bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of temperature-dependent multimode resonance Raman line shapes for harmonic potential surfacesa)
Authors: Friesner Richard , Pettitt Montgomery , Jean John M.
Published: July 2002

An exact form for the time dependent resonance Raman kernel at finite temperature is derived under the assumption that the ground and excited state potential surfaces of the system are harmonic. The resultant expression can be evaluated by matrix algebra and Fourier transformed numerically to recover Raman scattering and excitation profiles.
Photochemistry of toluene vapor at 193 nm. Direct measurements of formation of hot toluene and the dissociation rate to benzyl radical
Authors: Ikeda Noriaki , Nakashima Nobuaki , Yoshihara Keitaro
Published: July 2002

Toluene has been photolyzed with an ArF laser (193 nm) in the gas phase. Time-resolved absorption spectra show that vibrationally hot toluene (S**0) is formed after rapid internal conversion from the S3 state, and then dissociates to benzyl radical. The formation rate of benzyl radical from hot toluene has been measured to be (2.4±0.2)×106 s−1 under collision-free conditions. It has been shown that there are two photodissociation processes to benzyl radical at 193 nm. The major process is efficient internal conversion and formation of hot molecules which is followed by dissociation. The other process is a predissociation. Importance of ‘‘hot’’ intermediates is pointed out in the mechanism of ‘‘photochemical’’ reactions of benzene derivatives.
Phase transitions of semiflexible lattice polymers
Authors: Baumgärtner Artur
Published: July 2002

Dense systems of semiflexible polymers on square and cubic lattices, each chain comprising up to N=30 lattice sites, have been investigated by Monte Carlo methods. It is shown that semiflexibility only does not lead to a phase transition between an isotropic and a long-range orientationally ordered state, rather local orientation-dependent intermolecular interactions are necessary in order to observe a nematic transition, which is of first order in three dimensions.
Optical birefringence and temperature range of the biaxial nematic phase in a lyotropic liquid crystal
Authors: Galerne Y. , Neto A. M. Figueiredo , Liebert L.
Published: July 2002

A linear relation is found between the cube of the optical birefringence and the temperature range of the biaxial nematic phase in the lyotropic mixtures of potassium laurate, l-decanol, D2O. Such a relation which is consistent with the Landau model of the uniaxial to biaxial nematic phase transitions may be used as a criterion for preparing samples of the desired temperature range of the biaxial nematic phase.
Benchmark full configuration-interaction calculations on H2O, F, and F−
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Full configuration-interaction calculations are reported, and compared to other methods, for H2O at its equilibrium geometry and at two geometries with the H–O bonds stretched. Since the percentage of the SCF reference in the FCI wave function decreases greatly with the bond elongation, the accuracy of techniques based on a single reference do not compare well with the FCI results. However, the results from a CASSCF/MRCI treatment are in good agreement with the FCI. Correlation effects in F compared to Ne are far more similar than for F− compared to Ne, despite F− and Ne being isoelectronic. Since the importance of higher than double excitations is greater for F− than F, a very high percentage of the correlation must be obtained to accurately compute the electron affinity. In a CASSCF/MRCI treatment the higher than quadruple excitations contribute 0.02 eV to the E.A., even for modest basis sets.
Photoexcitation of reaction complexes in the reaction K+NaCl→KCl+Na
Authors: Maguire T. C. , Brooks Philip R. , Curl R. F. , Spence James H. , Ulvick Sydney J.
Published: July 2002

Emission at the Na D lines has been observed from the intersection of crossed molecular beams of K and NaCl irradiated by a cw dye laser at wavelengths from 590 and 735 nm. The three-beam signal exhibits a threshold near 735 nm and is linearly dependent on the laser and molecular beam intensities. This three-beam signal has been observed under experimental conditions in which all two-beam signals are accounted for, and is attributed to the formation of Na* by photoexcitation of the KNaCl reaction complex. The effective two-body cross section for the process is approximately 10−21 cm2 in a laser field of 1.5 kW/cm2.
A theoretical analysis of the main effects in magnetic vibrational circular dichroism
Authors: Walnut Thomas H.
Published: July 2002

An analysis is made of all the lowest order contributions to magnetic vibrational circular dichroism (MVCD). Three different contributing effects are found and characterized and are interpreted in terms of the interference and interaction of induced electronic currents.
Unimolecular dynamics of hydrogen atom release from excited cycloheptatriene, toluene, and p-xylene
Authors: Tsukiyama K. , Bersohn R.
Published: July 2002

Hydrogen atoms from the photodissociation of cycloheptatriene, toluene, and p-xylene were monitored by laser induced fluorescence using a tunable laser at 121.6 nm. The rates of formation of hydrogen atoms agree with a literature value for the rates of formation of benzyl radicals, the other fragments. The kinetic energy distributions of the nascent hydrogen atoms were derived from the Doppler broadening of the excitation spectra. The average fraction of the available energy released as translational energy was 11%–13%. The data are consistent with a statistical model in which at the instant of dissociation the remaining internal energy is distributed over only a fraction of the vibrational modes.
The influence of long-range force truncation on the thermodynamics of aqueous ionic solutions
Authors: Brooks Charles L.
Published: July 2002

