Xiu-Mei Pan

Theoretical Investigation of the Reactions of CF3CHFOCF3 with the OH Radical and Cl Atom

A dual-level direct dynamic method is employed to study the reaction mechanisms of CF(3)CHFOCF(3) (HFE-227 mc) with the OH radical and Cl atom. The geometries and frequencies of all the stationary points and the minimum energy paths (MEPs) are calculated at the BH&H-LYP/6-311G(d,p) level, and the energetic information along the MEPs is further refined by MC-QCISD theory. The classical energy profile is corrected by the interpolated single-point energies (ISPE) approach, incorporating the small-curvature tunneling effect (SCT) calculated by the variational transition state theory (VTST). The rate constants are in good agreement with the experimental data and are found to be k(1) = 2.87 x 10(-21)T(2.80) exp(-1328.60/T) and k(2) = 3.26 x 10(-16)T(1.65) exp(-4642.76/T) cm(3) molecule(-1) s(-1) over the temperature range 220-2000 K. The standard enthalpies of formation for the reactant CF(3)CHFOCF(3) and product radical CF(3)CFOCF(3) are evaluated via group-balanced isodesmic reactions, and the corresponding values are -454.06 +/- 0.2 and -402.74 +/- 0.2 kcal/mol, respectively, evaluated by MC-QCISD theory based on the BH&H-LYP/6-311G(d, p) geometries. The theoretical studies provide rate constants of the title reactions and the enthalpies of formation of the species, which are important parameters in determining the atmospheric lifetime and the feasible pathways for the loss of HFE-227 mc.

Mechanistic and Kinetic Study of CH2O+O3 Reaction

Both singlet and triplet potential energy surfaces for the reaction of ground-state formaldehyde (CH(2)O) and ozone (O(3)) are theoretically investigated at the BMC-CCSD//BHandHLYP/6-311+G(d,p) level. Various possible isomerization and dissociation pathways are probed. Hydrogen abstraction, oxygen abstraction, and C-addition/elimination are found on both the singlet and the triplet surfaces. The major products for the total reaction are HCO and HOOO, which are generated via hydrogen abstraction. The transition state theory (TST) and multichannel RRKM calculations have been carried out for the total and individual rate constants for determinant channels over a wide range of temperatures and pressures.

Theoretical studies on the mechanisms and dynamics of OH radicals with CH(2)FCF(2)OCHF(2) and CH(2)FOCH(2)F.

The mechanisms and dynamics studies of the multichannel reactions of CH(2)FCF(2)OCHF(2) + OH (R1) and CH(2)FOCH(2)F + OH (R2) have been carried out theoretically. Three hydrogen abstraction channels and two displacement processes are found for reaction R1, whereas there are two hydrogen abstraction channels and one displacement process for reaction R2. The minimum energy paths are optimized at the B3LYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies (ISPE) method at the BMC-QCISD level of theory. By means of canonical variational transition state theory with small-curvature tunneling correction, the rate constants of reactions R1 and R2 are obtained over the temperature range of 220-2000 K. The rate constants are in good agreement with the experimental data for reaction R1 and estimated data for reaction R2. The Arrhenius expression k(1) = 1.62 x 10(-20) T(2.75) exp(-1011/T) for reaction R1 and k(2) = 3.40 x 10(-21) T(3.04) exp(-384/T) for reaction R2 over 220-2000 K are obtained. Furthermore, to further reveal the thermodynamics properties, the enthalpies of formation of reactants CH(2)FCF(2)OCHF(2), CH(2)FOCH(2)F, and the product radicals CHFCF(2)OCHF(2), CH(2)FCF(2)OCF(2), and CHFOCH(2)F are calculated by using isodesmic reactions.

