Interpolated Single-point Energies Research Articles

Atmospheric chemistry of methoxyflurane: Reaction with Cl atoms and further degradation

AbstractA dual‐level direct dynamics technique is used to explore the kinetic properties of the reaction of Cl atoms with methoxyflurane (CH3OCF2CHCl2). The energy profiles of two reaction channels of the two conformers are refined with the interpolated single‐point energies (ISPE) method at the CCSD(T)/M06‐2X level. The canonical variational transition state theory (CVT) with a small‐curvature tunneling (SCT) correction is used to assess the rate coefficients over a wide temperature range (250–500 K) in order to get accurate results. At a temperature of 296 K, the calculated CVT/SCT rate coefficient kCl = 2.71 × 10−13 cm3 molecule−1 s−1 is found to be in reasonably good agreement with the experimental result. Our calculations demonstrate that CH3OCF2CHCl2 + Cl → CH2OCF2CHCl2 + HCl (R1) reaction is the key pathway and a prominent conduit for CH3OCF2CHCl2 degradation in the troposphere. The atmospheric loss of C•H2OCF2CHCl2 and CH3OCF2C•Cl2 radicals are discussed. The rate coefficient value of the oxidation of the primary product HC(O)OCF2CHCl2 with the Cl atoms is 1.8 × 10−14 cm3 molecule−1 s−1, and for the first time, the rate coefficient of HC(O)OCF2CHCl2 + OH reaction has also been reported here.

Kinetics of IO radicals with C1, C2 aliphatic alcohols in tropospherically relevant conditions.

Kinetics of the reaction of IO radicals with methanol (MeOH) and ethanol (EtOH) were experimentally studied in the gas phase using pulsed laser photolysis-cavity ring-down spectroscopy (PLP-CRDS). IO radicals were produced in situ at the reaction zone by photolysing a mixture of precursors (CH3I + O3 + N2) at 248nm and thereby electronically excited at 445.04nm. The rate coefficients for the reactions of (IO + MeOH) and (IO + EtOH) were measured at a total pressure of 60Torr/N2 in the range of 258-360K. At room temperature, the experimental rate coefficients of the title reactions were measured to be [Formula: see text] and [Formula: see text]. Dependencies of the kinetics with photolysis laser fluence and experimental pressures were verified. Effects of pressure over the kinetic behaviour of the studied systems were observed to be insignificant within the statistical uncertainties when studied in the range of ~ 30-150Torr/N2, whereas a minor and linear fluence dependency was observed within the studied limit. From the measured kinetic parameters, the atmospheric lifetimes of MeOH and EtOH were calculated in the tropospherically relevant conditions regarding their reactions with important atmospheric oxidants like Cl atom, OH and IO radicals. To complement experimental results, kinetics and thermochemistry for the title reactions were investigated theoretically via canonical variational transition state (CVT) theory in combination with small curvature tunnelling (SCT) corrections with a dual-level Interpolated Single Point Energy (ISPE) approach at the CCSD(T)/def2-QZVPP//M06-2X/def2-TZVPP level of theory/basis set in the temperatures between 200 and 400K. Good degree of agreement was encountered between experimentally measured and theoretically calculated rate coefficients. This article also discusses the thermochemical parameters and kinetic branching ratios (BRs) of all the pathways involved in the title reactions.

Theoretical investigations on the OH radical mediated kinetics of cis- and trans-CH3CF=CHF and CH3CH=CF2 over temperature range of 200-400K

Global reactivity for the OH-radical mediated kinetics of three Hydrofluoroolefins (HFOs), cis-CH3CF=CHF, trans-CH3CF=CHF and CH3CH=CF2 was evaluated at the CCSD(T)/cc-pVTZ//M062x/6-31G+(d,p) level of theory. The overall kinetics was computed using the Canonical Variational Transition State Theory (CVT) in combination with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE) corrections over the temperature range of 200 and 400 K. The total rate coefficients (k, in units of cm3 molecule−1 sec−1) for the reaction of OH radical with these HFOs at 298 K were computed to be, k298Kcis−CHF=CFCH3+OH=1.69×10−12, k298Ktrans−CHF=CFCH3+OH=3.12×10−12, and k298KCF2=CHCH3+OH=5.40×10−12. To further corroborate the computed kinetics, thermodynamic parameters and branching ratios were also computed for all the reaction pathways. Atmospheric lifetimes, Radiative forcing (RF), Global Warming Potentials (GWP) and Photochemical Ozone Creation Potentials (POCP) of these HFOs were also calculated and are presented in this manuscript.

Reaction kinetics of a series of alkanes with ClO and BrO radicals: A theoretical study

AbstractReactive halogen species (ClO and BrO) play a critical role in the stratospheric ozone depletion. However, the reactivity of these radical moieties with volatile organic compounds is also important in determining the composition of the Earth's atmosphere. In this study, the reactions of BrO and ClO radicals with a series of smaller linear alkanes (CnH2n+2, n = 1,2,3, and 4) were studied using the CCSD(T)/CBS//B3LYP/cc‐pVTZ level of theory. The kinetics of the title reactions were evaluated using the Canonical Variational Transition (CVT)/Small Curvature Tunneling (SCT)/Interpolated Single Point Energy (ISPE) methodology over the studied temperature range of 200‐3000 K. H‐abstraction from the nonterminal carbon was found to be thermodynamically and kinetically favorable in comparison to abstraction of H atom from the primary carbon. On a per‐hydrogen basis at 298 K, BrO and ClO are focused to abstract the secondary hydrogens of C3H8 faster than the primary hydrogens by factors of 40 and 110, respectively. On the other hand, for n‐C4H10 at the same temperature, the per‐hydrogen preferences for secondary H atoms are found to be factors of 220 and 270 for BrO and ClO, respectively. To aid in modeling the stratospheric environment and combustion regime, three parameter fits to the computed rate constants between 200 and 3000 K are provided. The branching ratios and thermochemical parameters for the various reaction channels are also discussed in this paper.

