Short-time maximum entropy method analysis of molecular dynamics simulation: Unimolecular decomposition of formic acid

We performed spectral analysis by using the maximum entropy method instead of the traditional Fourier transform technique to investigate the short-time behavior in molecular systems, such as the energy transfer between vibrational modes and chemical reactions. This procedure was applied to direct ab initio molecular dynamics calculations for the decomposition of formic acid. More reactive trajectories of dehydrolation than those of decarboxylation were obtained for Z-formic acid, which was consistent with the prediction of previous theoretical and experimental studies. Short-time maximum entropy method analyses were performed for typical reactive and non-reactive trajectories. Spectrograms of a reactive trajectory were obtained; these clearly showed the reactant, transient, and product regions, especially for the dehydrolation path.     

An ab initio quasi-diabatic potential energy matrix for OH(2Σ) + H2

A diabatic potential energy matrix for three electronic states of OH(3) has been constructed by interpolation of multi-reference configuration interaction electronic structure data. The reactive, exchange and non-reactive quenching dynamics are investigated using surface hopping classical trajectories. Classical trajectory simulations show good agreement with cross molecular beam data for the OH((2)Σ) + D(2) → HOD + D reaction.     

Impulsive inelastic scattering of O+(4S) by isotopic hydrogen molecules

We present experimental evidence that at relative energies above 10 eV the non-reactive inelastic scattering of O⁺ by H₂, D₂, and HD arises from impulsive elastic scattering of 0⁺ by individual H or D atoms. The relation of this impulsive non-reactive scattering to the reactive scattering from these systems is briefly discussed.     

Quasiperiodic motion in collinear HeH2+

The classical motion of the highly vibrationally excited HeH₂⁺ collinear complex has been investigated. An estimated of the fraction of phase containing quasiperiodic trajectories versus total energy is reported. At some energies it is found that dissociative and quasiperiodic trajectories occupy disjoint regions of phase space.     

A discontinuous Galerkin method to solve chromatographic models

This article proposes a discontinuous Galerkin method for solving model equations describing isothermal non-reactive and reactive chromatography. The models contain a system of convection-diffusion-reaction partial differential equations with dominated convective terms. The suggested method has capability to capture sharp discontinuities and narrow peaks of the elution profiles. The accuracy of the method can be improved by introducing additional nodes in the same solution element and, hence, avoids the expansion of mesh stencils normally encountered in the high order finite volume schemes. Thus, the method can be uniformly applied up to boundary cells without loosing accuracy. The method is robust and well suited for large-scale time-dependent simulations of chromatographic processes where accuracy is highly demanding. Several test problems of isothermal non-reactive and reactive chromatographic processes are presented. The results of the current method are validated against flux-limiting finite volume schemes. The numerical results verify the efficiency and accuracy of the investigated method. The proposed scheme gives more resolved solutions than the high resolution finite volume schemes.     

Mechanical and morphological investigations of reactive polysulfone toughened epoxy networks

The diglycidyl ether of bisphenol-A (DGEBA) epoxy resin was toughened by aminophenyl functional reactive polyethersulfones (R-PES) or by t-butyl terminated non-reactive polyethersulfones (T-PES). The molecular weights of PES were controlled to afford 5,000 to 20,000 g/mole and loadings were also varied from 5 to 30 wt.%. Epoxy networks cured with 4, 4'-diaminodiphenylsulfone (DDS) were subjected to Tg determinations, plane strain fracture toughness (K_1C) measurements, chemical resistance tests and morphological studies by SEM. Very significantly improved K_1C fracture toughness was obtained with reactive PES toughening without loss of chemical resistance, while non-reactive PES blended epoxy resins exhibited only slightly improved fracture toughness but poor chemical resistance. It was possible to load up to 30 wt.% of PES without utilizing solvent and the maximum K_1C fracture toughness with R-PES was around 2.2 MPa-m 0.5, which was equivalent to the neat thermoplastic resin. Ductile fracture of the PES phase is suggested as a major toughening mechanism and this is highly dependent of the excellent adhesion developed between the PES and epoxy phases due to the chemical bonds. The systems demonstrated that chemical resistance of thermosets can be combined with the tough characteristics of thermoplastics.     