The extended-RISM integral equation, with a renormalized HNC closure capable of accounting for truncated ion–solvent interactions, is used to examine the influence of electrostatic truncation on free energies and relative chloride and bromide ions at infinite dilution in SPC water are studied. Large effects in the ion–water distribution functions are observed for truncation distances between 7 and 12 Å. The resulting free energies and relative free energies of hydration display a slight oscillatory behavior as a function of truncation distance, with the maximum deviations occurring for the shortest truncation distances. The observed oscillatory behavior is in qualitative agreement with earlier Monte Carlo and molecular dynamics results.
Microscopical structure of the uniaxial and biaxial lyotropic nematics
Authors: Galerne Y. , Neto A. M. Figueiredo , Liébert L.
Published: July 2002

High-resolution x-ray measurements have been performed at the synchrotron of LURE (Orsay) to study both uniaxial (discotic and calamitic) and biaxial nematic lyomesophases of the mixture K-laurate, 1-decanol, and D2O. The variations of the densitometric profiles of the diffracted bands are analyzed vs temperature. They are shown to be consistent with the model of similar micellar aggregates in the three nematic phases, undergoing different orientational fluctuations. It is moreover discussed that the micelles have, on the average, the shape of biaxial platelets with a (1/2/3) anisotropy, and that they build the same pseudolamellar structure in each nematic phase.
Laser fluorescence excitation spectrum of jet-cooled tropolone: The Ã 1B2–X̃ 1A1 system
Authors: Redington Richard L. , Chen Yongqin , Scherer George J. , Field Robert W.
Published: July 2002

Five of the lowest frequency species a1, b1, and b2 vibrational modes of tropolone in the Ã 1B2 electronic state are studied using high resolution laser fluorescence excitation spectroscopy of the jet-cooled sample. The ν′26(b1) mode at 38 cm−1, most probably a ‘‘folding’’ vibration of the seven-membered and O⋅⋅⋅HO chelated ring systems of tropolone, is observed in the progression 26v0 with primarily even valued quantum numbers to v26=14. Tunneling doublets in the progression are observed to decrease from 18.90 cm−1 in the vibrationless state to unresolvable values when v′26 is larger than 8, thereby demonstrating highly mode-specific quenching of the ‘‘H atom’’ tunneling process by these low frequency, out-of-plane, ring deformation vibrations. The progression 251026v0 is observed to v26=9 and no tunneling doubling is resolved for any of the transitions. These progressions show that the tunneling energy level splitting of tropolone in the vibrationless level of the ground electronic state can be at most about 0.3 cm−1. Additional highly mode-specific interactions with the tunneling process are observed for the in-plane ring deformation modes ν′13(a1) and ν39(b2), but the ν′14(a1) mode at 378.24 cm−1, the lowest frequency a1 mode, is found to interact very little with the tunneling coordinate.
Hylleraas variational perturbation theory: Application to correlation problems in molecular systems
Authors: Cave Robert J. , Davidson Ernest R.
Published: July 2002

Hylleraas variational perturbation theory is applied through second order in energy to estimate the correlation energy in several molecular systems. The specific choices for H0 and V which are made lead to equations nearly identical to the multireference linearized coupled-cluster method of Laidig and Bartlett. The results obtained are in virtually exact agreement where comparisons have been made. Results from test calculations are presented for BeH2, CH2, and C2H4. In addition, the utility of perturbation theory for selecting correlating configurations is examined. This procedure is found to be quite accurate while significantly reducing the size of the system of linear equations to be solved.
Critical points of the volume phase transition in N-isopropylacrylamide gels
Authors: Hirotsu Shunsuke
Published: July 2002

Two critical points of the gel–gel transition in N-isopropylacrylamide gels were determined on the basis of the measurement of the coexistence curve. One of them was realized by changing the composition of solvent (water–methanol mixture) and the other by applying the osmotic pressure to a gel network, which was generated by adding large molecules to the outer solution. From the result the phase diagram in the xm(solvent composition)–T(temperature)–π(osmotic pressure) space was constructed. The order parameter of the gel–gel transition was determined for the first time as functions of temperature and solvent composition. The existence of the critical points was discussed in terms of recent theoretical studies.
Stochastic behavior and stirring rate effects in the chlorite–iodide reaction
Authors: Nagypál István , Epstein Irving R.
Published: July 2002

The autocatalytic reaction between chlorite and iodide ions in a closed system is a clock reaction, showing a sudden appearance of brown I2 followed by a rapid disappearance of the color. Under certain conditions, the reaction time displays a striking irreproducibility. This stochastic behavior is studied potentiometrically and spectrophotometrically as a function of initial [I− ], stirring rate and solution volume. The results imply that the irreproducibility is an inherent feature of the reaction generated by fluctuations in the solution after it is ‘‘well mixed.’’ The key contributors to the stochasticity are local concentration inhomogeneities resulting from imperfect stirring and the ‘‘supercatalytic’’ reaction kinetics. A qualitative explanation is given that incorporates these aspects.
Configuration interaction calculation of the electronic spectra of MgF2:V+2
Authors: Winter N. W. , Pitzer R. M.
Published: July 2002

