On the semiclassical initial value calculation of thermal rate coefficients for the N+N2 reaction

In this paper we compare semiclassical initial value representation, conventional transition state theory with Wigner and Eckart tunneling correction, quantum reduced dimensionality, and quasiclassical thermal rate coefficients for N+N(2) exchange reaction.     

On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients

Rate coefficients k(T) for dissociative electron attachment (DEA) to molecules in many cases exhibit a more or less strong rise with increasing temperature T (the electron temperature T(e) and the molecular temperature T(G) are assumed to be in thermal equilibrium, i.e., T = T(e) = T(G)). This rise is frequently modeled by the Arrhenius equation k(T) = k(A) exp[-E(a)∕(k(B)T)], and an activation energy E(a) is deduced from fits to the experimental data k(T). This behavior reflects the presence of an energy barrier for the anion on its path to the dissociated products. In a recent paper [J. Kopyra, J. Wnorowska, M. Forys?, and I. Szamrej, Int. J. Mass Spectrom. 268, 60 (2007)] it was suggested that the size of the rate coefficients for DEA reactions at room temperature exhibits an exponential dependence on the activation energy, i.e., k(E(a); T ≈ 300 K) = k(1) exp[-E(a)∕E(0)]. More recent experimental data for molecules with high barriers [T. M. Miller, J. F. Friedman, L. C. Schaffer, and A. A. Viggiano, J. Chem. Phys. 131, 084302 (2009)] are compatible with such a correlation. We investigate the validity and the possible origin of this dependence by analyzing the results of R-matrix calculations for temperature-dependent rate coefficients of exothermic DEA processes with intermediate barrier toward dissociation. These include results for model systems with systematically varied barrier height as well as results of molecule-specific calculations for CH(3)Cl, CH(3)Br, CF(3)Cl, and CH(2)Cl(2) (activation energies above 0.2 eV) involving appropriate molecular parameters. A comparison of the experimental and theoretical results for the considered class of molecules (halogenated alkanes) supports the idea that the exponential dependence of k(T = 300 K) on the activation energy reflects a general phenomenon associated with Franck-Condon factors for getting from the initial neutral vibrational levels to the dissociating final anion state in a direct DEA process. Cases are discussed for which the proposed relation does not apply.     

On the accuracy of non-equilibrium transport coefficients calculation

The algorithms of non-equilibrium transport coefficients calculation in reacting gas mixtures are discussed. The influence of the molecular interaction potential on the transport properties is estimated in the various temperature ranges.     

The rate coefficients for several ternary association reactions at thermal energies

The rate coefficients k for the ternary association reactions of CH₃⁺ and CD₃⁺ with H₂, N₂, O₂ CO and CO₂ N₂⁺ with N₂, and C⁺ with H₂ and D₂ have been measured within the temperature range 80–520 K in helium buffer gas. In every case, k ∝ AT^?n and the magnitudes of k are greater when the deuterated species are involved.     

Dynamically based fitting law for thermal rate coefficients in rotational relaxation

An earlier analytical, approximative result for the semi-classical, sudden limit of energy dependent j_o → j cross sections of rotational relaxation of homonuclear, diatomic molecules perturbed by an atom has been integrated to obtain dynamical fitting of temperature dependent rate coefficients. The result can be written by using two parameters, k_jjo = [(2j + 1)/(2j_o + 1)] ^1/2a (lj - 1]^?1 ? b), where the parameters a and b are given from the assumed intermolecular potential, The reduced mass for the collisions and the temperature. A comparison with several experimental results proves the validity of the above expression and gives some statements about the intermolecular potentials for the systems considered.     

Time-dependent approaches for the calculation of intersystem crossing rates

We present three formulas for calculating intersystem crossing rates in the Condon approximation to the golden rule by means of a time-dependent approach: an expression using the full time correlation function which is exact for harmonic oscillators, a second-order cumulant expansion, and a short-time approximation of this expression. While the exact expression and the cumulant expansion require numerical integration of the time correlation function, the integration of the short-time expansion can be performed analytically. To ensure convergence in the presence of large oscillations of the correlation function, we use a Gaussian damping function. The strengths and weaknesses of these approaches as well as the dependence of the results on the choice of the technical parameters of the time integration are assessed on four test examples, i.e., the nonradiative S(1) ? T(1) transitions in thymine, phenalenone, flavone, and porphyrin. The obtained rate constants are compared with previous results of a time-independent approach. Very good agreement between the literature values and the integrals over the full time correlation functions are observed. Furthermore, the comparison suggests that the cumulant expansion approximates the exact expression very well while allowing the interval of the time integration to be significantly shorter. In cases with sufficiently high vibrational density of states also the short-time approximation yields rates in good agreement with the results of the exact formula. A great advantage of the time-dependent approach over the time-independent approach is its excellent computational efficiency making it the method of choice in cases of large energy gaps, large numbers of normal modes, and high densities of final vibrational states.     