Mechanistic and Kinetic Study of CF3CH=CH2 + OH Reaction

The potential energy surfaces of the CF(3)CH═CH(2) + OH reaction have been investigated at the BMC-CCSD level based on the geometric parameters optimized at the MP2/6-311++G(d,p) level. Various possible H (or F)-abstraction and addition/elimination pathways are considered. Temperature- and pressure-dependent rate constants have been determined using Rice-Ramsperger-Kassel-Marcus theory with tunneling correction. It is shown that IM1 (CF(3)CHCH(2)OH) and IM2 (CF(3)CHOHCH(2)) formed by collisional stabilization are major products at 100 Torr pressure of Ar and in the temperature range of T < 700 K (at P = 700 Torr with N(2) as bath gas, T ≤ 900 K), whereas CH(2)═CHOH and CF(3) produced by the addition/elimination pathway are the dominant end products at 700-2000 K. The production of CF(3)CHCH and CF(3)CCH(2) produced by hydrogen abstractions become important at T ≥ 2000 K. The calculated results are in good agreement with available experimental data. The present theoretical study is helpful for the understanding the characteristics of the reaction of CF(3)CH═CH(2) + OH.

Theoretical Investigation of the Reaction of CF3CHFOCH3 with OH Radical

A direct ab initio dynamics method was used to study the mechanism and kinetics of the reaction CF(3)CHFOCH(3) + OH. Two reaction channels, R1 and R2, were found, corresponding to H-abstraction from a CH(3) group and a CHF group, respectively. The potential energy surface (PES) information was obtained at the G3(MP2)//MP2/6-311G(d,p) level. The standard enthalpies of formation for the reactant (CF(3)CHFOCH(3)) and products (CF(3)CHFOCH(2) and CF(3)CFOCH(3)) were evaluated via isodesmic reactions at the same level. Furthermore, the rate constants of two channels were calculated using the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions over a wide temperature range of 200-3000 K. The dynamic calculations demonstrate that reaction R1 dominates the overall reaction when the temperature is lower than 800 K whereas reaction R2 becomes more competitive in the higher temperature range. The calculated rate constants and branching ratios are both in good agreement with the available experimental values.

Theoretical study on the gas phase reaction of allyl alcohol with hydroxyl radical.

The complex potential energy surface of allyl alcohol (CH2CHCH2OH) with hydroxyl radical (OH) has been investigated at the G3(MP2)//MP2/6-311++G(d,p) level. On the surface, two kinds of pathways are revealed, namely, direct hydrogen abstraction and addition/elimination. Rice-Ramsperger-Kassel-Marcus theory and transition state theory are carried out to calculate the total and individual rate constants over a wide temperature and pressure region with tunneling correction. It is predicted that CH2CHOHCH2OH (IM1) formed by collisional stabilization is dominate in the temperature range (200-440 K) at atmospheric pressure with N2 (200-315 K at 10 Torr Ar and 100 Torr He). The production of CH2CHCHOH + H2O via direct hydrogen abstraction becomes dominate at higher temperature. The kinetic isotope effect (KIE) has also been calculated for the title reaction. Moreover, the calculated rate constants and KIE are in good agreement with the experimental data.

Theoretical study for the reaction of CH3CN with O(P3)

The low-lying triplet and singlet potential energy surfaces of the O((3)P)+CH(3)CN reaction have been studied at the G3(MP2)//B3LYP/6-311+G(d,p) level. On the triplet surface, six kinds of pathways are revealed, namely, direct hydrogen abstraction, C-addition/elimination, N-addition/elimination, substitution, insertion, and H-migration. Multichannel Rice-Ramsperger-Kassel-Marcus theory and transition-state theory are employed to calculate the overall and individual rate constants over a wide range of temperatures and pressures. It is predicted that the direct hydrogen abstraction and C-addition/elimination on triplet potential energy surface are dominant pathways. Major predicted end products include CH(3)+NCO and CH(2)CN+OH. At atmospheric pressure with Ar and N(2) as bath gases, CH(3)C(O)N (IM1) formed by collisional stabilization is dominated at T<700 K, whereas CH(3) and NCO produced by C-addition/elimination pathway are the major products at the temperatures between 800 and 1500 K; the direct hydrogen abstraction leading to CH(2)CN+OH plays an important role at higher temperatures in hydrocarbon combustion chemistry and flames, with estimated contribution of 64% at 2000 K. Furthermore, the calculated rate constants are in good agreement with available experimental data over the temperature range 300-600 K. The kinetic isotope effect has also been calculated for the triplet O((3)P)+CH(3)CN reaction. On the singlet surface, the atomic oxygen can easily insert into C-H or C-C bonds of CH(3)CN, forming the insertion intermediates s-IM8(HOCH(2)CN) and s-IM5(CH(3)OCN) or add to the carbon atom of CN group in CH(3)CN, forming the addition intermediate s-IM1(CH(3)C(O)N); both approaches were found to be barrierless. It is indicated that the singlet reaction exhibits a marked difference from the triplet reaction. This calculation is useful to simulate experimental investigations of the O((3)P)+CH(3)CN reaction in the singlet state surface.

