bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulation of dendrimers in variable solvent quality
Authors: Giupponi G. , Buzza D. M. A.
Published: May 2004

We study via lattice Monte Carlo simulation and Flory theory the properties of g=1–6 dendrimers in variable solvent quality. For all the generations studied, we find that the radius of gyration Rg collapses significantly (factor of 2) going from athermal to extreme poor solvent conditions, indicating that varying solvent quality is an effective means of controlling dendrimer size. We also find that in athermal, θ, and extreme poor solvent conditions, the radius of gyration of dendrimers scales with the total number of monomers roughly as Rg∼N1/3. However, a more careful analysis shows that in athermal and θ solvents, there is, in fact, a small but systematic deviation of Rg from Rg∼N1/3 scaling and the simulation data is described better by the Flory theory prediction of Rg∼N1/5[(g+1)m]2/5 in athermal solvents and Rg∼N1/4[(g+1)m]1/4 in θ solvents. We also find for our simulation data that stronger deviations from constant density scaling are possible, with scaling behavior as shallow as Rg∼N0.26 possible for solvent conditions in between θ and the completely collapsed state. It is evident therefore that dendrimers do not obey (or even approximately obey) Rg∼N1/3 scaling under all solvent conditions. Under all solvent conditions, we find that the intramolecular density is dense corelike (i.e., the density maximum is in the interior of the dendrimer) and terminal groups are delocalized throughout the dendrimer.
The role of attractive interactions in rod–sphere mixtures
Authors: Antypov Dmytro , Cleaver Douglas J.
Published: May 2004

We present a computer simulation study of binary mixtures of prolate Gay–Berne particles and Lennard-Jones spheres. Results are presented for three such rod–sphere systems which differ from each other only in the interaction between unlike particles. Both the mixing-demixing behavior and the transitions between the isotropic and any liquid crystalline phases are studied for each system, as a function of temperature and concentration ratio. For systems which show macroscopic demixing, the rod–sphere interaction is shown to give direct control over interfacial anchoring properties, giving rise to the possibility of micellar phase formation in the case of homeotropic anchoring. Additionally, it is shown that on incorporating high concentrations of spheres into a system of rods with weak demixing properties, microphase-separated structures can be induced, including bicontinuous and lamellar arrangements.
Efficient chemical kinetic modeling through neural network maps
Authors: Shenvi Neil , Geremia J. M. , Rabitz Herschel
Published: May 2004

An approach to modeling nonlinear chemical kinetics using neural networks is introduced. It is found that neural networks based on a simple multivariate polynomial architecture are useful in approximating a wide variety of chemical kinetic systems. The accuracy and efficiency of these ridge polynomial networks (RPNs) are demonstrated by modeling the kinetics of H2 bromination, formaldehyde oxidation, and H2+O2 combustion. RPN kinetic modeling has a broad range of applications, including kinetic parameter inversion, simulation of reactor dynamics, and atmospheric modeling.
Site-specific vibrational dynamics of the CD3ζ membrane peptide using heterodyned two-dimensional infrared photon echo spectroscopy
Authors: Mukherjee Prabuddha , Krummel Amber T. , Fulmer Eric C. , Kass Itamar , Arkin Isaiah T. , Zanni Martin T.
Published: May 2004

Heterodyned two-dimensional infrared (2D IR) spectroscopy has been used to study the amide I vibrational dynamics of a 27-residue peptide in lipid vesicles that encompasses the transmembrane domain of the T-cell receptor CD3ζ. Using 1–13C=18O isotope labeling, the amide I mode of the 49-Leucine residue was spectroscopically isolated and the homogeneous and inhomogeneous linewidths of this mode were measured by fitting the 2D IR spectrum collected with a photon echo pulse sequence. The pure dephasing and inhomogeneous linewidths are 2 and 32 cm−1, respectively. The population relaxation time of the amide I band was measured with a transient grating, and it contributes 9 cm−1 to the linewidth. Comparison of the 49-Leucine amide I mode and the amide I band of the entire CD3ζ peptide reveals that the vibrational dynamics are not uniform along the length of the peptide. Possible origins for the large amount of inhomogeneity present at the 49-Leucine site are discussed.
The nuclear magnetic resonance line shapes of Xe in the cages of clathrate hydrates
Authors: Jameson Cynthia J. , Stueber Dirk
Published: May 2004