Reactive bands and rebounding trajectories in collinear F + H2

Bands of reactive and unreactive trajectories have been mapped for classical collinear F + H₂ (υ = 0). The edges of the band are characterized by trajectories which undergo several reflections in the corner of the potential energy surface. Multiple reflections are seen to lead to less sharply defined band edges.     

Quasiclassical trajectory study of fast H-atom collisions with acetylene

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive chemical H-atom exchange, also via complex formation. The details of these outcomes are presented and correlated with the collision lifetime. Large energy transfer is found via complex formation, which can subsequently decay back to reactants, a non-reactive event, or to new products, a reactive event. For the present system, these two events are experimentally indistinguishable.     

NON-REACTIVE ORIENTATIONS OF MOLECULES AT SURFACES

The role of the orientation of a molecule in its interaction with a surface is examined for the specific case of NO interaction with Pt(111). For this system molecular chemisorption occurs, mediated by a strong chemisorption well. Experimental results concerning sticking, angular distributions of scattered molecules, steric effects in scattering, and rotational excitation will be presented. Classical trajectory calculations using a model potential energy hypersurface can reproduce most experimental findings. Analysis of the trajectories shows that there is a strong orientation dependence of rotational excitation and sticking. The O-end of the molecule turns out to be non-reactive. The N-end of the molecule is very reactive. Its behaviour can almost be described using statistical methods.     

An ab initio molecular dynamics study on the dissociative recombination reaction of HD2O+ + e−

An ab initio molecular dynamics simulations have been carried out for the dissociative recombination reaction of the deuterium-substituted hydronium cation, HD₂O⁺ + e ⁻, at the state-averaged multiconfigurational self-consistent field level. In the present simulations, five electronic states of HD₂O were included explicitly, and nonadiabatic transitions among adiabatic electronic states were taken into account by the Tully’s fewest switches algorithm. It is shown that the dominant products, OD + D + H, were generated in 63% of trajectories, while the products, OH + 2D, were generated in only 11% of trajectories, indicating that the release of a light fragment H is favored over the release of a heavy fragment D. This result is in conformity with the observation that there is a larger amount of deuterium substituted species than the non-substituted species in the interstellar space. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Theoretical study on activation mechanism of fluorine substitution reactions of Keggin-MAl12 in aqueous solutions

The fluorine substitution reactions of Keggin polymeric aluminum species K-MAl₁₂ (M = Al, Ga, Ge) were investigated using density functional theory. Two substitution pathways (associative and dissociative) were simulated. The results show that the replacement of charged tetrahedral center metals causes the change in energy barrier either for associative mechanism or dissociative mechanism. The preferred activation mechanisms are proposed by comparing associative and dissociative barriers for fluorine substitution reactions of three Keggin Al species. The fluorine substitution reactions of Al₁₃ and GaAl₁₂ are inclined to dissociative mechanism and GeAl₁₂ follows an associative mode, indicating a mechanistic variation induced by the alteration of tetrahedral metals.     

Reactive blending of poly(ethylene terephthalate) with ethylene-propylene rubber

A gram-scale miniature reactor designed for reactive blendind was introduced. A brief review was given on the light scattering method for morphology characterization and the ellipsometric analysis on polymer-polymer interface. These methods were applied for the reactive blending of poly(ethylene terephthalate) with ethylene-propylene rubber (70/30 PET/EPR). Compared with non-reactive system, the reactive one yielded finer dispersion of rubber particles. It showed better stability of particle dispersion during static annealing. The reactive system with thick interface showed higher impact strength than the non-reactive one having same particle size but thinner interface, suggesting that the thick interface may be a prerequisite to the rubber toughening.     

