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Presents a parallel treatment of the same phenomena by two approaches (steady-state approximation and approximate solution of the appropriate diffusion equation) to clarify the physical meaning of the important radiation boundary condition in the diffusional derivation. (JN)

Hydroperoxides are formed in the atmospheric oxidation of volatile organic compounds, in the combustion autoxidation of fuel, in the cold environment of the interstellar medium, and also in some catalytic reactions. They play crucial roles in the formation and aging of secondary organic aerosols and in fuel autoignition. However, the concentration of organic hydroperoxides is seldom measured, and typical estimates have large uncertainties. In this work, we developed a mild and environmental- friendly method for the synthesis of alkyl hydroperoxides (ROOH) with various structures, and we systematically measured the absolute photoionization cross-sections (PICSs) of the ROOHs using synchrotron vacuum ultraviolet-photoionization mass spectrometry (SVUV-PIMS). A chemical titration method was combined with an SVUV-PIMS measurement to obtain the PICS of 4-hydroperoxy-2-pentanone, a typical molecule for combustion and atmospheric autoxidation ketohydroperoxides (KHPs). We found that organic hydroperoxide cations are largely dissociated by loss of OOH. This fingerprint was used for the identification and accurate quantification of the organic peroxides, and it can therefore be used to improve models for autoxidation chemistry. The synthesis method and photoionization dataset for organic hydroperoxides are useful for studying the chemistry of hydroperoxides and the reaction kinetics of the hydroperoxy radicals and for developing and evaluating kinetic models for the atmospheric autoxidation and combustion autoxidation of the organic compounds. [ABSTRACT FROM AUTHOR]

We present quasiclassical trajectory simulations of the high-energy collision-induced O(³P) + N2 reaction. The simulation results show that reactive scattering may proceed on both the ³A′ and ³A" potential energy surfaces, but the reaction on ³A" surface overwhelmingly dominates. At total energies of collision slightly above the threshold, the NO(X²Π) product is found to be scattered primarily in the forward direction, and at higher collision energies, the fraction of NO products that scatter in the backward direction increases. Two-dimensional joint distributions of the center-of-mass differential cross section and translational energy have been examined to unravel the details of the scattering as functions of collision energy. [ABSTRACT FROM AUTHOR]

Accurate modeling of high-temperature hypersonic flows in the atmosphere requires consideration of collision-induced dissociation of molecular species and energy transfer between the translational and internal modes of the gas molecules. Here, we describe a study of the N2 + N2→N2 + 2N and N2 + N2→4N nitrogen dissociation reactions using the quasiclassical trajectory (QCT) method. The simulations used a new potential energy surface for the N4 system; the surface is an improved version of one that was presented previously. In the QCT calculations, initial conditions were determined based on a two-temperature model that approximately separates the translational-rotational temperature from the vibrational temperature of the N2 diatoms. Five values from 8000 K to 30000 K were considered for each of the two temperatures. Over 2.4 × 109 trajectories were calculated. We present results for ensemble-averaged dissociation rate constants as functions of the translational-rotational temperature T and the vibrational temperature T v. The rate constant depends more strongly on T when T v is low, and it depends more strongly on T v when T is low. Quasibound reactant states contribute significantly to the rate constants, as do exchange processes at higher temperatures. We discuss two sets of runs in detail: an equilibrium test set in which T = T v and a nonequilibrium test set in which T v < T. In the equilibrium test set, high-v and moderately-low-j molecules contribute most significantly to the overall dissociation rate, and this state specificity becomes stronger as the temperature decreases. Dissociating trajectories tend to result in a major loss of vibrational energy and a minor loss of rotational energy. In the nonequilibrium test set, as T v decreases while T is fixed, higher-j molecules contribute more significantly to the dissociation rate, dissociating trajectories tend to result in a greater rotational energy loss, and the dissociation probability's dependence on v weakens. In this way, as T v decreases, rotational energy appears to compensate for the decline in average vibrational energy in promoting dissociation. In both the equilibrium and nonequilibrium test sets, in every case, the average total internal energy loss in the dissociating trajectories is between 10.2 and 11.0 eV, slightly larger than the equilibrium potential energy change of N2 dissociation. [ABSTRACT FROM AUTHOR]