The ground and excited quartet states of divalent vanadium in a magnesium fluoride host have been determined from Hartree–Fock and configuration-interaction calculations. The potential energy curves for the symmetric stretch of the nearest-neighbor fluoride ions were determined and used to calculate the vibrational energy levels and wave functions. The ground-state absorption bandwidths were calculated from the Franck–Condon overlaps and compared to the experimental low-temperature spectra. The calculated vertical excited-state absorption energies from the first excited state are also found to be in good agreement with recent measurements.
Electronic spectra and response properties of BH and AlH
Authors: Scuseria Gustavo E. , Geertsen Jan , Oddershede Jens
Published: July 2002

We have calculated ground-state potential-energy curves, vertical electronic excitation energies, transition moments, radiative lifetimes, and indirect nulcear spin–spin coupling constants of BH and AlH. The coupling constants are predicted to be J(11B–H)=49.40 Hz and J(27Al–H)=10.01 Hz. Total energies are obtained using the coupled clusters singles and doubles method while excitation energies and response properties are calculated by means of the coupled cluster polarization propagator method. It is essential for the agreement with experiment that the coupled cluster rather than the second-order polarization propagator method is applied. The lowest excitation energies of each symmetry are nearly the same in a CISD (CI singles and doubles) calculation as in the polarization propagator calculation and both methods agree well with experiment. However, orbitals optimized for the individual states are needed in the CISD calculation, whereas ground-state self-consistent field (SCF) orbitals may be used for all states in the propagator calculation.
A new equation of state for athermal chains
Authors: Honnell Kevin G. , Hall Carol K.
Published: July 2002

A new equation of state for fluids containing athermal chain molecules is developed and compared to simulation results and existing theories in three, two, and one dimensions. The new expression, which builds upon the generalized Flory theory, contains no adjustable parameters and relates the compressibility factor of an n-mer fluid to the compressibility factors of monomer and dimer fluids at the same volume fraction. Comparisons with Monte Carlo results for three- and two-dimensional freely jointed chains show very good agreement, and the overall accuracy of the new expression appears comparable to Wertheim’s thermodynamic perturbation theory of polymerization. In one dimension the new expression reduces to the exact result. Application of the equation to chain models with internal constraints and overlapping hard sites is discussed and illustrated through comparisons with Monte Carlo results for rigid trimers. An extension of our approach to arbitrary reference fluids shows that the generalized Flory and new equations are the first two members of a family of increasingly accurate equations of state for chains.
Efficient pointwise representations for vibrational wave functions: Eigenfunctions of H+3
Authors: Whitnell Robert M. , Light J. C.
Published: July 2002

The successive truncation–diagonalization method described in previous work [Z. Bac̆ić, R. M. Whitnell, D. Brown and J. C. Light, Comp. Phys. Comm. (to be published)] is generalized to a three-dimensional discrete variable representation (DVR). The use of the 3D DVR leads to a sparse Hamiltonian matrix that makes the transformations used in the successive truncation-diagonalization technique very efficient. The method is applied to J=0 H+3 using a hyperspherical coordinate system. Full symmetry adaptation of the DVR is used allowing a complete resolution of the vibrational eigenfunctions into the D3h irreducible representations. Converged eigenvalues up to ∼20 000 cm−1 are reported for all representations. This method is thereby shown to be both efficient and accurate for calculating triatomic vibrational states with large amplitude motion.
Path integral versus conventional formulation of equilibrium classical statistical mechanics
Authors: Kholodenko Arkady L.
Published: July 2002

The partition function for the grand canonical ensemble of particles interacting via pairwise potential is presented in the field-theoretic path integral form. The Schwinger-type equation of motion for the above partition function produce in the lowest order approximation the famous mean spherical approximation. Use of the saddle point methods applied to the partition function produces known density functional results. The above field-theoretic form is further generalized to the case when the interaction between the particles depends upon their internal ‘‘quantum’’ states which are being modeled with the help of Potts-like variables.
A multireference coupled-cluster method for special classes of incomplete model spaces
Authors: Meissner Leszek , Kucharski Stanislaw A. , Bartlett Rodney J.
Published: July 2002

A multireference coupled-cluster (MRCC) method for special but general classes of incomplete model spaces is developed within a Hilbert space framework. The formulation avoids the valence universality requirement and related Fock space considerations that require hierarchical solutions for different number of electrons. Consequently, the Hilbert space approach has fewer amplitudes to determine. It is shown that diagonalization of the effective Hamiltonian leads to purely extensive energies, providing the formal basis for a general MR-CC methods for potential energy surfaces.
Electrical properties of polarizable ionic solutions. II. Computer simulation results
Authors: Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