Application of the GCA to a series of curve crossing topologies

The generator coordinate approximation (GCA ) for the calculation of spectra of diatomics is briefly reviewed. Results of this method applied to a series of curve crossings are presented. In all cases, the generator coordinate method performs significantly better than does the adiabatic approximation. When the energy gap becomes smaller, the GCA results improve. In the crossing limit, the GCA becomes exact. © 1994 John Wiley & Sons, Inc.     

On the validity of the Arrhenius equation for electron attachment rate coefficients

The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.     

Dynamical properties and thermal rate coefficients for the Na + HF reaction using genetic algorithm

The genetic algorithm optimization technique (GAOT) was used to build a new potential energy surface (PES) to the Na + HF → NaF + H reaction. Quasi‐Classical Trajectories and Transition State Theory methods were used to obtain the dynamical properties and thermal rate coefficients (TRCs), respectively, of this new PES. These features were compared with the dynamical properties and TRCs available in the literature. It was found that the GAOT PES agrees very well with other PESs, in which the maximum difference found is smaller than 1.0 Ų for the cross‐sections. These results endow the GAOT approach as a method to build PESs of reactive scattering processes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

乙醇-环己烷体系中NaI活度系数的测定与计算

由于非水溶剂的应用日趋广泛,促使物理化学工作者对于溶剂和溶液性质进行研究,同时开展了有关物理化学数据的测定和积累工作,以利于指导实践[1]。目前文献报道含水混合溶剂中的电解质活度系数较多,但非水混合溶剂中电解质活度系数报道较少。电解质浓溶液活度系数的计算方法目前应用较广的是Pitzer半经验算法[2]。Pitzer的算法中具体物系的β(0)、β(1)等系数需从实测活度系数数据拟合求得。孙仁义等通过测定汽液平衡盐效应的方法研究了盐在混合溶剂中的活度系数[3-4]。最近孙仁义等[5]提出了双液比固定条件下含盐体系汽液平衡数据热力学一…     

Time-dependent observables and diabatic curve crossing: a model study

We study the diabatic crossing of two electronic surfaces from the time-dependent point of view, looking at the time and frequency-resolved spontaneous emission spectra and to the total population of the excited state, which for a pump and-probe experiment with two δ-like pulses, is proportional to the total emission. The results of some computations on a model case are presented and discussed.     

On the curvature in logarithmic plots of rate coefficients for chemical reactions

In terms of the reduced potential energy barrier ζ = ΔuTS/kT, the rate coefficients for chemical reactions are usually expressed as proportional to e-ζ. The coupling between vibrational modes of the medium to the reaction coordinate leads to a proportionality of the regularized gamma function of Euler Q(a,ζ) = Γ(a,ζ)/Γ(a), with a being the number of modes coupled to the reaction coordinate. In this work, the experimental rate coefficients at various temperatures for several chemical reactions were fitted to the theoretical expression in terms of Q(a,ζ) to determine the extent of its validity and generality. The new expression affords lower deviations from the experimental points in 29 cases out of 38 and it accounts for the curvature in the logarithmic plots of rate coefficients versus inverse temperature. In the absence of tunneling, conventional theories predict the curvature of these plots to be identically zero.     

An equilibrium calculation of the thermal transport coefficients between two planes of arbitrary separation in a condensed phase

We present a method for the direct calculation at equilibrium of the shear viscosity and thermal conductivity over distances as short as one molecular diameter. The method is directly applicable to the calculation of viscosity and thermal conductivity in inhomogeneities such as the interface between coexisting phases. The method makes use of a novel extension of our recently developed boundary fluctuation theory.     

On the maximum reaction rate and inflexion points on the DTG curve

The authors present some theoretical considerations concerning the influence of the form of the conversion functionf() on the values of the degree of conversion corresponding to the maximum value of the reaction rate (_max) as well as on the inflexion points (_inf) of the DTG curve. The obtained equations are characterized by a general validity no matter the form off().