We report, for the first time, a prediction of the line shapes that would be observed in the Xe129 nuclear magnetic resonance (NMR) spectrum of xenon in the cages of clathrate hydrates. We use the dimer tensor model to represent pairwise contributions to the intermolecular magnetic shielding tensor for Xe at a specific location in a clathrate cage. The individual tensor components from quantum mechanical calculations in clathrate hydrate structure I are represented by contributions from parallel and perpendicular tensor components of Xe–O and Xe–H dimers. Subsequently these dimer tensor components are used to reconstruct the full magnetic shielding tensor for Xe at an arbitrary location in a clathrate cage. The reconstructed tensors are employed in canonical Monte Carlo simulations to find the Xe shielding tensor component along a particular magnetic field direction. The shielding tensor component weighted according to the probability of finding a crystal fragment oriented along this direction in a polycrystalline sample leads to a predicted line shape. Using the same set of Xe–O and Xe–H shielding functions and the same Xe–O and Xe–H potential functions we calculate the Xe NMR spectra of Xe atom in 12 distinct cage types in clathrate hydrates structures I, II, H, and bromine hydrate. Agreement with experimental spectra in terms of the number of unique tensor components and their relative magnitudes is excellent. Agreement with absolute magnitudes of chemical shifts relative to free Xe atom is very good. We predict the Xe line shapes in two cages in which Xe has not yet been observed.
Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: A finite size effects study
Authors: Ganster Patrick , Benoit Magali , Kob Walter , Delaye Jean-Marc
Published: May 2004

We study a calcium aluminosilicate glass of composition (SiO2)0.67–(Al2O3)0.12–(CaO)0.21 by means of molecular-dynamics simulations, using a potential made of two-body and three-body interactions. In order to prepare small samples that can subsequently be studied by first principles, the finite size effects on the liquid dynamics and on the glass structural properties are investigated. We find that finite size effects affect the Si–O–Si and Si–O–Al angular distributions, the first peaks of the Si–O, Al–O, and Ca–O pair correlation functions, the Ca coordination, and the oxygen atoms’ environment in the smallest system (100 atoms). We give evidence that these finite size effects can be directly attributed to the use of three-body interactions.
Minima hopping: An efficient search method for the global minimum of the potential energy surface of complex molecular systems
Authors: Goedecker Stefan
Published: May 2004

A method is presented that can find the global minimum of very complex condensed matter systems. It is based on the simple principle of exploring the configurational space as fast as possible and of avoiding revisiting known parts of this space. Even though it is not a genetic algorithm, it is not based on thermodynamics. The efficiency of the method depends strongly on the type of moves that are used to hop into new local minima. Moves that find low-barrier escape-paths out of the present minimum generally lead into low energy minima.
Modification of statistical threading in two-component pseudorotaxane melts using the amphiphilic approach and variations in the confinement geometry
Authors: Rane Sagar S. , Mattice Wayne L. , Pugh Coleen
Published: May 2004

Recently we described a coarse-grained model of poly(ethylene oxide) and then employed that model to study the amount of spontaneous threading of cyclic molecules by linear chains in the melt [C. A. Helfer, G. Xu, W. L. Mattice, and C. Pugh, Macromolecules 36, 10071 (2003)]. Since the amount of statistical threading at equilibrium is small, there is interest in identifying physical changes in the system that will increase the threading. We now use that coarse-grained model to investigate the effect on threading of various hypothetical (but feasible) modifications to the two-component system of macrocycles and linear chains in the melt, and different confinement geometries, that can bring about correlations in the arrangement of the rings. Our work follows on the concept of an amphiphilic approach [C. Pugh, J.-Y. Bae, J. R. Scott, and C. L. Wilkins, Macromolecules 30, 8139 (1997)] for increasing the statistical threading in homopolyrotaxane melts. We investigate whether introducing such correlations in the macrocycles can increase the spontaneous threading. This paper shows that some of our modifications can yield more than double the amount of threading seen in purely statistical mixing.
Early chemistry in hot and dense nitromethane: Molecular dynamics simulations
Authors: Riad Manaa M. , Reed Evan J. , Fried Laurence E. , Galli Giulia , Gygi François
Published: May 2004

We report density functional molecular dynamic simulations to determine the early chemical events of hot (T=3000 K) and dense (ρ=1.97 g/cm3, V/V0=0.68) nitromethane (CH3NO2). The first step in the decomposition process is an intermolecular proton abstraction mechanism that leads to the formation of CH3NO2H+ and the aci ion H2CNO2−. This event is also confirmed to occur in a fast annealing simulation to a final temperature of 4000 K at ρ=2.20 g/cm3. An intramolecular hydrogen transfer that transforms nitromethane into the aci acid form, CH2NO2H, accompanies this event. To our knowledge, this is the first confirmation of chemical reactivity with bond selectivity for an energetic material near the Chapman–Jouget state of the fully reacted material. We also report the decomposition mechanism followed up to the formation of H2O as the first stable product. We note that similarities in the global features of reactants, intermediates, and products of the reacting fluid seem to indicate a threshold for similar chemistry in the range of high densities and temperatures reported herein.
Fourier transform infrared emission spectra of MgH and MgD
Authors: Shayesteh A. , Appadoo D. R. T. , Gordon I. , Le Roy R. J. , Bernath P. F.
Published: May 2004

