MTA ELTE Research Group on Complex Chemical Systems

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MTA ELTE Research Group on Complex Chemical Systems

Hungary

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Varga T.,Eötvös Loránd University | Varga T.,MTA ELTE Research Group on Complex Chemical Systems | Zsely I.G.,Eötvös Loránd University | Turanyi T.,Eötvös Loránd University | And 2 more authors.
International Journal of Chemical Kinetics | Year: 2014

The optimization of a kinetic mechanism of the pyrolysis of ethyl iodide was carried out based on data obtained from reflected shock wave experiments with H-ARAS and I-ARAS detection. The analysis took into account also the measurements of Michael et al. (Chem. Phys. Lett. 2000, 319, 99-106) and Vasileiadis and Benson (Int. J. Chem. Kinet. 1997, 29, 915-925) of the reaction H2 + I = H + HI. The following Arrhenius parameters were determined for the temperature range 950-1400 K and the pressure range 1-2 bar: C 2H5I → C2H5 + I: log 10(A) = 13.53, E/R = 24,472 K; C2H5I → C2H4 + HI: log10(A) = 13.67, E/R = 27,168 K; H + HI → H2 + I: log10(A) = 13.82, E/R = 491 K; C 2H5I + H →C2H5 + HI: log 10(A) = 15.00, E/R = 2593 K (the units of A are cm3, mol, s). The joint covariance matrix of the optimized Arrhenius parameters was also determined. This covariance matrix was converted to the temperature-dependent uncertainty parameters f of the rate coefficients and also to the temperature-dependent correlation coefficients between pairs of rate coefficients. Each fitted rate coefficient was determined with much lower uncertainty compared to the estimated uncertainty of the data available in the literature. © 2013 Wiley Periodicals, Inc.


Varga T.,Eötvös Loránd University | Varga T.,MTA ELTE Research Group on Complex Chemical Systems | Nagy T.,Eötvös Loránd University | Nagy T.,Institute of Materials and Environmental Chemistry | And 10 more authors.
Proceedings of the Combustion Institute | Year: 2015

The Kéromnès et al. (2013) mechanism for hydrogen combustion has been optimized using a large set of indirect experimental data, consisting of ignition measurements in shock tubes (566 datapoints in 43 datasets) and rapid compression machines (219/19), and flame velocity measurements (364/59), covering wide ranges of temperature (800 K-2300 K), pressure (0.1 bar-65 bar) and equivalence ratio (φ = 0.2-5.0). According to the sensitivity analysis carried out at each experimental datapoint, 30 Arrhenius parameters and 3 third body collision efficiency parameters of 11 elementary reactions could be optimized using these experimental data. 1749 directly measured rate coefficient values in 56 datasets belonging to the 11 reaction steps were also utilized. Prior uncertainty ranges of the rate coefficients were determined from literature data. Mechanism optimization has led to a new hydrogen combustion mechanism, a set of newly recommended rate parameters with their covariance matrix, and temperature-dependent posterior uncertainty ranges of the rate coefficients. The optimized mechanism generated here was tested together with 13 recent hydrogen combustion mechanisms and proved to be the best one. © 2014 The Combustion Institute.


Olm C.,Eötvös Loránd University | Olm C.,MTA ELTE Research Group on Complex Chemical Systems | Zsely I.G.,Eötvös Loránd University | Palvolgyi R.,Eötvös Loránd University | And 5 more authors.
Combustion and Flame | Year: 2014