The abstraction and addition reactions of H with trans-N2H2 are studied by high-level ab initio methods and density functional theory. Rate constants were calculated for these two reactions by multistructural variational transition state theory with multidimensional tunneling and including torsional anharmonicity by the multistructural torsion method. Rate constants of the abstraction reaction show large variational effects, that is, the variational transition state yields a smaller rate constant than the conventional transition state; this results from the fact that the variational transition state has a higher zero-point vibrational energy than the conventional transition state. The addition reaction has a classical barrier height that is about 1 kcal/mol lower than that of the abstraction reaction, but the addition rates are lower than the abstraction rates due to vibrational adiabaticity. The calculated branching ratio of abstraction to addition is 3.5 at 200 K and decreases to 1.2 at 1000 K and 1.06 at 1500 K. [ABSTRACT FROM AUTHOR]

We find high multireference character for abstraction of H from the OH group of ethenol (also called vinyl alcohol); therefore we adopt a multireference approach to calculate barrier heights for the various possible reaction channels of OH+C2H3OH. The relative barrier heights of ten possible saddle points for reaction of OH with ethenol are predicted by multireference Mo\ller–Plesset perturbation theory with active spaces based on correlated participating orbitals (CPOs) and CPO plus a correlated π orbital (CPO+π). Six barrier heights for abstracting H from a C[Single_Bond]H bond range from 3.1 to 7.7 kcal/mol, two barrier heights for abstracting H from an O[Single_Bond]H bond are both 6.0 kcal/mol, and two barrier heights for OH addition to the double bond are -1.8 and -2.8 kcal/mol. Thus we expect abstraction at high-temperature and addition at low temperature. The factor that determines which H is most favorable to abstract is an internal hydrogen bond that constitutes part of a six-membered ring at one of the abstraction saddle points; the hydrogen bond contributes about 3 kcal/mol stabilization. [ABSTRACT FROM AUTHOR]

Homogeneous nucleation of clusters that exhibit magic numbers is studied numerically, using as an example aluminum at 2000 K, based on recent calculations of free energies [Li et al., J. Phys. Chem. C 111, 16227 (2007)] and condensation rate constants [Li and Truhlar, J. Phys. Chem. C 112, 11109 (2008)] that provide a database for Ali up to i=60. The nucleation behavior for saturation ratios greater than about 4.5 is found to be dominated by a peak in the free energy change associated with the reaction iAl→Ali at i=55, making it the critical size over a wide range of saturation ratios. Calculated steady-state nucleation rates are many orders of magnitude lower than predicted by classical nucleation theory (CNT). The onset of nucleation is predicted to occur at a saturation ratio of about 13.3, compared to about 5.1 in CNT, while for saturation ratios greater than about 25 the abundance of magic-numbered clusters becomes high enough to invalidate the assumption that cluster growth occurs solely by monomer addition. Transient nucleation is also predicted to be substantially different than predicted by CNT, with a much longer time required to reach steady state: about 10-4 s at a saturation ratio of 20, compared to about 10-7 s from CNT. Magic numbers are seen to play an important role in transient nucleation, as the nucleation currents for clusters of adjacent sizes become equal to each other in temporally successive groups, where the largest cluster in each group is the magic-numbered one. [ABSTRACT FROM AUTHOR]

A semiclassical trajectory method, called the self-consistent decay of mixing (SCDM) method, is presented for the treatment of electronically nonadiabatic dynamics. The SCDM method is a modification of the semiclassical Ehrenfest (SE) method (also called the semiclassical time-dependent self-consistent-field method) that solves the problem of unphysical mixed final states by including decay-of-mixing terms in the equations for the evolution of the electronic state populations. These terms generate a force, called the decoherent force (or dephasing force), that drives the electronic component of each trajectory toward a pure state. Results for several mixed quantum–classical methods, in particular the SCDM, SE, and natural-decay-of-mixing methods and several trajectory surface hopping methods, are compared to the results of accurate quantum mechanical calculations for 12 cases involving five different fully dimensional triatomic model systems. The SCDM method is found to be the most accurate of the methods tested. The method should be useful for the simulation of photochemical reactions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the stationary point of abstraction reaction at the fully optimized reaction space (FORS) level. Single-point calculations by unrestricted Moller-Plesset fourth order perturbation theory; Values of the activation energies.

