) splitting of the levels being considered due to the PNI effect. A similar effect can also occur in the electronic ground state of paramagnetic molecules, such as NO. The computation uses the consistent multiple-electron perturbation theory with a model single-electron central field bare potential. The computer code used is a modification of the original code developed for precision atomic calculations. All the computations boil down to the solution of a single set of ordinary differential equations, i.e., a unidimensional procedure.
Unraveling the dissociation of dimethyl sulfoxide following absorption at 193 nm
Authors: Blank David A. , North Simon W. , Stranges Domenico , Suits Arthur G. , Lee Yuan T.
Published: July 2002

We have studied the photodissociation of dimethyl sulfoxide, DMSO-h6 and DMSO-d6, at 193 nm using the technique of photofragment translational spectroscopy with a tunable vacuum ultraviolet product probe provided by undulator radiation on the Chemical Dynamics Beamline at the Advanced Light Source. In contrast to previous investigations we have found the dissociation to proceed via a stepwise mechanism involving multiple reaction channels. The primary dissociation, S–C bond cleavage to eliminate a methyl radical, was found to have two competing channels with distinct translational energy distributions. The translational energy distribution for the major primary dissociation channel suggests that it proceeds in a statistical manner on the ground electronic surface following internal conversion. In competition with this channel is a primary dissociation that exhibits a translational energy distribution suggestive of dissociation on an excited electronic surface with most of the available energy partitioned into translational and electronic degrees of freedom. Secondary decomposition of the CD3SO intermediate was found to proceed exclusively via C–S bond cleavage, CD3SO→CD3+SO. However, secondary decomposition of the CH3SO intermediate was found to exhibit competition between CH3SO→CH3+SO and CH3SO→CH2SO+H. The dissociation to CH3 and SO was the major secondary decomposition channel with the translational energy distribution indicating a barrier to recombination of >8 kcal/mol. While a minor hydrogen atom elimination channel was found to play a role in secondary decomposition of CH3SO intermediates, no analogous secondary C–D bond cleavage was detected from the CD3SO intermediates indicating the importance of tunneling in the secondary decomposition of CH3SO.
A new phenomenological approach to gas–liquid nucleation based on the scaling properties of the critical nucleus
Authors: Talanquer V.
Published: July 2002

The scaling properties of the critical nucleus, as proposed recently by McGraw and Laaksonen, are analyzed in the context of density functional results for simple Lennard-Jones systems. A simple phenomenological approach with no adjustable parameters is introduced to test the validity of these scaling relations in describing the nucleation behavior of nonpolar and weakly polar vapors.
Interpretation of helium atom scattering from isolated CO molecules on copper (001) based on an exact quantum mechanical model
Authors: Choi B. H. , Tang K. T. , Toennies J. P.
Published: July 2002

The previously reported scattering of He atoms from isolated CO molecules on Cu(001) has been reinterpreted using an exact quantum mechanical model assuming a hard hemisphere on a hard flat surface. From a best fit of all the observed oscillations for each incident energy an effective radius for the hemisphere is determined. The radius increases from a=2.4 to 4.4 Å as the incident energy decreases from 40.3 to 9.4 meV, as expected in view of the soft repulsion and long-range attraction of the true potential.
Mechanism of the C2H5+O2 reaction
Authors: Ignatyev Igor S. , Xie Yaoming , Allen Wesley D. , Schaefer Henry F.
Published: July 2002

The geometries, energies, and vibrational frequencies of the reactants, transition states, intermediates, and products of the reaction of ethyl radical with the oxygen molecule have been examined using density functional theory (DFT). Rather different theoretical predictions are obtained from the BLYP, B3LYP, and BHLYP methods. Comparisons with experimental deductions and high-level coupled cluster results suggest that the B3LYP method is superior for the C2H5+O2 problem. Using the B3LYP method with a triple-zeta plus double-polarization plus f function (TZ2Pf) basis set, a transition state between the ethylperoxy radical and products is discovered which lies 3.3 kcal mol−1 below reactants. This transition-state energy is consistent with the observed high yields of ethylene in the high-temperature reaction and is in good agreement with the height of the barrier estimated via modeling of the experimental kinetic data. However, this transition state (TS1) corresponds not to the internal proton transfer leading to the hydroperoxyethyl radical C2H4OOH but to the concerted elimination of ethylene. For the reverse reactionC2H4+HO2→C2H4OOH, the TZ2Pf UB3LYP classical barrier is 11.2 kcal mol−1.
Nuclear spin relaxation of spin one-half nuclei in the presence of neighboring higher-spin nuclei
Authors: Werbelow Lawrence G. , Kowalewski Jozef
Published: July 2002

The basis of scalar relaxation of the second kind and related features involving the secular approximation and separation of time scales are investigated in the motional regime where the extreme narrowing approximation fails. Using second order perturbation theory, it is demonstrated that significant differences exist between the derived results and conventional treatments. Extensions of the described approach are readily apparent.