High resolution Fourier transform infrared emission spectra of MgH and MgD have been recorded. The molecules were generated in an emission source that combines an electrical discharge with a high temperature furnace. Several vibration–rotation bands were observed for all six isotopomers in the X 2Σ+ ground electronic state: v=1→0 to 4→3 for MgH24, v=1→0 to 3→2 for MgH25 and MgH26, v=1→0 to 5→4 for MgD24, v=1→0 to 4→3 for MgD25 and MgD26. The new data were combined with the previous ground state data, obtained from diode laser vibration–rotation measurements and pure rotation spectra, and spectroscopic constants were determined for the v=0 to 4 levels of MgH24 and the v=0 to 5 levels of MgD24. In addition, Dunham constants and Born–Oppenheimer breakdown correction parameters were obtained in a combined fit of the six isotopomers. The equilibrium vibrational constants (ωe) for MgH24 and MgD24 were found to be 1492.776(7) cm−1 and 1077.298(5) cm−1, respectively, while the equilibrium rotational constants (Be) are 5.825 523(8) cm−1 and 3.034 344(4) cm−1. The associated equilibrium bond distances (re) were determined to be 1.729 721(1) Å for MgH24 and 1.729 157(1) Å for MgD24.
Quantum scattering and adiabatic channel treatment of the low-energy and low-temperature capture of a rotating quadrupolar molecule by an ion
Authors: Dashevskaya E. I. , Litvin I. , Nikitin E. E. , Troe J.
Published: May 2004

The capture rate coefficients of homonuclear diatomic molecules (H2 and N2) in the rotational state j=1 interacting with ions (Ar+ and He+) are calculated for low collision energies assuming a long-range anisotropic ion-induced dipole and ion–quadrupole interaction. A comparison of accurate quantum rates with quantum and state-specific classical adiabatic channel approximations shows that the former becomes inappropriate in the case when the cross section is dominated by few partial contributions, while the latter performs better. This unexpected result is related to the fact that the classical adiabatic channel approximation artificially simulates the quantum effects of tunneling and overbarrier reflection as well as the Coriolis coupling and it suppresses too high values of the centrifugal barriers predicted by a quantum adiabatic channel approach. For H2(j=1)+Ar+ and N2(j=1)+He+ capture, the rate constants at T→0 K are about 3 and 6 times higher than the corresponding values for H2(j=0)+Ar+ and N2(j=0)+He+ capture.
Accurate reaction paths using a Hessian based predictor–corrector integrator
Authors: Hratchian Hrant P. , Schlegel H. Bernhard
Published: May 2004

Central to the theoretical description of a chemical reaction is the reaction pathway. The intrinsic reaction coordinate is defined as the steepest descent path in mass weighted Cartesian coordinates that connects the transition state to reactants and products. In this work, a new integrator for the steepest descent pathway is presented. This method is a Hessian based predictor–corrector algorithm that affords pathways comparable to our previous fourth order method at the cost of a second order approach. The proposed integrator is tested on an analytic surface, four moderately sized chemical reactions, and one larger organometallic system.
Sum-frequency vibrational spectroscopy of chiral liquids off and close to electronic resonance and the antisymmetric Raman tensor
Authors: Belkin M. A. , Shen Y. R. , Harris R. A.
Published: May 2004

The strength of the chiral vibrational peaks in infrared-visible sum-frequency (SF) vibrational spectra from isotropic chiral liquids is proportional to the square of the corresponding antisymmetric Raman element. Under the Born–Oppenheimer adiabatic approximation with nonadiabatic corrections, the antisymmetric Raman tensor is much weaker than the symmetric counterpart, but becomes significantly stronger as the input frequency (or the sum-frequency in SF generation) approaches electronic resonance. We verify the theory with experimental results obtained from infrared-visible doubly resonant sum-frequency generation from an isotropic solution of chiral molecules.
Electric fields with ultrasoft pseudo-potentials: Applications to benzene and anthracene
Authors: Tóbik Jaroslav , Dal Corso Andrea
Published: May 2004

We present density-functional perturbation theory for electric field perturbations and ultra-soft pseudopotentials. Applications to benzene and anthracene molecules and surfaces are reported as examples. We point out several issues concerning the evaluation of the polarizability of molecules and slabs with periodic boundary conditions.
The structure of methane hydrate under geological conditions a combined Rietveld and maximum entropy analysis
Authors: Baumert J. , Gutt C. , Johnson M. R. , Tse J. S. , Klug D. D. , Press W.
Published: May 2004

We present a study of the structure of a fully deuterated methane hydrate under the geological conditions found in the world’s oceans. In situ high-resolution neutron diffraction experiments have been performed at temperatures of 220, 275, and 280 K and a pressure of 100 bar, corresponding to the conditions at 1000 m water depth. The data were analyzed with a combination of Rietveld refinement and maximum entropy methods. From the Rietveld refinement, precise atomic parameters of the host lattice could be determined, indicating increasing distortions of the structure of the cages at elevated temperatures and pressures. Debye–Waller factors of the encaged CD4 molecules have been found to exceed the values of the Debye–Waller factors of the D2O molecules considerably. In the large cage of structure type I the thermal center-of-mass displacements of the guests are 5–10 times larger than those of the water molecules. From the maximum entropy analysis maps of the scattering length density have been obtained, showing details of the vibrational amplitudes of the atoms in methane hydrate. The Debye–Waller factors of all molecules have been found to deviate considerably from a simple spherical geometry.
Monte Carlo simulation of the self-assembly and phase behavior of semiflexible equilibrium polymers
Authors: Lü Xinjiang , Kindt James T.
Published: May 2004

