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

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. Source

Csaszar A.G.,Eotvos Lorand 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

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. Source

Czako G.,Eotvos Lorand University | Csaszar A.G.,Eotvos Lorand University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems | Schaefer H.F.,University of Georgia
Journal of Physical Chemistry A

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. Source

Varga T.,Eotvos Lorand University | Varga T.,MTA ELTE Research Group on Complex Chemical Systems | Zsely I.G.,Eotvos Lorand University | Turanyi T.,Eotvos Lorand University | And 2 more authors.
International Journal of Chemical Kinetics

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. Source

Szabo I.,Eotvos Lorand University | Csaszar A.G.,Eotvos Lorand University | Csaszar A.G.,MTA ELTE Research Group on Complex Chemical Systems | Czako G.,Eotvos Lorand University
Chemical Science

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. Source

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