Interpolated variational transition state theory calculations with centrifugal-dominant, small-curvature tunneling coefficients have been carried out for the case of the deuterium kinetic isotope effect (KIE) in the reaction OH+12CD4→HDO+12CD3 and for the 13C KIE for the reaction OH+13CH4→H2O+13CH3. The interpolated variationally optimized generalized transition states predict notably different nontunneling KIEs than the conventional ones, and factorization analyses of the KIEs are presented to illustrate the origin of the differences. The zero-point energies at the variational transition states differ from those at the saddle point by up to 0.19 kcal/mol for the OH+12CD4 reaction and by up to 0.34 kcal/mol for the OH+13CH4 reaction. The incorporation of multidimensional tunneling effects partly cancels the effect of variational optimization of the transition state. [ABSTRACT FROM AUTHOR]

In this paper we optimize several algorithms for the computation of reaction rates based on information calculated along minimum energy reaction paths and we evaluate the efficiencies of the optimized algorithms. The investigations are based on the calculation of chemical reaction rate constants using variational transition state theory and multidimensional semiclassical transmission coefficients including reaction path curvature. Several methods are evaluated and compared by a systematic set of applications to test cases involving the hydrogen-atom transfer reactions CH3+H2→CH4+H and OH+H2→H2O+H. For each method we present general recommendations for all algorithmic choices other than gradient step size so that future calculations may be carried out reasonably efficiently by varying only one parameter. In the process of these optimizations we have found that the accuracy of the Euler stabilization method can be significantly increased by choosing the auxiliary parameters differently than in previous work; the optimized algorithm is called ES1*. Our final recommendations for future work are (i) when the Hessian/gradient computational cost ratio is low (<=3): the Page–McIver algorithm with the Hessian recalculated at every step, with a cubic starting step, and with curvature calculated from the derivative of the gradient, and (ii) when the Hessian/gradient computational cost ratio is moderate or large: the ES1* algorithm with a Hessian step size three times larger than the gradient step size, with a quadratic starting step, and with curvature calculated from the derivative of the gradient. [ABSTRACT FROM AUTHOR]

State-to-state reaction probabilities are found to be highly final-state specific at state-selected threshold energies for the reactions O+H2→OH+H and H+H2→H2+H. The study includes initial rotational states with quantum numbers 0–15, and the specificity is especially dramatic for the more highly rotationally excited reactants. The analysis is based on accurate quantum mechanical reactive scattering calculations. Final-state specificity is shown in general to increase with the rotational quantum number of the reactant diatom, and the trends are confirmed for both zero and nonzero values of the total angular momentum. [ABSTRACT FROM AUTHOR]

We present improved techniques and well-optimized basis sets for application of the outgoing wave variational principle to calculate converged quantum mechanical reaction probabilities, and we illustrate them with calculations for the reactions D+H2→HD+H with total angular momentum J=3 and F+H2→HF+H with J=0 and 3. The optimization involves the choice of distortion potential, the grid for calculating half-integrated Green’s functions, the placement, width, and number of primitive distributed Gaussians, and the computationally most efficient partition between dynamically adapted and primitive basis functions. We present benchmark calculations with 224–1064 channels. [ABSTRACT FROM AUTHOR]

We have carried out ab initio calculations using Mo\ller–Plesset perturbation theory, scaling all correlation energy in second order (MP-SAC2) with several large basis sets, for the reaction OH+CH4→H2O+CH3. We found that correlation has a large effect on the geometry, barrier height, and vibrational frequencies of the transition state. The final calculated values, obtained with a correlation-balanced basis set, for the forward and reverse classical barrier heights are 7.9 and 21.2 kcal/mol, respectively. We have used these with transition state theory and an Eckart model for semiclassical tunneling calculations of the rate constants for the above reaction in the temperature range from 200 to 2000 K. We found that the present model, which requires information only at the reactants, transition state, and products, predicts rate constants of the same order of magnitude as the experimental data for this wide temperature range. [ABSTRACT FROM AUTHOR]

We present global 3A’ and 3A‘ potential energy surfaces for the reaction O(3P)+H2→OH+H and its isotopic analogs. The new surfaces are based in part on the surface of Lee et al. [J. Chem. Phys. 76, 3563 (1982)] for collinear O–H–H, which is modified to give accurate properties for reactants and products. The potentials for noncollinear O–H–H geometries are based on bending energies of Bowman et al. [J. Chem. Phys. 81, 1739 (1984)] as fit previously and called surface set M2 by two of the authors [B. C. Garrett and D. G. Truhlar, Int. J. Quantum Chem. 29, 1463 (1986)], and the potentials for H–O–H geometries are based on a new interpolation scheme. The new surfaces treat the approach of an O to either end of H2 equivalently. We used improved canonical variational transition state theory calculations with the least-action ground-state tunneling approximation (ICVT/LAG) to recalibrate the classical barrier height to 13.0 kcal/mol. The ICVT/LAG rate constants calculated using the new surfaces are in excellent agreement with experimental values over the temperature range 298–2400 K. Rate constants and kinetic isotope effects for reactions involving D2 and HD also are presented and analyzed. Here again, the theoretical results are in very good agreement with experiments. [ABSTRACT FROM AUTHOR]