We present molecular dynamics simulations for two limiting models of ionic solutions: one where the solvent molecules are polar, but nonpolarizable; the other where they are only polarizable (but have no permanent dipole moment). For both models, the static two-body correlation functions, the frequency-dependent dielectric constant and conductivity are calculated and the statistical uncertainty on these quantities estimated for molecular dynamics runs of the order of 105 integration steps. For the case of the polar solvent, the accuracy of the computed static interionic correlation functions allows a valuable test of the hypernetted chain integral equation theory at an ionic concentration of 0.04. The quantitative variation of the fluctuations of polarization and electrical current with change of boundary conditions is evaluated within the context of the second model (polarizable nonpolar solvent). Applying the relationships derived in Part I between the phenomenological coefficients and susceptibilities, it is shown that consistent values for the dielectric constant and electrical conductivity are obtained. The sum rules which generalize the Stillinger–Lovett conditions to ionic solutions are computed and shown to be satisfied in our simulations. The evaluation of these sum rules constitutes an important test of the convergence of the electrolyte system to an equilibrium state.
Stochastic dynamics of the chlorite–iodide reaction
Authors: Sagués F. , Ramírez-Piscina L. , Sancho J. M.
Published: July 2002

A recently proposed theoretical framework appropriate to the study of the stochastic behavior of several chemical systems is used to analyze the irreproducibility of the observed reaction times in the chlorite–iodide clock reaction. Noise terms are incorporated through the kinetic constants and their intensity is further correlated with the inverse of the stirring rate. Analytical and simulation results are obtained for the first moments of the reaction time distribution. These results are compared with recent experimental data obtained by Nagypál and Epstein.
A general model-space coupled-cluster method using a Hilbert-space approach
Authors: Meissner Leszek , Bartlett Rodney J.
Published: July 2002

An extension of the recently reported multireference coupled-cluster method [L. Meissner, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 91, 6187 (1989)] to the general incomplete model space is presented. Connectedness of the cluster operators and the effective Hamiltonian is achieved by the admission of a special kind of internal excitation in the standard definition of the cluster operators and by setting corresponding matrix elements of the effective Hamiltonian equal to zero. This special type of similarity transformation leads to a unique partition of the excitations from the model space into those generated by the cluster operators and those generated by the effective Hamiltonian. The new form of the effective Hamiltonian preserves the size extensivity of the method. A discussion of the application of the method to the problem of the simultaneous calculation of energies of the ground closed-shell state and excited states is considered.
Ion–electron pairs in condensed polar media treated as H-like atoms
Authors: Schiller Robert
Published: July 2002

A model of radiation produced ion–electron pairs with an electron energy below about 1 eV is proposed for the description of electron moderation, self-trapping, and escape in condensed polar media. A discrete ion–electron pair is regarded as a ground-state H-like atom which interacts with the medium through polarization forces and by experiencing energy fluctuations. Slowly relaxing polarization makes electron energy decrease on the time scale of constant-charge dielectric relaxation. A strongly attracting polarization well is formed immediately upon ion pair formation owing to instantaneous polarization. Energy fluctuations can make the electron escape from the field of the ion, but not from that of the polarization well, i.e., the electron is already self-trapped when moderation and escape take place. The rate of electron moderation was found to agree with earlier estimates made by Fröhlich and Platzman. Solvated electron yields, by allowing also for initial recombination of energetically independent ion–electron pairs, were evaluated for a series of polar liquids and for ice as a function of εs2cp1/2, where εs is static relative permittivity and cp is specific heat. By making use of one common adjustable parameter for all the systems discussed, good agreement was obtained with the majority of the experimental data.
The structure and the dynamics of the ordered solid phase of CBr4, a rigid molecule model
Authors: Zieliński P. , Fouret R. , Foulon M. , More M.
Published: July 2002

Various known parameters of the atom–atom potential for the bromine atoms interaction have been used to model the experimental structure and the cohesive energy of the ordered phase II of CBr4. The set of parameters corresponding to krypton has turned out to give the best agreement with the experimental observations. The frequencies of the lattice vibrations at the zone center and the elastic constants have then been calculated and shown to be in a good quantitative agreement with the existing experimental data. The validity of the rigid molecule approach has been briefly discussed.
Isothermal–isobaric molecular dynamics simulation of liquid water
Authors: Ruff Imre , Diestler Dennis J.
Published: July 2002

Results of the first isothermal–isobaric molecular dynamics simulation of water are reported. Water molecules are assumed to be flexible and to interact via a sum of pairwise interatomic potentials. Extensive runs for 256 molecules at 298 and 373 K and 1 bar give satisfactory agreement with observed properties of liquid water as diverse as density, internal energy, heat capacity, compressibility, pair correlation functions, diffusion coefficient, vibrational spectrum, relative permittivity, and dielectric relaxation spectra. The nonlinear dependence of the relative permittivity on the electric field is also computed by means of fluctuation–dissipation relations.
Rotation–vibration interactions in formaldehyde: Results for low vibrational excitations
Authors: Aoyagi Mutsumi , Gray Stephen K.
Published: July 2002