A large set of experimental data was accumulated for hydrogen combustion: ignition measurements in shock tubes (770 data points in 53 datasets) and rapid compression machines (229/20), concentration-time profiles in flow reactors (389/17), outlet concentrations in jet-stirred reactors (152/9) and flame velocity measurements (631/73) covering wide ranges of temperature, pressure and equivalence ratio. The performance of 19 recently published hydrogen combustion mechanisms was tested against these experimental data, and the dependence of accuracy on the types of experiment and the experimental conditions was investigated. The best mechanism for the reproduction of ignition delay times and flame velocities is Kéromnès-2013, while jet-stirred reactor (JSR) experiments and flow reactor profiles are reproduced best by GRI3.0-1999 and Starik-2009, respectively. According to the reproduction of all experimental data, the Kéromnès-2013 mechanism is currently the best, but the mechanisms NUIG-NGM-2010, ÓConaire-2004, Konnov-2008 and Li-2007 have similarly good overall performances. Several clear trends were found when the performance of the best mechanisms was investigated in various categories of experimental data. Low-temperature ignition delay times measured in shock tubes (below 1000K) and in RCMs (below 960K) could not be well-predicted. The accuracy of the reproduction of an ignition delay time did not change significantly with pressure and equivalence ratio. Measured H2 and O2 concentrations in JSRs could be better reproduced than the corresponding H2O profiles. Large differences were found between the mechanisms in their capability to predict flow reactor data. The reproduction of the measured laminar flame velocities improved with increasing pressure and total diluent concentration, and with decreasing equivalence ratio. Reproduction of the flame velocities measured using the flame cone method, the outwardly propagating spherical flame method, the counterflow twin-flame technique, and the heat flux burner method improved in this order. Flame cone method data were especially poorly reproduced. The investigation of the correlation of the simulation results revealed similarities of mechanisms that were published by the same research groups. Also, simulation results calculated by the best-performing mechanisms are more strongly correlated with each other than those of the weakly performing ones, indicating a convergence of mechanism development. An analysis of sensitivity coefficients was carried out to identify reactions and ranges of conditions that require more attention in future development of hydrogen combustion models. The influence of poorly reproduced experiments on the overall performance was also investigated. © 2014 The Combustion Institute.


Olm C.,Eötvös Loránd University | Olm C.,MTA ELTE Research Group on Complex Chemical Systems | Olm C.,TU Bergakademie Freiberg | Zsely I.G.,Eötvös Loránd University | And 4 more authors.
Combustion and Flame | Year: 2015

A large set of experimental data was accumulated for syngas combustion: ignition studies in shock tubes (732 data points in 62 datasets) and in rapid compression machines (492/47), flame velocity determinations (2116/217) and species concentration measurements from flow reactors (1104/58), shock tubes (436/21) and jet-stirred reactors (90/3). In total, 4970 data points in 408 datasets from 52 publications were collected covering wide ranges of temperature T, pressure p, equivalence ratio ϕ, CO/H2 ratio and diluent concentration Xdil. 16 recent syngas combustion mechanisms were tested against these experimental data, and the dependence of their predictions on the types of experiment and the experimental conditions was investigated. Several clear trends were found. Ignition delay times measured in rapid compression machines (RCM) and in shock tubes (ST) at temperatures below 1000K could not be well-predicted. Particularly for shock tubes, facility effects at temperatures below 1000K could not be excluded. The accuracy of the reproduction of ignition delay times did not change significantly with pressure. The agreement of measured and simulated laminar flame velocities is better at low initial (i.e. cold side) temperatures, at fuel-lean conditions, for CO-rich and highly diluted mixtures. The reproduction of the experimental flame velocities is better when these were measured using the heat flux method or the counterflow twin-flame technique, compared to the flame cone method and the outwardly propagating spherical flame approach. With respect to all data used in this comparison, five mechanisms were identified that reproduce the experimental data similarly well. These are the NUIG-NGM-2010, Kéromnès-2013, Davis-2005, Li-2007 and USC-II-2007 mechanisms, in decreasing order of their overall performance. The influence of poorly reproduced experiments and weighting on the performance of the mechanisms was investigated. Furthermore, an analysis of local sensitivity coefficients was carried out to determine the influence of selected reactions at the given experimental conditions and to identify those reactions that require more attention in future development of syngas combustion models. © 2014 The Combustion Institute.