Variational transition state theory (VTST) with an adiabatic approximation for excited vibrational modes and semiclassical transmission coefficients to account for tunneling is used to calculate state-specific reaction rates for the collinear reaction H+F2(n=1)→HF+H and the three-dimensional reaction Cl+H2(n=1–4,7)→HCl+H, where n is the initial vibrational quantum number and the rate constants are summed over final vibrational states. We also consider deuterated and tritiated analogs for both reactions for n=1. The results are compared to previous state-selected rate constants calculated without transition state assumptions. Comparisons to accurate quantal results for the H+F2 series test both the semiclassical tunneling methods and the validity of the adiabatic approximation for excited-state dynamical-bottleneck locations. Comparison to quasiclassical trajectory calculations for the Cl+H2 series provide further tests of the adiabatic approximation. In general the VTST/adiabatic-plus-tunneling calculations are quite successful. [ABSTRACT FROM AUTHOR]

We have calculated state-to-state reaction cross sections for two new energies and state-to-state vibrationally inelastic cross sections for four energies for H+D2 collisions on the most accurate available potential energy surface. The dynamics calculations are based on the quasiclassical trajectory quadratic smooth sampling method. We present a detailed analysis of the theoretical inelastic and reaction cross sections and compare the trends in the reactive scattering results to those calculated from Gerrity and Valentini’s experiments and extrapolations thereof. The agreement between theory and experiment for the reactive scattering at 0.98–1.3 eV relative translational energy is stupendously good. [ABSTRACT FROM AUTHOR]

We use diatomics-in-molecules theory to estimate the nuclear-motion contributions to atom–diatom potential energy surfaces for reactive systems, with special emphasis on the saddle point region. We examine the reactions of H, O, F, and Cl with H2 and H with Cl2. Nuclear-motion corrections at the saddle point are in the range 0.007–0.07 kcal/mol for cases with one or two hydrogens and classical barrier heights in the range 0.7–3 kcal/mol and are about 0.2 kcal/mol for cases with two hydrogens and classical barrier heights in the range 8–12 kcal/mol. For F+H2 with parameters such that the predicted nuclear-motion correction is 0.03 kcal/mol at the saddle point, the correction is as large as 0.2 kcal/mol elsewhere on the surface. Isotopic substitution of D for H changes the classical barrier height by 0.003–0.1 kcal/mol for the cases studied. [ABSTRACT FROM AUTHOR]

In many cases, variational transition states for a chemical reaction are significantly displaced from a saddle point because of zero-point and entropic effects that depend on the reaction coordinate. Such displacements are often controlled by the competition between the potential energy along the minimum-energy reaction path and the energy requirements of one or more vibrational modes whose frequencies show a large variation along the reaction path. In calculating reaction rates from potential-energy functions we need to take account of these factors and—especially at lower temperatures—to include tunneling contributions, which also depend on the variation of vibrational frequencies along a reaction path. To include these effects requires more information about the activated complex region of the potential-energy surface than is required for conventional transition-state theory. In the present article we show how the vibrational and entropic effects of variational transition-state theory and the effective potentials and effective masses needed to calculate tunneling probabilities can be estimated with a minimum of electronic structure information, thereby allowing their computation at a higher level of theory than would otherwise be possible. As examples, we consider the reactions OH+H2, CH3+H2, and Cl+CH4 and some of their isotopic analogs. We find for Cl+CH4→HCl+CH3 that the reaction rate is greatly enhanced by tunneling under conditions of interest for atmospheric chemistry. [ABSTRACT FROM AUTHOR]

We present equations for generalized-normal-mode vibrational frequencies in reaction-path calculations based on various sets of coordinates for describing the internal motions of the system in the vicinity of a reaction path. We consider two special cases in detail as examples, in particular three-dimensional atom–diatom collisions with collinear steepest descent paths and reactions of the form CX3+YZ→CX3 Y+Z with reaction paths having C3v symmetry. We then present numerical comparisons of the differences in harmonic reaction-path frequencies for various coordinate choices for three such systems, namely, H+H2→H2+H, O+H2→OH+H, and CH3+H2→CH4+H. We test the importance of the differences in the harmonic frequencies for dynamics calculations by using them to compute thermal rate constants using variational transition state theory with semiclassical ground-state tunneling corrections. We present a new coordinate system for the reaction CH3+H2 that should allow for more accurate calculations than the Cartesian system used for previous reaction-path calculations on this and other polyatomic systems. [ABSTRACT FROM AUTHOR]