We have carried out large-scale variational calculations on formaldehyde including all six vibrational modes and the rotational mode associated with K (the projection of total angular momentum on a body-fixed axis). A complete form of the Watson Hamiltonian and a realistic potential function based on ab initio data are used. Emphasis in this paper is placed on the low vibrational energy regime and the effect of Coriolis interaction. Comparison is made with other theoretical calculations and experiment, where available. Analysis of some of our full mode rovibrational states shows instances of simultaneous vibration and rotation mixing due to a strong a-axis Coriolis interaction. We show that such mixing can be understood from the perspective of a classical nonlinear resonance between the rotational frequency and the difference in two bending mode (out-of-plane and in-plane) frequencies. This confirms previous full dimension classical and reduced dimension quantum studies and represents an alternative way of understanding Coriolis interaction in the low vibrational energy regime.
The vibronic state distribution of the NCO(X̃ 2Π) product from the CN+O2 reaction
Authors: Sauder Deborah G. , Patel-Misra Dipti , Dagdigian Paul J.
Published: July 2002

The vibronic state distribution of the NCO product from the CN+O2 reaction has been extracted from a laser fluorescence experiment in a cell at a total pressure of 140 mTorr. The CN reagent was prepared by 193 nm photolysis of cyanogen, and individual NCO vibronic levels were interrogated by fluorescence excitation in its Ã 2Σ+−X̃ 2Π band system after a variable delay. A finite induction time was observed for the formation of the NCO product in all detected vibronic levels, indicative of the necessity to moderate the translational and rotational energy of the CN photolysis fragment before appreciable reaction can occur. The NCO product was observed in 80 different vibronic levels, with energies up to 51.5 kJ/mol. A nascent vibronic state distribution among the various (v1,v2) Renner–Teller components was estimated from the relative intensities of the various bands. While it was not possible to determine the distribution in the v3 levels because of insufficiently accurate spectroscopic data, it is nevertheless clear that the NCO product from CN+O2 is formed with considerable vibrational excitation. These results are consistent with previous kinetic studies which suggest that the reaction occurs on an attractive potential energy surface, with no activation barrier.
Growth and electronic properties of ultrathin lutetium–diphthalocyanine films studied by electron spectroscopy
Authors: Bufler J. , Abraham M. , Bouvet M. , Simon J. , Göpel W.
Published: July 2002

We report on a systematic study of the thin-film growth and the electronic structure of the intrinsic molecular semiconductor lutetium–diphthalocyanine (LuPc2). The films were grown in ultrahigh vacuum. Starting from submonolayer coverage, thickness-dependent x-ray and ultraviolet photoemission spectroscopy (UPS) measurements have been carried out. Information about the growth mechanism, the work function, and the evolution of the valence density of states with increasing coverage have been obtained. By combination of UPS with high-resolution electron-energy-loss spectroscopy an assignment of the spectroscopic structures to recently published molecular-orbitals calculations is proposed.
Computer simulation of the dynamics of the plastic phase of succinonitrile
Authors: Cardini Gianni , Righini Roberto , Califano Salvatore
Published: July 2002

The complex dynamics of the plastic phase of crystalline succinonitrile CN–CH2–CH2–CN, involving trans–gauche isomerization as well as molecular reorientations along the diagonals of the cubic unit cell, is investigated by means of molecular dynamics simulation. A simple molecular model in which the end CN groups and the CH2 groups are represented by single pseudoatoms is proposed. The model includes, in addition to the external degrees of freedom, three degrees of internal freedom described by two bending and a torsion coordinate. The intermolecular interactions are treated in terms of Lennard-Jones potentials. Force constants are used for the bending coordinates and the intramolecular torsion is described by a two minima potential function with three parameters. The dynamical properties of the crystal are investigated by calculation of the time dependence of single molecule autocorrelation functions. It is found that the most suitable autocorrelation functions are those involving the torsion and the orientation of the central C–C bond and of the vector joining the two CN end atoms. Three distinct regimes are observed for the orientational relaxation of the model system: (a) a very fast relaxation with a time constant τ<1 ps due to the overdamped librations and torsional vibrations; (b) a fast decay with τ≊3–4 ps due to the spinning motion of the trans conformers around the long inertia axis; (c) a slow diffusive regime with τ≊20–30 ps due to the molecular tumbling motions and to the trans–gauche isomerization.
Interaction energies associated with short intermolecular contacts of C–H bonds. A
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o computational study of the C–H⋅⋅⋅O contact interaction in CH4⋅⋅⋅OH2
Authors: Novoa Juan J. , Tarron Benito , Whangbo Myung-Hwan , Williams Jack M.
Published: July 2002