Grand canonical Monte Carlo simulations of a simple model semiflexible equilibrium polymer system, consisting of hard sphere monomers reversibly self-assembling into chains of arbitrary length, have been performed using a novel sampling method to add or remove multiple monomers during a single MC move. Systems with two different persistence lengths and a range of bond association constants have been studied. We find first-order lyotropic phase transitions between isotropic and nematic phases near the concentrations predicted by a statistical thermodynamic theory, but with significantly narrower coexistence regions. A possible contribution to the discrepancy between theory and simulation is that the length distribution of chains in the nematic phase is bi-exponential, differing from the simple exponential distribution found in the isotropic phase and predicted from a mean-field treatment of the nematic. The additional short length-scale characterizing the distribution appears to arise from the lower orientational order of short chains. The dependence of this length-scale on chemical potential, bond association constant, and total monomer concentration has been examined.
On the importance of the “density per particle” (shape function) in the density functional theory
Authors: De Proft F. , Ayers P. W. , Sen K. D. , Geerlings P.
Published: May 2004

The central role of the shape function σ(r_) from the density functional theory (DFT), the ratio of the electron density ρ(r_) and the number of electrons N of the system (density per particle), is investigated. Moreover, its relationship with DFT based reactivity indices is established. In the first part, it is shown that an estimate for the chemical hardness can be obtained from the long range behavior of the shape function and its derivative with respect to the number of electrons at a fixed external potential. Next, the energy of the system is minimized with the constraint that the shape function should integrate to unity; the associated Lagrange multiplier is shown to be related to the electronic chemical potential μ of the system. Finally, the importance of the shape function for both molecular structure, reactivity, and similarity is outlined.
Investigation of photoinduced electron transfer in model system of vitamin E-duroquinone by time-dependent density functional theory
Authors: Duan Xiao-Hui , Li Ze-Rong , Li Xiang-Yuan , Li Liu-Ming
Published: May 2004

Photoinduced electron transfer of the model system composed of vitamin E and duroquinone has been investigated using time-dependent density functional theory. Calculations for the excited states tell that the photoexcitation of the model system can directly yield the charge transfer states in which the vitamin E moiety is positively charged but the duroquinone moiety is negatively charged. Our theoretical investigations indicate that the second charge transfer state of the model system can also be produced through the decay of higher locally excited state S4. Since S4 state in the model system corresponds to S1 state of the isolated duroquinone used as a model for peroxyl radical, and S2 state has the character of electron transfer from the tertiary amine group of the vitamin E moiety to the duroquinone moiety, the decay from S4 to S2 corresponds to the dynamic process following the photoexcitation of the duroquinone moiety of the model system, i.e., the initial stage of antioxidant reaction of vitamin E. Calculations of the kinetic parameters for the electron transfer have been carried out in the framework of the Marcus–Jortner–Levich formalism. Our calculations confirm that the electron transfer from S4 to S2 possesses the character of the inverted regime and the barrier is negligibly small.
Systematic ab initio calculations on the energetics and stability of covalent O4
Authors: Hernández-Lamoneda Ramón , Ramı́rez-Solı́s Alejandro
Published: May 2004

Ab initio calculations with highly correlated methods together with extensive basis sets have been used to obtain the most accurate heat of formation and stability with respect to dissociation (into molecular oxygen) for the chemically bound tetraoxygen molecule. Our calculations show that the heat of formation is significantly smaller and that the barrier to dissociation is larger than previously assumed. In particular, we have shown that the previous theoretical estimate for the heat of formation of tetraoxygen was in error by a significant amount (18%–24%) owing to lack of accuracy in the theoretical method then used. Our best estimates places that value in the range 93–95 kcal/mol and this should be taken into consideration when discussing the possible relevance of tetraoxygen in a variety of experiments, as well as in the fundamental atmospheric chemical processes where oxygen species participate.
Theoretical study of the longitudinal first hyperpolarizability of polysilaacetylene
Authors: Jacquemin Denis , Perpète Eric A. , André Jean-Marie
Published: May 2004

With the help of ab initio tools taking into account dynamic electron correlation effects, we study the longitudinal electronic first hyperpolarizability of carbon–silicon analogues to polyacetylene. It turns out that the MP2/6-31G(d)//HF/6-31G(d) scheme is suitable to obtain a semiquantitative accuracy for the first hyperpolarizability of long polysilaacetylene oligomers. The conformation of the chain has a crucial impact on its second-order nonlinear optical properties. We also show that, for some chain lengths, the frequency dispersion effects may have a huge impact, even when far away from resonance. These phenomena are rationalized in terms of delocalization and asymmetry.
Interaction of benzene thiol and thiolate with small gold clusters
Authors: Letardi Sara , Cleri Fabrizio
Published: May 2004