We analyze potential energy surfaces that have been proposed by one of the authors, Bunker, and Chapman and by Raff for the reaction CH3+H2↔CH4+H. The surfaces are modified to remove discontinuities and zero frequencies, where present, and the modified surfaces are compared to each other in terms of reaction-path properties and to ab initio calculations for stationary point properties. They are also used for rate constant calculations which are compared to experiment. The rate constants were calculated by improved canonical variational transition state theory with small-curvature semiclassical adiabatic ground-state transmission coefficients (ICVT/SCSAG) over a wide temperature range, 298–1340 K. Both surfaces yield rate constants in poor agreement with experimental values. The reaction-path analysis leads to a list of potential energy surface features that are important for the rate constants but inaccurate in the existing surfaces and that should be improved in subsequent work. [ABSTRACT FROM AUTHOR]

We present a sequence of three successively improved new semiempirical potential energy surfaces for the reaction CH3+H2→CH4+H. The semiempirical calibration is based on ab initio electronic structure calculations and experimental thermochemical data, vibrational frequencies, reaction rate constants, Arrhenius parameters, and kinetic isotope effects (KIE’s). To compare to the experimental kinetic data we apply variational transition state theory and semiclassical estimates of tunneling probabilities. We also provide detailed factorization analyses of the KIE’s to illustrate the way in which various surface features contribute to the overall KIE’s, and we discuss the substantial difficulties in attributing specific kinetic results to isolated potential energy surface features. Each of the three new surfaces, called J1, J2, and J3, has a thinner barrier than the one before. In addition, we provide one example, called surface J2A, showing the effect of making the barrier even thinner than on the best surface. The best surface yields rate constants for the forward and reverse reaction, activation energies, and KIE’s that are consistent with most of the available experimental data. [ABSTRACT FROM AUTHOR]

We report large-basis-set CASSCF/MR-CISD/SEC (complete active space self-consistent-field orbitals used for multireference configuration interaction with all single and double excitations and scaled external correlation) and MP4 (Mo\ller–Plesset fourth order perturbation theory) calculations of the FH2 potential energy surface for collinear and bent geometries in the vicinity of the F---H--H saddle point. These calculations indicate that higher order correlation effects become much more important as the generalized transition states are bent, and that the unrestricted saddle point for this reaction is noncollinear. This means that the sterically allowed cone of reactive configurations is much broader than either previously available ab initio calculations or the present lower-order ones would predict. [ABSTRACT FROM AUTHOR]

The small-curvature semiclassical adiabatic (SCSA) approximation, which is based on a reaction-path Hamiltonian, is used to calculate the rate of interconversion of the ammonia invertomers. In these calculations, we employ two realistic potential energy surfaces for which accurate tunneling splittings have been calculated previously. The semiclassical method is shown to be accurate within 16% for this prototype quantal isomerization rate. [ABSTRACT FROM AUTHOR]

We report and compare two sets of thermal rate constants for the collinear reaction Cl+HBr → HCl+Br, which is dominated by the n=0 vibrational state of reactants and the n=2 vibrational state of products. One set is based on converged multichannel quantal calculations carried out using hyperspherical coordinates. The other set is based on variational transition state theory (VTST) with a least-action (LA) ground-state (G) transmission coefficient (VTST/LAG). The two sets of rate constants agree within a factor of 2.2 over the whole factor-of-12 temperature range (200–2400 K) studied. The error is comparable to what was found previously for the symmetric reaction Cl+HCl′ → HCl+Cl′, which is dominated by n=0 states of reactants and products. This indicates that the VTST/LAG method is as applicable to reactions that proceed by tunneling into excited states as it is for ground-state-to-ground-state tunneling reactions. We also show that the VTST/LA approximation provides useful accuracy for the rate constant of the collinear excited-state reaction Cl+HBr(n=1) → HCl+Br. [ABSTRACT FROM AUTHOR]