To accurately evaluate the C–H⋅⋅⋅O contact interaction energy, ab initio calculations were performed on CH4⋅⋅⋅OH2 at the self-consistent-field molecular orbital (SCF-MO) and MP2 levels with various quality basis sets including a near Hartree–Fock limit (HFL) one. The interaction energies calculated with truncated basis sets have small basis set superposition errors and become close to the near HFL results only when they include diffuse functions. This is attributed to the necessity of properly representing the lone pair electrons of oxygen that make short contact interaction with the C–H bond. Our MP2 calculations suggest that the conformation of CH4⋅⋅⋅OH2 with one C–H⋅⋅⋅O contact is the only minimum-energy structure. At the MP2 level with the near HFL basis set, the binding energy of CH4⋅⋅⋅OH2 is calculated to be 0.59±0.05 kcal/mol. The potential energy curves of CH4⋅⋅⋅OH2 calculated as functions of the H⋅⋅⋅O contact angles are consistent with the observation that the most frequently found H⋅⋅⋅O contact angles of organic solids are in the range ‖φ‖≤∼60° and ‖θ‖≤∼30°.
Folding kinetics of proteins and copolymers
Authors: Miller Robert , Danko Craig A. , Fasolka Michael J. , Balazs Anna C. , Sun Chan Hue , Dill Ken A.
Published: July 2002

We model the kinetic processes by which globular proteins and other heteropolymers fold to compact states. We perform Monte Carlo dynamics simulations on short self-avoiding copolymer chains on two-dimensional square lattices. The driving force for collapse is the aversion of nonpolar monomers for water. The chain monomers are of two types, H and P; favorable interactions occur among HH contacts. One respect in which this study differs from previous Monte Carlo folding studies is that the chains are sufficiently short that: (i) we can know unequivocally which conformations are at global minima (the ‘‘native’’ states) and which are at local minima of free energy, (ii) we can explore ‘‘pathway space’’ densely to determine the relative probabilities of all the possible pathways, and thus we establish that the model is ergodic and gives the equilibrium distribution in the long-time limit. We find that any individual molecule passes through a wide range of conformational states, often many times. Nevertheless, this meandering is not inconsistent with the observation that proteins fold through specific pathways involving particular sequences of events. Some pathways are strongly favored; i.e., folding is ‘‘cooperative’’ in that a ‘‘nucleating’’ HH contact acts as a constraint that restricts local conformational freedom and speeds the ‘‘zipping up’’ of other contacts nearby. Which pathways are favored depends on the sequence and the solvent. How does a chain fold to its native state so quickly? For these short chains of fixed length, we observe three regimes of folding kinetics. In the ‘‘Levinthal limit,’’ as the HH attraction approaches zero, folding is slow because the molecule searches randomly through the large ensemble of open conformations. In the ‘‘multiple minima’’ limit, when the HH attraction is very strong, folding is also slow because the chain becomes stuck in local minima of wrong compact conformations. However, we find that folding is relatively fast for intermediate HH attraction because the driving force ‘‘directs’’ the molecule toward a small ensemble of compact states, and yet incorrect potential wells are not too deep to slow this process.
Structure and energetics of Cr(CO)6 and Cr(CO)5
Authors: Barnes Leslie A. , Liu Bowen , Lindh Roland
Published: July 2002

The geometric structure of Cr(CO)6 is optimized at the modified coupled-pair functional (MCPF), single and double excitation coupled-cluster (CCSD), and CCSD(T) levels of theory (including a perturbational estimate for connected triple excitations), and the force constants for the totally symmetric representation are determined. The geometry of Cr(CO)5 is partially optimized at the MCPF, CCSD, and CCSD(T) levels of theory. Comparison with experimental data shows that the CCSD(T) method gives the best results for the structures and force constants, and that remaining errors are probably due to deficiencies in the one-particle basis sets used for CO. The total binding energies of Cr(CO)6 and Cr(CO)5 are also determined at the MCPF, CCSD, and CCSD(T) levels of theory. The CCSD(T) method gives a much larger total binding energy than either the MCPF or CCSD methods. An analysis of the basis set superposition error (BSSE) at the MCPF level of treatment points out limitations in the one-particle basis used here and in a previous study. Calculations using larger basis sets reduce the BSSE, but the total binding energy of Cr(CO)6 is still significantly smaller than the experimental value, although the first CO bond dissociation energy of Cr(CO)6 is well described. An investigation of 3s3p correlation reveals only a small effect. In the largest basis set, the total CO binding energy of Cr(CO)6 is estimated to be 140 kcal/mol at the CCSD(T) level of theory, or about 86% of the experimental value. The remaining discrepancy between the experimental and theoretical value is probably due to limitations in the one-particle basis, rather than limitations in the correlation treatment. In particular, an additional d function and an f function on each C and O are needed to obtain quantitative results. This is underscored by the fact that even using a very large primitive set (1042 primitive functions contracted to 300 basis functions), the superposition error for the total binding energy of Cr(CO)6 is 22 kcal/mol at the MCPF level of treatment.
Catalytic oxidation of CO on Pt(335): A study of the active site
Authors: Xu Jiazhan , Yates John T.
Published: July 2002