Theoretical Studies of the Reactions CFxH3−xCOOR+Cl and CF3COOCH3+OH

The mechanism and kinetics of the reactions of CF(3)COOCH(2)CH(3), CF(2)HCOOCH(3), and CF(3)COOCH(3) with Cl and OH radicals are studied using the B3LYP, MP2, BHandHLYP, and M06-2X methods with the 6-311G(d,p) basis set. The study is further refined by using the CCSD(T) and QCISD(T)/6-311++G(d,p) methods. Seven hydrogen-abstraction channels are found. All the rate constants, computed by a dual-level direct method with a small-curvature tunneling correction, are in good agreement with the experimental data. The tunneling effect is found to be important for the calculated rate constants in the low-temperature range. For the reaction of CF(3)COOCH(2)CH(3) +Cl, H-abstraction from the CH(2) group is found to be the dominant reaction channel. The standard enthalpies of formation for the species are also calculated. The Arrhenius expressions are fitted within 200-1000 K as kT(1) =8.4×10(-20) T (2.63) exp(381.28/T), kT(2) =2.95×10(-21) T (3.13) exp(-103.21/T), kT(3) =1.25×10(-23) T (3.37) exp(791.98/T), and kT(4) =4.53×10(-22) T (3.07) exp(465.00/T).

Theoretical study on the kinetics of OH radical reactions with CH3OOH and CH3CH2OOH

The mechanisms and kinetics studies of the OH radical with alkyl hydroperoxides CH3OOH and CH3CH2OOH reactions have been carried out theoretically. The geometries and frequencies of all the stationary points are calculated at the UBHandHLYP/6‐311G(d,p) level, and the energy profiles are further refined by interpolated single‐point energies method at the MC‐QCISD level of theory. For two reactions, five H‐abstraction channels are found and five products (CH3OO, CH2OOH, CH3CH2OO, CH2CH2OOH, and CH3CHOOH) are produced during the above processes. The rate constants for the CH3OOH/CH3CH2OOH + OH reactions are corrected by canonical variational transition state theory within 250–1500 K, and the small‐curvature tunneling is included. The total rate constants are evaluated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained.

Theoretical Study on the Reactions of (CF3)2CFOCH3 + OH/Cl and Reaction of (CF3)2CFOCHO with Cl Atom

Reactions of (CF3)2CFOCH3 and (CF3)2CFOCHO with hydroxyl radical and chlorine atom are studied at the B3LYP and BHandHLYP/6-311+G(d,p) levels along with the geometries and frequencies of all stationary points. This study is further refined by CCSD(T) and QCISD(T)/6-311+G(d,p) methods in the minimum energy paths. For the reaction (CF3)2CFOCH3 + OH, two hydrogen abstraction channels are found. The total rate constants for the reactions (CF3)2CFOCH3 + OH/Cl and (CF3)2CFOCHO + Cl are followed by means of the canonical variational transition state with the small-curvature tunneling correction. The comparison between the hydrogen abstraction rate constants by hydroxyl and chlorine atom is discussed. Calculated rate constants are in reasonable agreement with the available experiment data. The standard enthalpies of formation for the reactants, (CF3)2CFOCH3 and (CF3)2CFOCHO, and two products, (CF3)2CFOCH2 and (CF3)2CFOCO, are evaluated by a series of isodesmic reactions. The Arrhenius expressions for the title reactions are given as follows: k1= 1.08 × 10(-22) T(3.38) exp(-213.31/T), k2= 3.55 × 10(-22) T(3.61) exp(-240.26/T), and k3= 3.00 × 10 (-19) T(2.58) exp(-1294.34/T) cm(3) molecule(-1) s(-1).