We present an improved analytic potential energy surface for the F+H2→FH+H and H+FH′→HF+H′ reactions. The final surface is obtained in two stages. First we create a surface, called No. 4, which is based in the F–H–H barrier region on a previous partly empirical and partly theoretical fit and is based on the F–H···H exit channel and H–F–H barrier regions on new large-basis-set configuration interaction calculations. The final surface, called No. 5 incorporates more empirical information for collinear geometries in both the F–H–H and FH···H regions but remains a good representation of the ab initio calculations for bending potentials and in the strong-interaction regions. Variational-transition-state theory rate constants and WKB adiabatic barrier heights indicate that the final surface is more accurate than previous surfaces for thermal rate constants and overall reaction thresholds for F+H2→HF+H, F+D2→DF+D, and F+HD→HF+D and for product-state thresholds for HF (n′=3) and DF(n′=4), where n′ is the final vibrational quantum number. [ABSTRACT FROM AUTHOR]

We present a general formulation that allows physically intuitive curvilinear internal coordinates to be used for the calculation of potential energy expansions and generalized normal-mode vibrational frequencies in reaction-path calculations. The reaction path is defined, as usual, as the minimum-energy path in the mass-scaled Cartesian coordinate system, and curvilinear coordinates are used for vibrational frequency calculations at nonstationary points. The method is well adapted for use in variational transition state theory with semiclassical multidimensional tunneling (VTST/MT) approximations to calculate thermal rate constants. We present VTST/MT calculations for five reactions, H+H2→H2+H, O+H2→OH+H, CH3+H2→CH4+H, H+O2→HO2, and Cl+HBr→HCl+Br, to illustrate the use of the new curvilinear coordinates, and we compare the results to calculations employing rectilinear coordinates. We make detailed comparisons not only of the calculated rate constants but also of the vibrationally adiabatic ground-state potential energy curves and bound-state vibrational frequencies as functions of the reaction coordinate. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Variational transition state theory with multidimensional tunneling contributions (VTST/MT) is tested against quantum mechanical rate constants for the reaction H+CH[sub 4]→H[sub 2]+CH[sub 3] at temperatures up to 1000 K. The VTST/MT method can be and has been applied to many reactions that cannot be treated by rigorous quantum dynamics methods. Studying the accuracy of VTST/MT by comparison with accurate quantal results that are becoming available for systems of increasing size is important for validating the theory. In the present study, covering a factor of five in temperature, the VTST/MT method is found to have a mean deviation from accurate quantal rate constants for a six-body reaction of only 13% and maximum deviation of only 23%. [ABSTRACT FROM AUTHOR]

We present the revM06-L functional, which we designed by optimizing against a larger database than had been used for Minnesota 2006 local functional (M06-L) and by using smoothness restraints. The optimization strategy reduced the number of parameters from 34 to 31 because we removed some large terms that increased the required size of the quadrature grid and the number of self-consistent-field iterations. The mean unsigned error (MUE) of revM06-L on 422 chemical energies is 3.07 kcal/mol, which is improved from 3.57 kcal/mol calculated by M06-L. The MUE of revM06-L for the chemical reaction barrier height database (BH76) is 1.98 kcal/mol, which is improved by more than a factor of 2 with respect to the M06-L functional. The revM06-L functional gives the best result among local functionals tested for the noncovalent interaction database (NC51), with an MUE of only 0.36 kcal/mol, and the MUE of revM06-L for the solid-state lattice constant database (LC17) is half that for M06-L. The revM06-L functional also yields smoother potential curves, and it predicts moreaccurate results thanM06-L for seven out of eight diversified test sets not used for parameterization. We conclude that the revM06-L functional is well suited for a broad range of applications in chemistry and condensed-matter physics. [ABSTRACT FROM AUTHOR]

We have applied improved canonical and microcanonical variational transition state theories to the calculation of the rate constants for the polyatomic reaction OH+H2→H2O+H over the temperature range 200–2400 K using the Schatz–Elgersma fit to the Walch–Dunning ab initio potential energy surface. The results are compared to canonical variational transition state theory calculations that employed the same potential energy surface and to experiment. We find that the new results, which are in good agreement with experiment, differ very slightly from those obtained with canonical variational theory. One explanation for this agreement is that the microcanonical variational transition states have a rather weak energy dependence and lie within 0.04a0 of the ground-state adiabatic barrier maximum. We also find that quantum mechanical effects and the inclusion of reaction-path curvature are important at lower temperatures, and that the transition state theory treatment of this reaction breaks down for temperatures higher than about 2400 K. [ABSTRACT FROM AUTHOR]