The catalytic reaction CO(a)+O(a)→CO2(g) has been studied on Pt(335) using infrared reflection absorption spectroscopy (IRAS) and temperature programmed reaction (TPR) methods. Both dissociative O2 adsorption and CO adsorption occur preferentially on the step sites. It has been found that chemisorbed CO on the (111) terrace sites is more reactive than chemisorbed CO on the (100) step sites. In contrast, chemisorbed O on the step sites is more reactive at high CO coverages than chemisorbed O on the terrace sites. The results indicate that at high CO coverages the most reactive geometry involves step site O[O(S)] interacting with terrace sites CO [CO(T)]. This new information provides a conceptual basis for understanding the interplay between geometrical and energetic factors influencing the CO oxidation reaction.
HNSi: Hot bands in emission around 3 μm
Authors: Elhanine M. , Hanoune B. , Guelachvili G.
Published: July 2002

Four new hot bands of HNSi, 2ν1−ν1, ν1+ν3−ν3, 2ν1+ν3−(ν1+ν3), and ν1+ν2−ν2, are observed in emission from a radio frequency excited plasma with a high resolution Fourier transform interferometer. The equilibrium rotational and vibrational parameters are determined for the first time. About 400 lines positions, including improved data on the ν1 band, are provided.
Charge transfer of O2+ in helium at thermal and low electron volt energies
Authors: Gargaud M. , Bacchus-Montabonel M. C. , McCarroll R.
Published: July 2002

Cross sections and rate coefficients in the thermal-eV energy range are calculated for the charge transfer reaction of O2+(3P) ions with helium. In agreement with earlier predictions, it is confirmed that the reaction populates preferentially the metastable (2P) state of the O+ ion. The computed cross sections are, however, considerably larger than previous experimental and theoretical estimations. A discussion is given of the reasons for this discrepancy.
A test of different rotational Raman linewidth models: Accuracy of rotational coherent anti-Stokes Raman scattering thermometry in nitrogen from 295 to 1850 K
Authors: Martinsson Lars , Bengtsson Per-Erik , Aldén Marcus , Kröll Stefan , Bonamy Jeanine
Published: July 2002

Rotational Raman linewidths calculated from three different models have been used in temperature measurements by rotational coherent anti-Stokes Raman scattering (CARS)—a semiclassical ab initio model, the modified exponential energy gap model (MEG), and the energy corrected sudden scaling law (ECS). Experimental rotational CARS spectra were generated, using the dual-broadband approach, in pure nitrogen at atmospheric pressure in a heat pipe in the temperature range from 295 to 1850 K. Below 1500 K, the temperatures evaluated using the ECS linewidths agreed with the heat-pipe temperatures to within 20 K. Above 1500 K, the errors in the evaluated temperatures increased steeply for all linewidth models, reaching errors of several hundreds of Kelvins at 1850 K. This behavior of the evaluated temperature is probably caused by the uncertainty in the values of the rotational Raman linewidths for high rotational states at high temperatures. This work therefore illustrates that rotational CARS can be used for experimentally studying Raman linewidths and in particular their dependence on temperature and rotational quantum number. The influence of different experimental parameters on the evaluated temperatures is discussed, and the spectral synthesis program is presented.
Prediction of the transitional properties of liquid crystal dimers. A molecular field calculation based on the surface tensor parametrization
Authors: Ferrarini A. , Luckhurst G. R. , Nordio P. L. , Roskilly S. J.
Published: July 2002

The transitional properties of the liquid crystal dimers formed by two mesogenic groups linked by a flexible chain exhibit a pronounced dependence on the number of atoms in the spacer. Here we present the results of a theoretical calculation of the nematic–isotropic transition temperature, the entropy of transition, and the second rank orientational order parameters at the transition for two homologous series of cyanobiphenyl dimers. In one the alkyl spacer is attached directly to the mesogenic groups while in the other the alkyl chain is linked via ether units to the two cyanobiphenyl groups. At the heart of the molecular field theory is its parametrization based on the surface tensor approach. In this theory the potential of mean torque for each conformer is related to its topology by the surface tensor, which ensures that molecules are aligned such that their surfaces overlap to a maximal extent. The transitional properties predicted by the theory are found to depend in a sensitive manner on the geometry of the cyanobiphenyl group and that of its link to the alkyl spacer. By using realistic geometries and allowing for the various conformational states within the rotational isomeric state model the theory is able to account for the behavior of homologous series of dimers as the spacer length is changed, and for the dependence of the transitional properties on the nature of the link between the alkyl chain and the mesogenic groups.
Diffusion of ionic penetrants in charged disordered media
Authors: Chakraborty Arup K. , Bratko D. , Chandler David
Published: July 2002

We consider the diffusion of ionic species in technologically relevant materials such as zeolites. These materials are characterized by a disordered density distribution of charged sites that couple with the diffusing species. We present a model for ion diffusion in a specific form of charged disorder. This is a primitive model for ion diffusion in charged or acidic zeolites. The theory relies on a path integral representation of the propagator, and a Gaussian field theory for the effects of the disorder. We use the Feynman–Bogoliubov variational method to treat the model, and calculate the diffusion coefficient for ions in a medium characterized by randomly located charges. Numerical solution of our equations, and asymptotic analyses of the same, show that in our theory there is a crossover from diffusive to subdiffusive behavior beyond a threshold value for the average density of the disorder. This threshold coincides with the actual diffusion changing from processes well approximated by Gaussian paths to those involving escapes from deep potential wells and barrier crossings. These results are discussed in the context of recent field-theoretic and renormalization group approaches to the problem of diffusion in random media. Our approach to diffusion in random media appears reasonably general and should be applicable to many technologically relevant problems, and is not compute intensive.
The formation and destruction of H3O−
Authors: Miller Thomas M. , Viggiano A. A. , Miller Amy E. Stevens , Morris Robert A. , Henchman Michael , Paulson John F. , Van Doren Jane M.
Published: July 2002