Furtenbacher T.,Eötvös Loránd University | Furtenbacher T.,MTA ELTE Research Group on Complex Chemical Systems | Szidarovszky T.,Eötvös Loránd University | Szidarovszky T.,MTA ELTE Research Group on Complex Chemical Systems | And 4 more authors.
Journal of Chemical Theory and Computation | Year: 2013

On the basis of both experiment and theory, accurate rotational-vibrational line positions and energy levels, with associated critically reviewed labels and uncertainties, are reported for the ground electronic state of the H3 + molecular ion. An improved MARVEL algorithm is used to determine the validated experimental levels and their self-consistent uncertainties from a set of 1610 measured transitions and associated uncertainties, coming from 26 sources. 1410 transitions have been validated for ortho-H3+ and para-H3+, 78 belong to floating components of the spectroscopic network (SN) investigated and thus left unvalidated, while 122 measured transitions had to be excluded from the MARVEL analysis for one reason or another. The spectral range covered by the experiments is 7-16 506 cm -1. Altogether 13 vibrational band origins are reported, the highest J value, where J stands for the rotational quantum number, for which energy levels are validated is 12. The MARVEL energy levels are checked against ones determined from accurate variational nuclear motion computations employing the best available adiabatic ab initio potential energy surface and exact kinetic energy operators. The number of validated and thus recommended experimental-quality rovibrational energy levels is 652, of which 259 belong to ortho-H3+ and 393 to para-H3+. There are 105 further energy levels within floating components of the SN. The variational computations have been performed both without and with a simple nonadiabatic correction, whereby nonadiabaticity is modeled by the use of a non-nuclear vibrational mass. The lists of validated lines and levels for H3+ are deposited in the Supporting Information to this paper. © 2013 American Chemical Society.


Fabri C.,Eötvös Loránd University | Furtenbacher T.,MTA ELTE Research Group on Complex Chemical Systems | Csaszar A.G.,Eötvös Loránd University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems
Molecular Physics | Year: 2014

A hybrid variational-perturbational nuclear motion algorithm based on the perturbative treatment of the Coriolis coupling terms of the Eckart-Watson kinetic energy operator following a variational treatment of the rest of the operator is described. The algorithm has been implemented in the quantum chemical code DEWE. Performance of the hybrid treatment is assessed by comparing selected numerically exact variational vibration-only and rovibrational energy levels of the C2H4, C2D4, and CH4 molecules with their perturbatively corrected counterparts. For many of the rotational-vibrational states examined, numerical tests reveal excellent agreement between the variational and even the first-order perturbative energy levels, whilst the perturbative approach is able to reduce the computational cost of the matrix-vector product evaluations, needed by the iterative Lanczos eigensolver, by almost an order of magnitude. © 2014 Taylor & Francis.


Rudolph H.D.,University of Ulm | Demaison J.,Lille University of Science and Technology | Csaszar A.G.,Eötvös Loránd University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems
Journal of Physical Chemistry A | Year: 2013

Accurate equilibrium, re, structures of the monosubstituted benzene molecules benzonitrile, C6H5CN, and phenylacetylene, C6H5CCH, have been determined using two different, to some extent complementary techniques. The semiexperimental, r eSE, structural parameters are the result of a least-squares fit to equilibrium rotational constants derived from experimental effective ground-state rotational constants and rovibrational corrections based principally on an ab initio cubic force field. The composite ab initio Born-Oppenheimer, reBO, structural parameters are obtained from frozen-core and all-electron MP2 and the CCSD(T) geometry optimizations using Gaussian basis sets up to quintuple-zeta quality. The DFT(B3LYP) method, with two different Gaussian basis sets, 6-31G* and 6-311+G(3df,2pd), was used to calculate the cubic force field employed during the re SE structure determination. With the 6-31G* basis set, the error of the rovibrational correction is to a large extent random, whereas with the 6-311+G(3df,2pd) basis set it is mainly systematic. As shown here, systematic errors do not have a significant effect on the accuracy of the derived structure; the quality of the structural fit, however, is sensitive to the true accuracy of the ground-state rotational constants. An even more important general conclusion of this study is that the addition of extra rotational constants from multisubstituted species does not seem to improve the accuracy of the reSE structures, quite in contrast to the highly desirable availability of data corresponding to all singly substituted species. © 2013 American Chemical Society.