We studied the interaction between benzene thiol and thiolate molecules, and gold clusters made of 1 to 3 atoms, by means of ab initio density functional theory in the local density approximation. We find that the thiolate is energetically more stable than the thiol, however the process of detachment of H from the thiol appears to be possibly mediated by the intermediate step of H chemisorption on Au. Cleavage of the S–H bond is accompanied by a 90° rotation of the molecule around the S–Au bond, showing a strong steric specificity. Such a rotation is induced by the relative energy shift of the S atom p orbitals with respect to the benzene π ring and the Au d orbitals. By analyzing the correlation of the bond energy, bond lengths, and HOMO–LUMO gap with the number of S–Au bonds, we find that the thiolate S atom appears to prefer a low-coordination condition on Au clusters.
The gas-phase chemiionization reaction between samarium and oxygen atoms: A theoretical study
Authors: Paulovič Jozef , Gagliardi Laura , Dyke John M. , Hirao Kimihiko
Published: May 2004

The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO+. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin–orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction.
Theoretical studies of the kinetics of methane hydrate crystallization in external electromagnetic fields
Authors: English Niall J. , MacElroy J. M. D.
Published: May 2004

Nonequilibrium molecular-dynamics (MD) simulations have been performed for the growth and dissolution of a spherical methane hydrate crystallite, surrounded by a saturated water–methane liquid phase, in both the absence and presence of external electromagnetic (e/m) fields in the microwave to far infrared range (5–7500 GHz) at root-mean square (rms) electric field intensities of up to 0.2 V/Å. A rigid/polarizable potential was used to model water and a rigid/nonpolarizable model was utilized for methane. In the absence of a field, it was found that the average growth rate of the crystallite was ∼0.32 water and 0.045 methane molecules per picosecond, evaluated over a 500 ps NPT simulation for three different initial geometries. Upon the application of an e/m field, it was found that no significant deviations from the zero-field crystal growth patterns were observed for rms electric field intensities of less than about 0.1 V/Å, regardless of the field frequency. At, and above, this “threshold” intensity, it was found that dissolution took place. The mobility of the molecules in the system was enhanced by the e/m field, to the greatest extent for frequencies of 50–100 GHz. Furthermore, it was observed that there was a systematic frequency variation in the pattern of dipole alignment with the external field and this led to marked differences in the rate of dissolution.
Coarse-grained controllability of wavepackets by free evolution and phase shifts
Authors: Shapiro E. A. , Ivanov Misha Yu. , Billig Yuly
Published: May 2004

We describe an approach to controlling wavepacket dynamics and a criterion of wavepacket controllability based on discretized properties of the wavepacket’s localization on the orbit. The notion of “coarse-grained control” and the coarse-grained description of the controllability in infinite-dimensional Hilbert spaces are introduced and studied using the mathematical apparatus of loop groups. We prove that 2D rotational wavepackets are controllable by only free evolution and phase kicks by AC Stark shift implemented at fractional revivals. This scheme works even if the AC Stark shifts can have only a smooth coordinate dependence, correspondent to the action of a linearly polarized laser field.
Infrared photodissociation spectroscopy of V+(CO2)n and V+(CO2)nAr complexes
Authors: Walker N. R. , Walters R. S. , Duncan M. A.
Published: May 2004

V + ( CO 2 ) n and V+(CO2)nAr complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected within a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced (IR-REPD) photodissociation spectroscopy. Photofragmentation proceeds exclusively through loss of intact CO2 molecules from V+(CO2)n complexes or by elimination of Ar from V+(CO2)nAr mixed complexes. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of free CO2 at 2349 cm−1. A linear geometry is confirmed for V+(CO2). Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO2, consistent with structures in which all ligands are bound directly to the metal ion. Fragmentation of the larger clusters terminates at the size of n=4, and a new vibrational band at 2350 cm−1 assigned to external ligands is observed for V+(CO2)5 and larger cluster sizes. These combined observations indicate that the coordination number for CO2 molecules around V+ is exactly four. Fourfold coordination contrasts with that seen in condensed phase complexes, where a coordination number of six is typical for V+. The spectra of larger complexes provide evidence for an intracluster insertion reaction that produces a metal oxide-carbonyl species.
Highly accurate evaluation of atomic three-electron integrals of lowest orders
Authors: Harris Frank E. , Frolov Alexei M. , Smith Vedene H.
Published: May 2004

