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Direct Dynamics Method (direct + dynamics_method)
Selected AbstractsDirect ab initio dynamics calculations of the rate constants for the reaction of CHF2CF2OCH3 with ClINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2007Lei Yang A dual-level direct dynamics method is employed to reveal the dynamical properties of the reaction of CHF2CF2OCH3 (HFE-254pc) with Cl atoms. The optimized geometries and frequencies of the stationary points and the minimum energy path (MEP) are calculated at the B3LYP/6-311G(d,p) level by using GAUSSIAN 98 program package, and energetic information is further refined by the G3(MP2) method. Two H-abstraction channels have been identified. For the reactant CHF2CF2OCH3 and the two products, CHF2CF2OCH2 and CF2CF2OCH3, the standard enthalpies of formation are evaluated with the values of ,256.71 ± 0.88, ,207.79 ± 0.12, and ,233.43 ± 0.88 kcal/mol, respectively, via group-balanced isodesmic reactions. The rate constants of the two reaction channels are evaluated by means of canonical variational transition-state theory (CVT) including the small-curvature tunneling (SCT) correction over a wide range of temperature from 200 to 2000 K. The calculated rate constants agree well with the experimental data, and the Arrhenius expressions for the title reaction are fitted and can be expressed as k1 = 9.22 × 10,19 T2.06 exp(219/T), k2 = 4.45 × 10,14T0.90 exp(,2220/T), and k = 4.71 × 10,22 T3.20) exp(543/T) cm3 molecule,1 s,1. Our results indicate that H-abstraction from CH3 group is the main reaction pathway in the lower temperature range, while H-abstraction from CHF2 group becomes more competitive in the higher temperature range. © 2007 Wiley Periodicals, Inc. 39: 221,230, 2007 [source] Ab initio direct dynamics studies on the reactions of chlorine atom with CH3,nFnCH2OH (n = 1,3)JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 16 2007Ying Wang Abstract The hydrogen abstraction reactions of Cl atom with a series of fluorinated alcohols, i.e., CH3,nFnCH2OH + Cl (n = 1,3) (R1,R3) have been studied systematically by ab initio direct dynamics method and the canonical variational transition state theory (CVT). The potential energy surface information is calculated at the MP2/6-311G(d,p) level. Energies along the minimum energy paths are improved by a series of single-point calculations at the higher modified GAUSSIAN-2 (G2M) level of theory. Theoretical analysis shows that three kinds of hydrogen atoms can be abstracted from the reactants CH2FCH2OH and CHF2CH2OH, and for CF3CH2OH, two possible pathways are found. The rate constants for each reaction channel are evaluated by CVT with the small-curvature tunneling correction (SCT) over a wide range of temperature from 200 to 2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values for the reactions CHF2CH2OH + Cl and CF3CH2OH + Cl. However, for the reaction CH2FCH2OH + Cl, there is negative temperature dependence below 500 K, which is different from the experimental fitted. It is shown that in the low temperature ranges, the three reactions all proceed predominantly via H-abstraction from the methylene positions, and with the increase of the temperature the H-abstraction channels from the fluorinated-methyl positions should be taken into account, while the H-abstraction channels from the hydroxyl groups are negligible over the whole temperature ranges. Also, the reactivity decreases substantially with fluorine substitution at the methyl position of alcohol. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Theoretical study on the Br + CH3SCH3 reactionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2007Hui Zhang Abstract The multiple-channel reactions Br + CH3SCH3 , products are investigated by direct dynamics method. The optimized geometries, frequencies, and minimum energy path are all obtained at the MP2/6-31+G(d,p) level, and energetic information is further refined by the G3(MP2) (single-point) theory. The rate constants for every reaction channels, Br + CH3SCH3 , CH3SCH2 + HBr (R1), Br + CH3SCH3 , CH3SBr + CH3 (R2), and Br + CH3SCH3 ,CH3S + CH3Br (R3), are calculated by canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200,3000 K. The total rate constants are in good agreement with the available experimental data, and the two-parameter expression k(T) = 2.68 × 10,12 exp(,1235.24/T) cm3/(molecule s) over the temperature range 200,3000 K is given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smallest barrier height among three channels considered, and the other two channels to yield CH3SBr + CH3 and CH3S + CH3Br are minor channels over the whole temperature range. © 2007 Wiley Periodicals, Inc. J Comput Chem 2007 [source] Theoretical study and rate constant calculation for reaction of CF3CH2OH with OHJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2007Ying Wang Abstract The reaction mechanism of CF3CH2OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF3CH2OH, CF3CHOH, and CF3CH2O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200,2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250,430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel. © 2007 Wiley Periodicals, Inc. J Comput Chem 28: 802,810, 2007 [source] | ||||||||||