Czako G.,Eötvös Loránd University | Csaszar A.G.,Eötvös Loránd University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems | Schaefer H.F.,University of Georgia
Journal of Physical Chemistry A | Year: 2014

The H2O⋯X complexes, with X = F, Cl, Br, and I, show considerable viability with nonspin-orbit De(D0) dissociation energy values of 3.73(2.42), 3.60(2.68), 3.54(2.72), and 3.36(2.77) kcal mol-1 for X = F, Cl, Br, and I, respectively, obtained at the CCSD(T)-F12b/aug-cc-pVTZ(-PP) level of theory using relativistic pseudopotentials (PPs) for Br and I. Spin-orbit (SO) corrections, computed with the Breit-Pauli operator in the interacting states approach at the all-electron MRCI+Q/aug-cc-pwCVTZ(-PP) level, are found to depend sensitively and unpredictably on the O⋯X separations. 96% (F), 87% (Cl), 54% (Br), and 30% (I) quenching of the SO corrections significantly reduces the dissociation energies of the H2O⋯X complexes, resulting in De(D0) values of 3.38(2.06), 2.86(1.94), 1.64(0.83), and 1.23(0.64) kcal mol-1 for X = F, Cl, Br, and I, respectively. (Graph Presented). © 2014 American Chemical Society.


Szabo I.,Eötvös Loránd University | Csaszar A.G.,Eötvös Loránd University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems | Czako G.,Eötvös Loránd University
Chemical Science | Year: 2013

Though bimolecular nucleophilic substitution (SN2) reactions play a fundamental role in chemistry, chemically accurate full-dimensional global analytical potential energy surfaces (PESs) have not been developed for these systems. These PESs govern the motion of the atoms in a chemical reaction; thus, the knowledge of the PES is essential to study the dynamics and atomic-level mechanisms. Here, we report a full-dimensional ab initio PES for the F- + CH3Cl → Cl- + CH3F reaction, the PES has an estimated average accuracy of about 0.5 kcal mol -1. Quasiclassical trajectories on this PES reveal that the direct rebound mechanism dominates at high collision energies, whereas the reaction is mainly indirect at low collision energies, where the formation of long-lived hydrogen-bonded and C3v ion-dipole entrance-channel complexes play a major role in the dynamics. A direct stripping mechanism is also found at large impact parameters resulting in significant forward scattering, whereas the direct rebound mechanism scatters towards backward directions. At high collision energies the reaction can be controlled by orienting the reactants into a reactive F- + H3CCl orientation, whereas at low collision energies the initial orientation is not always maintained, because the long-range ion-dipole interactions efficiently steer the reactants into a reactive orientation even if F- initially approaches the non-reactive side of CH3Cl. Mode-specific vibrational distributions show that the reaction produces vibrationally hot CH3F molecules with excited CF stretching, especially at low collision energies. This journal is © 2013 The Royal Society of Chemistry.


Csaszar A.G.,Eötvös Loránd University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems | Demaison J.,Lille University of Science and Technology | Rudolph H.D.,University of Ulm
Journal of Physical Chemistry A | Year: 2015

Up to six different techniques are utilized to estimate the equilibrium structures (re) of a series of mostly unsaturated, N-containing heterocycles. Accurate Born-Oppenheimer (reBO) and, if allowed, semiexperimental (reSE), as well as empirical (rm-type) estimates of the equilibrium structures of three-membered (1H- and 2H-azirine, aziridine), four-membered (azete), five-membered (pyrrole, pyrazole, imidazole), six-membered (pyridine, pyrimidine, uracil), and seven-membered (1H-azepine) rings, containing usually one but in some cases two N atoms, are determined. The agreement among the structural results of the different techniques is very satisfactory. It is shown that it is possible to use the CCSD(T) electronic structure method with the relatively small wCVTZ basis set, with all electrons correlated, and the effect of further basis set enlargement, wCVTZ → wCVQZ, computed at the MP2 level, to obtain reliable equilibrium structures for the semirigid molecules investigated. Extension to larger basis sets does not significantly improve the accuracy of the computed results. Although all molecules investigated are oblate, and their principal axis system is subject to large rotations upon isotopic substitution, the semiexperimental method, when applicable, provides accurate results, though in the difficult cases it must be augmented with the mixed regression method. Finally, it is noteworthy that the empirical mass-dependent (rm) method also delivers surprisingly accurate structures for this class of compounds. © 2014 American Chemical Society.

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