Calculations of three-electron atomic systems in Hylleraas coordinates require integrals involving all the interparticle distances rij, which have usually been evaluated by introducing series expansions. For integrals with the smallest powers of rij these expansions do not converge at a satisfactory rate, leading some investigators to introduce convergence-acceleration procedures. This paper recommends the alternative of evaluating these integrals in closed form and presents stable explicit formulas for so doing. Some of the formulas are more compact versions of those in the literature; others have not been previously reported. It is also shown that finite-difference methods can be used with advantage to obtain additional low-order integrals. Sample integral values have been provided for test purposes.
Variation of the secondary relaxation strength in a binary glass former
Authors: Kasper Gernot , Reiser Andreas
Published: May 2004

We have performed dielectric permittivity measurements in the glass transition region of mixtures of meta-toluidine with aniline. The dielectric loss of the mixtures shows a secondary relaxation peak which increases with the logarithm of the number of aniline molecules. There is increasing evidence that the relaxation mechanism involves motions of clusters made of up to eight molecules.
Transition state dynamics of N2O4⇌2NO2 in liquid state
Authors: Katō Toshiko
Published: May 2004

Transition state dynamics of dissociation and association reactions N2O4⇌2NO2 in liquid state are studied by classical molecular dynamics simulations of reactive liquid NO2 at 298 K. An OSPP+LJ potential between NO2 molecules proposed in paper I [J. Chem. Phys. 115, 10852 (2001)], which takes into account the orientational sensitivity of the chemical bond, has been used in the simulation. The trajectory and energy evolution of various reactions are studied in the OSPP+LJ liquid, which reproduces both the observed liquid phase equilibrium constant and Raman band shape of the dissociation mode. It is found that a NO2 pair in reactive liquid NO2 is bound when ET<0 and dissociates when ET>0, and the dissociation of a reactant pair occurs when the transition state (TS) surface of ET=0 is crossed from negative to positive, where ET is the sum of the potential and kinetic energies of intermolecular motion of the pair. Two types of dissociation are found depending on the source of energy for dissociation; the first type D is the dissociation via collisional activation of the reactive mode by solvent molecules, and the second type T is the dissociation via bond transfer from a dimer to a monomer NO2 through the TS of NO2 trimer. It is concluded that the type T dissociation is found to be much more probable than the type D dissociation because of easy energy conservation. The reactant experiences the TS of NO2 trimer for a long time (1–10 ps) in NO2 mediated bond transfer reactions, and crossing and recrossing trajectories and dynamics in the TS neighborhood are studied.
Shear stress relaxation in liquids
Authors: Petravic Janka
Published: May 2004

We show that at high densities, as the system size decreases, liquid becomes able to permanently sustain increasing internal shear stress after a constant deformation, although the other characteristic liquid properties, such as the pair distribution function and diffusion coefficient do not change under strain. The system size necessary for observation of this effect increases with the decrease in temperature, and it is stronger in pair potentials with steeper repulsive part. We relate this result to the size of the “cooperatively rearranging regions” of the Adam–Gibbs theory of glass transition.
Measurement-assisted coherent control
Authors: Gong Jiangbin , Rice Stuart A.
Published: May 2004

Two advantageous roles of the influence of measurement on a system subject to coherent control are exposed using a five-level model system. In particular, a continuous measurement of the population in a branch state in the Kobrak–Rice extended stimulated Raman adiabatic passage scheme is shown to provide a powerful means for controlling the population transfer branching ratio between two degenerate target states. It is demonstrated that a measurement with a large strength may be used to completely shut off the yield of one target state and that the same measurement with a weak strength can dramatically enhance the robustness of the controlled branching ratio against dephasing.
A discrete time-dependent method for metastable atoms and molecules in intense fields
Authors: Peng Liang-You , McCann J. F. , Dundas Daniel , Taylor K. T. , Williams I. D.
Published: May 2004

The full-dimensional time-dependent Schrödinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium, the hydrogen atom and the hydrogen molecular ion are presented. At very high laser intensity, above the saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments.
Effect of anisotropic diffusion and external electric field on the rate of diffusion-controlled reactions
Authors: Traytak S. D. , Barzykin A. V. , Tachiya M.
Published: May 2004

In this paper we investigate theoretically the effect of an external electric field on the rate constant of steady-state bulk diffusion-controlled reactions. We generalize previously derived results for isotropic diffusion in the absence of interparticle interaction [J. Chem. Phys. 87, 4622 (1987)] to the case where translational diffusion is anisotropic. A frequently occurring situation of transverse isotropy where Dx=Dy≠Dz is considered in detail. We derive the first-order expansion for the reaction rate constant in terms of the electric field strength E, k(E)=k(0) (1+1/2εγ), where γ=k(0)/4πRD⊥, ε=qER/kBT, q is the charge, R is the contact distance, and D⊥ is the transverse diffusion coefficient. Numerical calculations show that this first-order expansion works well in the whole range of applicability of the Nernst–Einstein relation, i.e., for ε<1.
Negative contributions in the velocity correlation function of supercooled liquid water
Authors: De Santis Alberto , Ercoli Alessandro , Rocca Dario
Published: May 2004