Ab initio calculations to the reactions of HF and HCl with Si(OH)4 and (HO)3SiOSi(OH)3: Modeling of SiO2 etching reactions

The local mechanisms of dissociative chemisorption and reactive etching are investigated by molecular models modeling ?SiOH groups and ?SiOSi? bridges at SiO₂ surfaces. The elementary reaction paths for the reactions of HF and HCl with Si? O bonds of the models, Si(OH)₄ and (HO)₃SiOSi(OH)₃, are calculated by the Hartree–Fock method. The local transition-state structures are nearly plane quadrangles with neighboring pentacoordinated silicon and tricoordinated oxygen atoms. For the reaction and activation energies, the electron correlation is considered by the Møller–Plesset perturbation theory. The results agree with experimental findings. © 1994 John Wiley & Sons, Inc.     

State(R)-state(TS)-state(P) theoretical study of H + H<Subscript>2</Subscript> system

The calculation of H + H₂ system by symplectic quasiclassical trajectory (SQCT) shows that there are two types of collision trajectories A and B, i.e., type A trajectory passes the saddle point of transition state (TS), whereas type B trajectory does not pass the saddle point of transition state. Not all the reactants of type A trajectory are reactive, while not all of type B trajectory are nonreactive. The partition and reactivity of these two types of trajectories are affected by reactant state(R), furthermore, the types of trajectories affect the state and angle distributions of products. Not only the rudiment framework for theoretical study on state(R)-state(TS)-state(P) is established, but also the further understanding of transition state theory (TST) of Eyring is investigated in this paper.     

Two-Temperature Two-Dimensional Non Chemical Equilibrium Modeling of Ar–CO<Subscript>2</Subscript>–H<Subscript>2</Subscript> Induction Thermal Plasmas at Atmospheric Pressure

Here the authors developed a two-dimensional two-temperature chemical non-equilibrium (2T-NCE) model of Ar–CO₂–H₂ inductively coupled thermal plasmas (ICTP) around atmospheric pressure (760 torr). Assuming 22 different particles in this model and by solving mass conservation equations for each particle, considering diffusion, convection and net production terms resulting from 198 chemical reactions, chemical non-equilibrium effects were taken into account. Species density of each particle or simply particle composition was also derived from the mass conservation equation of each one taking the non chemical equilibrium effect into account. Transport and thermodynamic properties of Ar–CO₂–H₂ thermal plasmas were self-consistently calculated using the first order approximation of the Chapman–Enskog method at each iteration point implementing the local particle composition and temperature. Calculations at reduced pressure (500 and 300 torr) were also done to investigate the effect of pressure on non-equilibrium condition. Results obtained by the present model were compared with results from one temperature chemical equilibrium (1T-CE) model, one-temperature chemically non equilibrium (1T-NCE) model and finally with 2T-NCE model of Ar–N₂–H₂ plasmas. Investigation shows that consideration of non-chemical equilibrium causes the plasma volume radially wider than CE model due to particle diffusion. At low pressure with same input power, presence of diffusion is relatively stronger than at high pressure. Comparison of present reactive model with non-reactive Ar–N₂–H₂ plasmas shows that maximum temperature reaches higher in reactive C–H–O molecular system than non-reactive plasmas due to extra contribution of reaction heat.     