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Zusammenfassung Es werden einige theoretische Überlegungen bezüglich des Einflusses der Form der Konversionsfunktionf() auf die Werte der Konversionsrate bei maximalen Reaktionsgeschwindigkeiten (_max) sowie auf Inflexionspunkte () der DTG-Kurve angestellt. Die erhaltenen Gleichungen sind unabhängig von der Form vonf() durch eine all-gemeine Gültigkeit charakterisiert.

     

On measurement of thermal diffusion coefficients in multicomponent mixtures

We investigate the steady-state separation of the individual components of an incompressible multicomponent liquid mixture in a narrow two-dimensional thermogravitational column. Analytic working equations for measuring thermal diffusion coefficients analogous to the existing equations for a binary mixture are derived. Similar to the binary results, we find that when compositional variation has negligible effect on fluid density and vertical diffusive flux can be ignored, molecular diffusion does not affect steady-state separation. However, when compositional effects on density are taken into account, molecular diffusion does affect the bulk convective flow and the steady-state separation of the components. There may be also two distinct trends in the velocity and separation profiles. With one or more negative thermal diffusion coefficients, there may be more than one convection cell resulting in oscillatory behavior of separation. The working equations presented can be used to measure thermal diffusion coefficients of multicomponent mixtures. Such measurements have not yet been reported in the literature.     

On measurement of molecular and thermal diffusion coefficients in multicomponent mixtures

We have developed the theory for using the deflection of laser beams of various wavelengths to determine molecular and thermal diffusion coefficients in multicomponent mixtures. In the past, simultaneous determination of molecular and thermal diffusion coefficients have only been achieved for binaries. Our procedure is faster and more accurate than the current techniques to determine molecular diffusion coefficients. The analysis for an N-component mixture requires deflections from (N - 1) beams of different wavelengths. We show in an example that the molecular and thermal diffusion coefficients can be determined reliably in a ternary mixture.     

Relativistic DMRG calculations on the curve crossing of cesium hydride

Over the past few years, it has been shown in various studies on small molecules with only a few electrons that the density-matrix renormalization group (DMRG) method converges to results close to the full configuration-interaction limit for the total electronic energy. In order to test the capabilities of the method for molecules with complex electronic structures, we performed a study on the potential-energy curves of the ground state and the first excited state of 1sigma+ symmetry of the cesium hydride molecule. For cesium relativistic effects cannot be neglected, therefore we have used the generalized arbitrary-order Douglas-Kroll-Hess protocol up to tenth order, which allows for a complete decoupling of the Dirac Hamiltonian. Scalar-relativistic effects are thus fully incorporated in the calculations. The potential curves of the cesium hydride molecule feature an avoided crossing between the ground state and the first excited state, which is shown to be very well described by the DMRG method. Compared to multireference configuration-interaction results, the potential curves hardly differ in shape, for both the ground state and the excited state, but the total energies from the DMRG calculations are in general consistently lower. However, the DMRG energies are as accurate as corresponding coupled cluster energies at the equilibrium distance, but convergence to the full configuration-interaction limit is not achieved.     

Freezing point and heating curve behavior for an organic reciprocal salt pair system

Binary and quasi-binary freezing point data have been obtained to establish the freezing point diagram for the ternary reciprocal salt pair system involving cyclohexylamine stearate, cyclohexylamine palmitate, 2,2'-dipyridylamine stearate, and 2,2'-dipyridylamine palmitate. The compositions and primary freezing points of the ternary peritectic and eutectic mixtures have been determined Apparent anomalies in the heating curves for selected compositions have been explained on the basis of the existence of metastable equilibria.     

On the calculation of diffusion coefficients and aggregation numbers of nonionic surfactants in micellar solutions

The calculation of the diffusion coefficients of nonionic surfactants as functions of their concentrations in micellar solutions has been analyzed within the framework of the quasi-chemical version of the law of mass action. The methods of the introduction of initial calculation parameters, calculation scheme for an ideal mixture of monomeric molecules and micelles, and corrections for varying solution viscosity have been considered. Numerical estimations have been performed using aqueous tetraoxyethylene octyl ether, pentaoxyethylene hexyl ether, and octyl-β-D-glucopyranoside solutions as examples.     

The O + O2 reaction: quantum detailed probabilities and thermal rate coefficients

The detailed quantum probabilities of the O + O₂ reactive system have been computed at zero total angular momentum using the time-independent quantum program ABC thanks to the restructuring of the code and its implementation on the EGEE production Grid. Their main features are discussed and out of them J-shifting thermal rate coefficients have been computed to compare with the experiment and quasiclassical trajectory results over a wide range of temperatures.