We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3 s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6 s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride–water bond energy D0298(H−−H2O)=14.4±1.0 kcal mol−1 (0.62±0.04 eV). The heat of formation of H3O−, −37.5±1.0 kcal mol−1, and the proton affinity of H3O−, 386.0±1.0 kcal mol−1, are derived from these results. Dissociation of H3O− into OH− and H2 requires 4.5±1.0 kcal mol−1 energy.
Lattice field theory for spherical macroions in solution: Calculation of equilibrium pair correlation functions
Authors: Walsh A. M. , Coalson R. D.
Published: July 2002

A recently developed lattice field theory (LFT) formulation of the statistical mechanics of a classical Coulomb gas interacting with a fixed charge distribution [J. Chem. Phys. 97, 5653 (1992)] is utilized to compute equilibrium pair correlation functions, g(r), of aqueous suspensions of charged spherical macroions (‘‘polyballs’’). These results are compared to the predictions of the widely used Derjaguin et al. (DLVO) theory. In addition an effective pair potential for the colloid–colloid interaction is obtained by applying the LFT approach to a two-colloid system. This pair potential is then used in a many-colloid simulation to obtain g(r). All simulations are performed using the Metropolis Monte Carlo procedure together with the appropriate interpolyball potential function. Even in the regime in which a pair potential description might be expected to be adequate, differences exist between the LFT and pair potential results.
The interaction potential of a symmetric helium trimer
Authors: Bhattacharya Aniket , Anderson James B.
Published: July 2002

The interaction potential of three helium atoms in symmetric linear configurations is calculated under the Born–Oppenheimer approximation using an ‘‘exact’’ Green’s function quantum Monte Carlo method. The results are compared with those of ‘‘exact’’ quantum Monte Carlo calculations for the helium dimer and the Axilrod–Teller triple-dipole energy expression. For nearest neighbor separations of 5.6 bohrs, the equilibrium distance for a helium dimer, the corresponding trimer energy is −21.5±1.9 K, compared to −22.3±0.2 K, calculated for pairwise additive behavior. For all internuclear separations in the neighborhood of the van der Waals well (∼5.6 bohrs), the nonadditive contribution is found to be very small.
Temperature dependence of ultrafast intermolecular electron transfer faster than solvation process
Authors: Nagasawa Yutaka , Yartsev Arkadiy P. , Tominaga Keisuke , Johnson Alan E. , Yoshihara Keitaro
Published: July 2002

Temperature dependence of intermolecular electron transfer (ET) between oxazine 1 (OX 1) in the excited state and electron donor solvents such as aniline (AN) and N,N-dimethylaniline (DMA) was studied by observing fluorescence dynamics of OX1. The fluorescence decay of OX1 in DMA showed a single exponential behavior with a time constant of ∼280 fs which was independent of temperature over the range of 280 to 353 K. In AN, the ET caused nonexponential fluorescence decay whose time constants range from a few hundred femtoseconds to a few picoseconds depending on temperature. The time constants of these ET were smaller than the solvation times obtained by dynamic fluorescence Stokes shift of coumarin 102. This indicates the importance of vibrational nuclear motion in ET. The extended Sumi–Marcus two-dimensional reaction coordinate model which concerns the effect of high frequency mode was applied to explain the experimental observations. Good agreements were obtained between the experiments and calculations in terms of necessary parameters such as electronic matrix element and solvent reorganization energy. The difference between the ET of OX1 in DMA and AN can be mainly explained by the free energy difference between the reactant and the product.
Near-infrared femtosecond photoionization/dissociation of cyclic aromatic hydrocarbons
Authors: DeWitt Merrick J. , Levis Robert J.
Published: July 2002

Pulses of 780 nm light of duration 170 fs and power densities up to 3.8×1013 W cm−2 are used to study the photoionization/dissociation processes in the series of gas phase, cyclic aromatic hydrocarbons including benzene, naphthalene, phenanthrene, and anthracene. The near-infrared ionization process leads to the production of intact molecular ions for all of the molecules studied. Measurements of the ion intensity as a function of laser fluence revealed the order of the ultrafast ionization process to be 8.0±0.1 for anthracene, 6.9±0.1 for phenanthrene, 8.5±0.1 for naphthalene, and 8.1±0.1 for benzene. The relative femtosecond photoionization cross section decreased from 1.0 for anthracene to 0.2 for phenanthrene to 0.1 for naphthalene to ∼0.005 for benzene. The relative order and cross section of the femtosecond ionization processes suggest that a field ionization mechanism is operative.