This article is an essay that discusses the concepts underlying the application of modern transition state theory to reactions in enzymes. Issues covered include the potential of mean force, the quantization of vibrations, the free energy of activation, and transmission coefficients to account for nonequilibrium effect, recrossing, and tunneling. [ABSTRACT FROM AUTHOR]

Isotopic substitution of muonium for hydrogen provides an unparalleled opportunity to deepen our understanding of quantum mass effects on chemical reactions. A recent topical review in this journal of the thermal and vibrationally state-selected reaction of Mu with H2raises a number of issues that are addressed here. We show that some earlier quantum mechanical calculations of the Mu + H2reaction, which are highlighted in this review, and which have been used to benchmark approximate methods, are in error by as much as 19% in the low-temperature limit. We demonstrate that an approximate treatment of the Born–Oppenheimer diagonal correction that was used in some recent studies is not valid for treating the vibrationally state-selected reaction. We also discuss why vibrationally adiabatic potentials that neglect bend zero-point energy are not a useful analytical tool for understanding reaction rates, and why vibrationally non-adiabatic transitions cannot be understood by considering tunnelling through vibrationally adiabatic potentials. Finally, we present calculations on a hierarchical family of potential energy surfaces to assess the sensitivity of rate constants to the quality of the potential surface. [ABSTRACT FROM AUTHOR]

In a recent paper, Wang et al. found an iridium-containing compound with a formal oxidation state of 9. This is the highest oxidation state ever found in a stable compound. To learn if this is the highest chemical oxidation state possible, Kohn-Sham density functional theory was used to study various compounds, including PdO42+, PtO42+, PtO3F22+, PtO4OH+, PtO5, and PtO4SH+, in which the metal has an oxidation state of 10. It was found that PtO42+ has a metastable state that is kinetically stable with a barrier height for decomposition of 31 kcal mol−1 and a calculated lifetime of 0.9 years. All other compounds studied would readily decompose to lower oxidation states. [ABSTRACT FROM AUTHOR]

This essay provides a perspective on several issues in valence bond theory: the physical significance of semilocal bonding orbitals, the capability of valence bond concepts to explain systems with multireferences character, the use of valence bond theory to provide analytical representations of potential energy surfaces for chemical dynamics by the method of semiempirical valence bond potential energy surfaces (an early example of specific reaction parameters), by multiconfiguration molecular mechanics, by the combined valence bond-molecular mechanics method, and by the use of valence bond states as coupled diabatic states for describing electronically nonadiabatic processes (photochemistry). The essay includes both ab initio and semiempirical approaches. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

Equilibrium free-energy cycles relating oxidation and reduction potentials in solution to ionization potentials and electron affinities in the gas phase are constructed and the utilities of various levels of theory for computing particular free-energy changes within these cycles are discussed within the context of several examples. Emphasis is placed on the use of quantum-mechanical continuum solvation models to compute free energies of solvation. Key systems discussed include quinones, substituted anilines, substituted phenols, and reductive dechlorination reactions. [ABSTRACT FROM AUTHOR]

We have studied the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) and the coenzyme nicotinamide adenine dinucleotide phosphate (NADPH); the substrate is 5-protonated 7,8-dihydrofolate, and the product is tetrahydrofolate. The potential energy surface is modeled by a combined quantum mechanical-molecular mechanical (QM/MM) method employing Austin model 1 (AMI) and a simple valence bond potential for 69 QM atoms and employing the CHARMM22 and TIP3P molecular mechanics force fields for the other 21 399 atoms; the QM and MM regions are joined by two boundary atoms treated by the generalized hybrid orbital (GHO) method. All simulations are carried out using periodic boundary conditions at neutral pH and 298 K. In stage 1, a reaction coordinate is defined as the difference between the breaking and forming bond distances to the hydride ion, and a quasithermodynamic free energy profile is calculated along this reaction coordinate. This calculation includes quantization effects on bound vibrations but not on the reaction coordinate, and it is used to locate the variational transition state that defines a transition state ensemble. Then, the key interactions at the reactant, variational transition state, and product are analyzed in terms of both bond distances and electrostatic energies. The results of both analyses support the conclusion derived from previous mutational studies that the M20 loop of DHFR makes an important contribution to the electrostatic stabilization of the hydride transfer transition state. Third, transmission coefficients (including recrossing factors and multidimensional tunneling) are calculated and averaged over the transition state ensemble. These averaged transmission coefficients, combined with the quasithermodynamic free energy profile determined in stage 1, allow us to calculate rate constants, phenomenological free energies of activation, and primary and secondary kinetic isotope effects. A primary kinetic isotope effect (KIE) of 2.8 has been obtained, in good agreement with the experimentally determined value of 3.0 and with the value 3.2 calculated previously. The primary KIE is mainly a consequence of the quantization of bound vibrations. In contrast, the secondary KIE, with a value of 1.13, is almost entirely due to dynamical effects on the reaction coordinate, especially tunneling. [ABSTRACT FROM AUTHOR]

