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Variational Transition State Theory (variational + transition_state_theory)
Kinds of Variational Transition State Theory Selected AbstractsKinetic isotope effects for the H2 + C2H , C2H2 + H reaction based on the ab initio calculations and a global potential energy surfaceINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2010Liping Ju In the present paper, kinetic isotope effects of the title reaction are studied with canonical variational transition state theory on the modified Wang Bowman (MWB) potential energy surface (PES) (Chem Phys Lett 2005, 409, 249) and the ab initio calculations at the quadratic configuration interaction (QCISD (T, full))/aug-cc-pVTZ//QCISD (full)/cc-pVTZ level. The calculated rate constants for the isotopic variants of this title reaction on the MWB PES have good agreement with those of the present ab initio calculations over the temperature range of 20,5000 K for the forward reactions and 800,5000 K for the reverse reactions, respectively. In particular, the forward rate constants for the title reaction and its isotopically substituted reactions have negative temperature dependences at about 40 K. Rate expressions are presented for all the studied reactions. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 289,298, 2010 [source] Ab initio chemical kinetics for the NH2 + HNOx reactions, part II: Kinetics and mechanism for NH2 + HONOINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2009Shucheng Xu The kinetics and mechanism for the reaction of NH2 with HONO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The reaction producing the primary products, NH3 + NO2, takes place via precomplexes, H2N,,,c -HONO or H2N,,,t -HONO with binding energies, 5.0 or 5.9 kcal/mol, respectively. The rate constants for the major reaction channels in the temperature range of 300,3000 K are predicted by variational transition state theory or Rice,Ramsperger,Kassel,Marcus theory depending on the mechanism involved. The total rate constant can be represented by ktotal = 1.69 × 10,20 × T2.34 exp(1612/T) cm3 molecule,1 s,1 at T = 300,650 K and 8.04 × 10,22 × T3.36 exp(2303/T) cm3 molecule,1 s,1 at T = 650,3000 K. The branching ratios of the major channels are predicted: k1 + k3 producing NH3 + NO2 accounts for 1.00,0.98 in the temperature range 300,3000 K and k2 producing OH + H2NNO accounts for 0.02 at T > 2500 K. The predicted rate constant for the reverse reaction, NH3 + NO2 , NH2 + HONO represented by 8.00 × 10,26 × T4.25 exp(,11,560/T) cm3 molecule,1 s,1, is in good agreement with the experimental data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 678,688, 2009 [source] Direct dynamic study on the hydrogen abstraction reaction of H2CO with NCOINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2009Hao Sun A direct ab initio dynamics method is used to investigate the hydrogen-abstraction reaction of H2CO with NCO. The potential energy surface information is obtained at the MP2/6-311G(d,p) level. More accurate single-point energy is refined at the G3(MP2)//MP2/6-311G(d,p) level. Furthermore, the rate constants of reaction H2CO + NCO are evaluated by using the canonical variational transition state theory with small-curvature tunneling contributions over a wide temperature range of 200,2000 K. The calculated reaction enthalpy and rate constants are in good agreement with the available experimental values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 394,400, 2009 [source] Ab initio study of the OH + CH2O reaction: The effect of the OH··OCH2 complex on the H-abstraction kineticsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2006Shucheng Xu Kinetics for the reaction of OH radical with CH2O has been studied by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on the geometries optimized at the B3LYP/6-311+G(3df, 2p) and CCSD/6-311++G(d,p) levels. The rate constant for the reaction has been computed in the temperature range 200,3000 K by variational transition state theory including the significant effect of the multiple reflections above the OH··OCH2 complex. The predicted results can be represented by the expressions k1 = 2.45 × 10 -21T2.98 exp (1750/T) cm3 mol,1 s,1 (200,400 K) and 3.22 × 10 -18T2.11 exp(849/T) cm3 mol,1 s,1 (400,3000 K) for the H-abstraction process and k2 = 1.05 × 10 -17T1.63 exp(,2156/T) cm3 mol,1 s,1 in the temperature range of 200,3000 K for the HO-addition process producing the OCH2OH radical. The predicted total rate constants (k1 + k2) can reproduce closely the recommended kinetic data for OH + CH2O over the entire range of temperature studied. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 322,326, 2006 [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] Analysis of Classical and Quantum Paths for Deprotonation of Methylamine by Methylamine DehydrogenaseCHEMPHYSCHEM, Issue 12 2007Kara E. Ranaghan Abstract The hydrogen-transfer reaction catalysed by methylamine dehydrogenase (MADH) with methylamine (MA) as substrate is a good model system for studies of proton tunnelling in enzyme reactions,an area of great current interest,for which atomistic simulations will be vital. Here, we present a detailed analysis of the key deprotonation step of the MADH/MA reaction and compare the results with experimental observations. Moreover, we compare this reaction with the related aromatic amine dehydrogenase (AADH) reaction with tryptamine, recently studied by us, and identify possible causes for the differences observed in the measured kinetic isotope effects (KIEs) of the two systems. We have used combined quantum mechanics/molecular mechanics (QM/MM) techniques in molecular dynamics simulations and variational transition state theory with multidimensional tunnelling calculations averaged over an ensemble of paths. The results reveal important mechanistic complexity. We calculate activation barriers and KIEs for the two possible proton transfers identified,to either of the carboxylate oxygen atoms of the catalytic base (Asp428,),and analyse the contributions of quantum effects. The activation barriers and tunnelling contributions for the two possible proton transfers are similar and lead to a phenomenological activation free energy of 16.5±0.9 kcal,mol,1 for transfer to either oxygen (PM3-CHARMM calculations applying PM3-SRP specific reaction parameters), in good agreement with the experimental value of 14.4 kcal,mol,1. In contrast, for the AADH system, transfer to the equivalent OD1 was found to be preferred. The structures of the enzyme complexes during reaction are analysed in detail. The hydrogen bond of Thr474,(MADH)/Thr172,(AADH) to the catalytic carboxylate group and the nonconserved active site residue Tyr471,(MADH)/Phe169,(AADH) are identified as important factors in determining the preferred oxygen acceptor. The protein environment has a significant effect on the reaction energetics and hence on tunnelling contributions and KIEs. These environmental effects, and the related clearly different preferences for the two carboxylate oxygen atoms (with different KIEs) in MADH/MA and AADH/tryptamine, are possible causes of the differences observed in the KIEs between these two important enzyme reactions. [source] | ||||||||||