The translational dynamics of supercooled and normal liquid water is investigated via a specific correlation function ΔB with the aim of explaining the behavior of the centers of mass velocity correlation function (VCF). ΔB is divided into diffusive and nondiffusive parts that yield separated contributions to the VCF, namely an Enskog-type diffusive one, modeled by an exponential function, and a nondiffusive one, made up by damped oscillations of a vanishing time integral. In the translational density of states (DOS), the oscillations yield the bands at ω1≅50 cm−1, ω3≅240 cm−1 (the two well-known experimental bands of the Raman spectra) and ω2≅160 cm−1 (the Einstein frequency of the liquid). It is shown that the chief negative lobe of the VCF is mainly due to the DOS component at the lowest frequency ω1. The study of the relative pair dynamics shows that this lobe is due to the transverse dynamics, while the longitudinal one determines the fast DOS component at ω3. The presence of a negative tail is highlighted. Its contribution extends beyond the region of the fast dynamics (t<0.7 ps) up to about 1.5 ps and is due to a low-frequency oscillating mode that produces a low-frequency DOS band centered at about ω0=20 cm−1.
Infrared spectra of X−⋅CO2⋅Ar cluster anions (X=Cl,Br,I)
Authors: Weber J. Mathias , Schneider H.
Published: May 2004

Ion–molecule clusters of the heavier halide anions X−⋅CO2 (X=Cl−,Br−,I−) with CO2 have been studied by gas phase infrared photodissociation spectroscopy, using Ar evaporation from the complexes X−⋅CO2⋅Ar upon infrared excitation. We observe that the asymmetric stretch vibrational mode of the CO2 molecule is red-shifted from the frequency of free CO2, with the red-shift increasing toward the lighter halide ions. A similar trend is repeated in the region of the Fermi resonance of the combination bands of the asymmetric stretch vibration with two quanta of the bending vibration and the symmetric stretch vibration. We discuss our findings in the framework of ab initio and density functional theory calculations.
Phase coexistence in polydisperse charged hard-sphere fluids: Mean spherical approximation
Authors: Kalyuzhnyi Yurij V. , Kahl Gerhard , Cummings Peter T.
Published: May 2004

Taking advantage of the availability of the analytic solution of the mean spherical approximation for a mixture of charged hard spheres with an arbitrary number of components we show that the polydisperse fluid mixture of charged hard spheres belongs to the class of truncatable free energy models, i.e., to those systems where the thermodynamic properties can be represented by a finite number of (generalized) moments of the distribution function that characterizes the mixture. Thus, the formally infinitely many equations that determine the parameters of the two coexisting phases can be mapped onto a system of coupled nonlinear equations in these moments. We present the formalism and demonstrate the power of this approach for two systems; we calculate the full phase diagram in terms of cloud and shadow curves as well as binodals and discuss the distribution functions of the coexisting daughter phases and their charge distributions.
Spinodal and ideal glass limits for metastable liquids
Authors: Speedy Robin J.
Published: May 2004

A spinodal line provides a high-temperature limit to the stability of a superheated liquid and a line of ideal glass transitions may provide a low-temperature limit for a supercooled liquid. For models in which the thermodynamic properties depend on only one independent external variable the line of ideal glass transition can be viewed as a second-order transition between two equilibrated phases and it meets the spinodal with the same slope at the maximum tension a stretched liquid can sustain. For real materials there are two independent external variables, temperature and pressure; the line of ideal glass transitions cannot be viewed as a second-order phase transition and the two lines need not have the same slope where they meet.
The outer valance orbital electron densities of cyclopentane by binary (e,2e) spectroscopy
Authors: Deng J. K. , Li G. Q. , Wang F. , Su G. L. , Ning C. G. , Zhang T. , Ren X. G. , Wang Y. , Zheng Y.
Published: May 2004

The binding energy spectra and electron distributions in momentum space of the valence orbitals of cyclopentane (C5H10) are studied by Electron Momentum Spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 1.2 eV. The experimental momentum profiles of the outer valence orbitals are compared with the theoretical momentum distributions calculated using Hartree–Fock and density functional theory (DFT) methods. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree–Fock and DFT calculations when the large and diffuse basis sets are used.
Adsorption and vibrational spectroscopy of ammonia at mordenite: Ab initio study
Authors: Bučko T. , Hafner J. , Benco L.
Published: May 2004

The adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving Brønsted acid (BA) sites, terminal silanol groups, and Lewis sites has been investigated using periodic ab initio density-functional theory. It is shown that ammonia forms an ammonium ion when adsorbed at strong BA sites. The calculated adsorption energies for different BA sites vary in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The lowest adsorption energy is found for a monodentate complex in the main channel, the highest for a tetradentate configuration in the side pocket. At weak BA sites such as terminal silanol groups or a defect with a BA site in a two-membered ring ammonia is H bonded via the N atom. Additional weak H bonds are formed between H atoms of ammonia and O atoms of neighboring terminal silanol groups. The calculated adsorption energies for such adducts range between 61.7 and 70.9 kJ/mol. The interaction of ammonia with different Lewis sites is shown to range between weak (ΔEads=17.8 kJ/mol) and very strong (ΔEads=161.7 kJ/mol), the strongest Lewis site being a tricoordinated Al atom at the outer surface. Our results are in very good agreement with the distribution of desorption energies estimated from temperature-programmed desorption (TPD) and microcalorimetry experiments, the multipeaked structure of the TPD spectra is shown to arise from strong and weak Brønsted and Lewis sites. The vibrational properties of the adsorption complexes are investigated using a force-constant approach. The stretching and bending modes of NH4+ adsorbed to the zeolite are strongly influenced by the local environment. The strongest redshift is calculated for the asymmetric stretching mode involving the NH group hydrogen bonded to the bridging O atom of the BA site, the shift is largest for a monodentate and smallest for a tetradentate adsorption complex. The reduced symmetry of the adsorbate also leads to a substantial splitting of the stretching and bending modes. In agreement with experiment we show that the main vibrational feature which differentiates coordinatively bonded ammonia from a hydrogen-bonded ammonium ion is the absence of bending modes above 1630 cm−1 and in the region between 1260 and 1600 cm−1, and a low-frequency bending band in the range from 1130 to 1260 cm−1. The calculated distribution of vibrational frequencies agrees very well with the measured infrared adsorption spectra. From the comparison of the adsorption data and the vibrational spectra we conclude that due to the complex adsorption geometry the redshift of the asymmetric stretching is a better measure of the acidity of an active sites than the adsorption energy.
Relation between pressure shift and electric-field shift of single-molecule lines in a polymer glass
Authors: Bauer M. , Kador L.
Published: May 2004

The pressure shifts and the electric-field shifts of individual chromophores in an amporphous matrix are—due to strong disorder—subject to broad distributions. By means of single-molecule spectroscopy we measured both the pressure and the electric-field shift of about 800 tetra-tert-butylterrylene molecules in polyisobutylene. We found a significant correlation of 0.52 (Kendall’s correlation coefficient) between the two observables. Analytical calculations and Monte Carlo simulations based on a model by Laird and Skinner predict a nonzero, yet, distinctly smaller correlation. The Monte Carlo simulations showed that the usual assumptions of a spherical shape and isotropic polarizability of the chromophores in glassy systems is an oversimplification of the complex nanoscopic structure and cannot reproduce our experimental results. By taking the molecular anisotropy into account, we obtain agreement of the simulated and the measured correlation between pressure shift and electric-field shift parameter.
Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals
Authors: Kitchin J. R. , Nørskov J. K. , Barteau M. A. , Chen J. G.
Published: May 2004

The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.
Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite
Authors: van den Berg A. W. C. , Bromley S. T. , Flikkema E. , Wojdel J. , Maschmeyer Th. , Jansen J. C.
Published: May 2004

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700–1200 K, the diffusion coefficient is found to range from 1.6⋅10−10 to 1.8⋅10−9 m2/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.
Physisorption of molecular oxygen on C60 thin films
Authors: Niklowitz P. G. , Li Z. Y. , Jardine A. P. , Luo M. F. , Allison W.
Published: May 2004

The interaction of oxygen molecules with a fullerene surface has been studied using high resolution electron energy loss spectroscopy and temperature programmed desorption. Vibrational excitation of the adsorbed oxygen is observed at 190 meV, an energy value comparable with that for molecular oxygen in the gas phase. We take this to indicate physisorption of molecular oxygen on the C60 surface. Thermal desorption results also show that the bonding of oxygen molecules to the C60 overlayer is comparable to that on a graphite surface. A detailed study of the energy dependence of the vibrational excitation reveals an inelastic electron resonance scattering process. The angular dependence of the resonant vibrational excitation exhibits features distinctively different from those for molecular oxygen physisorbed on the related graphite surface, at a comparable coverage. One possible reason is that the corrugated surface potential, due to the curvature of the C60 molecules, promotes the preferential ordering of the physisorbed oxygen molecules perpendicular to the surface plane of the C60 overlayer.
Preparation and photoelectron spectrum of the glycine molecular anion: Assignment to a dipole-bound electron species with a high-dipole moment, non-zwitterionic form of the neutral core
Authors: Diken Eric G. , Hammer Nathan I. , Johnson Mark A.
Published: May 2004

We report the gas-phase preparation of negatively charged glycine as well as the Gly⋅(H2O)1,2− complexes by entrainment of the neutral precursor into an ionized supersonic expansion tuned to optimize the (H2O)n−⋅Arm clusters. The photoelectron spectrum of Gly− displays the signature of a dipole-bound species, with sufficient vibrational fine structure to characterize the core neutral as a higher energy, non-zwitterionic isomer of the amino acid.