Reports on selected experiments about enzyme-catalyzed reactions which show convex Arrhenius plots. Analysis of the effect of activation energy; Interpretation of the data on proposed microscopic models; Requirement for the consistency of the microcanonical rate coefficient as a decreasing function of energy.

Adequate polarization functions reduce the error of density functional theory (DFT) for the heat of reaction for CF4+ SiCl4from ∼9–12 kcal mol−1to ∼2–4 kcal mol−1, and using an improved density functional further reduces it to ∼1 kcal mol−1. This reaction was previously identified as a stumbling block for DFT, but we show that the problem with the previous calculations was not DFT but rather inadequate basis sets to account for intramolecular charge polarization. [ABSTRACT FROM AUTHOR]

We present an efficient algorithm for generating semiglobal potential energy surfaces of reactive systems. The method takes as input molecular mechanics force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locations are determined by an iterative process. These Hessian expansions might come, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A 2x2 electronic diabatic Hamiltonian matrix is constructed from these data such that, by construction, the lowest eigenvalue of this matrix provides a semiglobal approximation to the lowest electronically adiabatic potential energy surface. The theory is illustrated and tested by applications to rate constant calculations for three gas-phase test reactions, namely, the isomerization of 1,3-cis-pentadiene, OH+CH[sub 4]→H[sub 2]O+CH[sub 3], and CH[sub 2]Cl+CH[sub 3]F→CH[sub 3]Cl+CH[sub 2]F. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents corrections to the study 'Ab Initio State Theory Calculations of the Reaction Rate for OH+Ch[sub4]--H[sub2]O+Ch[sub3],' by Tanh N. Truong and Donald G. Truhlar from the 1990 issue of the 'Journal of Chemical Physics.'

Presents an erratum of an article on nuclear-motion of Born-Oppenheimer barrier heights for chemical reactions.

The article offers the author's insights on a study conducted by Claire Adjiman and colleagues which developed a method that allows identification of solvent to improve the kinetics of a reaction. He says that the focus on free energies is one key advantage in modern reaction dynamics, and describes the transition state theory. He states the definition of an inverse catalyst design as described by the Purdue University Center for Catalyst Design.

This Perspective presents a personal overview of the current status of the theory of chemical kinetics and mechanisms for complex processes. We attempt to assess the status of the field for reactions in the gas phase, at gas-solid interfaces, in liquid solutions, in enzymes, and for protein folding. Some unifying concepts such as potential energy surfaces, free energy, master equations, and reaction coordinates occur in more than one area. We hope this Perspective will be useful for highlighting recent advances and for identifying important areas for future research. [ABSTRACT FROM AUTHOR]

Distinguishing between the concerted second-order mechanism for β-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated α,β-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cBI) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces. [ABSTRACT FROM AUTHOR]

We present a new model for predicting branching ratios of chemical reactions when a branching of the reaction path occurs after the dynamical bottleneck, including the case where it occurs after an intermediate. The model is based on combining nonstatistical phase space theory for the direct component of a reaction with variational transition-state theory for an indirect component of reaction. The competition between direct and indirect processes is treated by an extension of the unified statistical model. This new method provides a way to understand the factors that control this kind of chemical reaction and to perform calculations using high-level electronic structure methods for complex systems. The model is based on quantized energy levels of transition states and products, and it involves the same information as required for calculating transition-state rate constants and equilibrium constants plus a phenomenological relaxation time, which was taken from previous work. For the textbook reaction of the hydroboration of propene by BH3 it has recently been inferred that the selectivity can only be understood by consideration of dynamical trajectories. However, the calculated branching fraction of this prototype reaction increases from 2%-3% when calculated under the inappropriate assumption of complete equilibration of the intermediate to from 8%-9% when calculated with the new theory, which requires only limited information about the system and does not involve running trajectories. The calculated result is in reasonable agreement with experiment (∼10%). [ABSTRACT FROM AUTHOR]