Novel hybrid systems based on poly(propylene-g-maleic anhydride) and Ti-POSS by direct reactive blending

Novel hybrid systems based on maleic anhydride-grafted polypropylene (PPgMA) and home-made Ti-containing amino polyhedral oligomeric silsesquioxanes (Ti-POSS-NH₂) have been prepared by one-step reactive blending, and their properties have been compared with those of systems based on a non-reactive POSS (POSS). The occurrence of a reaction between PPgMA and the reactive POSS molecules has been assessed by Fourier Transform Infrared Resonance (FTIR) measurements, whereas dispersion of POSS into the polymer was evaluated by Scanning Electron Microscopy (SEM), showing a nanometric dispersion only for the reactive POSS. Thermo-oxidative behaviour was studied by Thermogravimetric Analysis (TGA), showing a delayed volatilization of the PPgMA/Ti-POSS-NH₂ with respect to both PPgMA/POSS and pristine PPgMA, which is attributed to the chemical activity of Ti in Ti-POSS-NH₂. To highlight the mechanism of the hybrid system decomposition, samples which underwent a thermal treatment at 250 °C, i.e. the onset temperature for polymer matrix decomposition in thermo-oxidative conditions, have been studied by FTIR and X-Ray Photoelectron Spectroscopy (XPS) measurements.     

Interpretation of collision induced dissociative charge-transfer processes in rare-gas molecule ions

We propose an interpretation of experimental measurements of dissociative charge-transfer processes X₂⁺+Y→X+X+Y⁺ (X=rare-gas atom) in terms of avoided-crossings of adiabatic potential surfaces. Model potential energy surfaces for a typical system (X=He, Y=Ne) are computed by the method of diatomics-in-molecules (DIM). The qualitative shapes of the surfaces suggest dynamical simplifications which can be embodied in a classical-mechanical trajectory model with “surface-hopping”. Analogy with earlier surface-hopping trajectory calculations and with trajectories for endothermic ion-molecule reactons provides a basis for understanding some of the major experimental findings for the He₂⁺-Ne reaction. The model viewpoint is also able to rationalize the observance (or non-observance) of other rare-gas reactions and can be extended to the case where Y=N₂, X=He.     

The chemisorption of hydrogen on Cu(111): A dynamical study

Ab initio band-structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top, bridge, and threefold sites, so that the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H₂ molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The results show that (a) when H₂ is in the ground vibrational state the dissociative adsorption probability P_a increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.     

A comparative study of particle swarm optimization and its variants for phase stability and equilibrium calculations in multicomponent reactive and non-reactive systems

Particle swarm optimization is a novel evolutionary stochastic global optimization method that has gained popularity in the chemical engineering community. This optimization strategy has been successfully used for several applications including thermodynamic calculations. To the best of our knowledge, the performance of PSO in phase stability and equilibrium calculations for both multicomponent reactive and non-reactive mixtures has not yet been reported. This study introduces the application of particle swarm optimization and several of its variants for solving phase stability and equilibrium problems in multicomponent systems with or without chemical equilibrium. The reliability and efficiency of a number of particle swarm optimization algorithms are tested and compared using multicomponent systems with vapor–liquid and liquid–liquid equilibrium. Our results indicate that the classical particle swarm optimization with constant cognitive and social parameters is a reliable method and offers the best performance for global minimization of the tangent plane distance function and the Gibbs energy function in both reactive and non-reactive systems.     

Influence of Reactive Compatibilization on the Morphology of Polypropylene/Polystyrene Blends

To study the efficiency of different mechanisms for reactive compatibilization of polypropylene/polystyrene blends (PP/PS blends), main chain or terminal-functionalized PP and terminal-functionalized PS have been synthesized by different methods. While the in-situ block and graft copolymer formation results in finer phase morphologies compared to the corresponding non-reactive blends, the morphology development in the ternary blend system PP/PS + HBP (hyperbranched polymer) is a very complex process. HBP with carboxylic acid end groups reacts preferably with the reactive sites of the oxazoline functionalized PS (PS-Ox) and locates mainly within the dispersed PS-Ox phase. A bimodal size distribution of the PS-Ox particles within the oxazoline modified PP (PP-Ox) matrix phase is observed with big PS-Ox particles (containing the HBP as dispersed phase) and small PS-Ox particles similar in size to the unimodal distributed particles in the non-reactive PP-Ox/PS-Ox blends. Factors influencing the morphology are discussed.