Pilgrim: A thermal rate constant calculator and a chemical kinetics simulator

Pilgrim is a program written in Python and designed to use direct dynamics in the calculation of thermal rate constants of chemical reactions by the variational transition state theory (VTST), based on electronic structure calculations for the potential energy surface. Pilgrim can also simulate reaction mechanisms using kinetic Monte Carlo (KMC).For reaction processes with many elementary steps, the rate constant of each of these steps can be calculated by means of conventional transition state theory (TST) or by using VTST. In the current version, Pilgrim can evaluate thermal rates using the canonical version of reaction-path VTST, which requires the calculation of the minimum energy path (MEP) associated with each elementary step or transition structure. Multi-dimensional quantum effects can be incorporated through the small-curvature tunneling (SCT) approximation. These methodologies are available both for reactions involving a single structure of the reactants and the transition state and also for reactions involving flexible molecules with multiple conformations of the reactant and/or of the transition state. For systems with many conformers, the program can evaluate each of the elementary reaction rate constants by multipath canonical VTST or multi-structural VTST. Moreover, the reactant can be unimolecular or bimolecular.Torsional anharmonicity can be incorporated through either the MSTor or the Q2DTor programs. Dual-level calculations are also available in Pilgrim: automatic high-level single-point energies can be used to correct the energy of reactants, transition states, products, and MEP points using the interpolated single-point energies (ISPE) algorithm.When the rate constants of all the chemical processes of interest are known, by means of their calculation using Pilgrim or alternatively through analytical fits to the rate constants as functions of temperature, it is possible to simulate a multistep mechanism under specified laboratory conditions using KMC. This algorithm allows performing a kinetic simulation to monitor the evolution of each chemical species with time and obtain the product yields. Program summaryProgram Title: PilgrimCPC Library link to program files:http://dx.doi.org/10.17632/24cj4dwxvg.1Developer’s repository link:https://github.com/cathedralpkg/pilgrim/releases; https://comp.chem.umn.edu/pilgrim; https://conservancy.umn.edu/handle/11299/166578Licensing provisions: MITProgramming language: Python 3Nature of problem: Calculation of thermal rate constants for bimolecular and unimolecular chemical reactions and simulation of reaction mechanismsSolution method: The program uses variational transition state theory to calculate thermal rate constants and kinetic Monte Carlo to simulate reaction mechanisms.Restrictions and unusual features: The program cannot treat reactions without saddle points. Unimolecular reactions are calculated only in the high-pressure limit. Direct dynamics calculations with Pilgrim require an electronic structure package to be supplied by the user; currently, Pilgrim supports the Gaussian [Frisch et al. (2003), Frisch et al. (2016a), Frisch et al. (2016b)] and Orca [Neese (2011)] electronic structure packages. Pilgrim has an especially powerful suite of options for handling torsional anharmonicity and multistructural effects.

Tropospheric chemistry of ethyl tiglate initiated by Cl atoms

Temperature dependent rate coefficients for the reaction of ethyl tiglate (ET) with Cl atoms, were investigated using relative rate method. The rate coefficients were calculated using Canonical Variational Transition State Theory incorporated with Small Curvature Tunneling (CVT/SCT) coupled with interpolated single point energies (ISPE) method over the temperature range of 200–400 K. The thermochemistry, branching ratios for each reaction channels, atmospheric effects and cumulative lifetime of ET were also calculated and discussed. The possible degradation mechanism for the title reactions were also explored.

Cl-Initiated Photo-oxidation Studies of Methyl Valerate and Methyl Isovalerate under Tropospherically Relevant Conditions.

Rate coefficients for the reactions of methyl valerate (MV) and methyl isovalerate (MIV) were investigated using the relative rate (RR) technique in the temperature range of 268-363 K and at 760 Torr of N2. The rate coefficients at 298 ± 2 K were measured to be kMV+Cl = (1.69 ± 0.40) × 10-10 cm3 molecule-1 s-1 and kMIV+Cl = (1.06 ± 0.23) × 10-10 cm3 molecule-1 s-1. To comprehend the experimentally measured rate coefficients, kinetic parameters for the title reactions were computed at the CCSD(T)/cc-pVTZ//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and the interpolated single-point energy (ISPE) method. Additionally, the degradation mechanisms were proposed based on the analyzed products. Moreover, to further understand the atmospheric fate of these molecules in the troposphere, atmospheric lifetimes, radiative forcings (RFs), and global warming potentials (GWPs) were calculated.

Cl Atom-Initiated Photo-Oxidation Reactions of Vinyl Trifluoroacetate and Allyl Trifluoroacetate in Tropospheric Conditions.

Relative rate (RR) technique coupled with gas chromatography (GC) was used to investigate the kinetics of the reactions of two hydrofluoroesters (allyl trifluoroacetate (CF3C(O)OCH2CH═CH2, ATFA) and vinyl trifluoroacetate (CF3C(O)OCH═CH2, VTFA)) with Cl atoms between 268 and 363 K and at 760 Torr of N2 (or air). The temperature-dependent Arrhenius expressions for the title reactions were obtained to be k268-363KVTFA+Cl = [(7.83 ± 2.26) × 10-12 exp((974 ± 89)/T)] cm3 molecule-1 s-1 and k268-363KATFA+Cl = [(9.03 ± 1.92) × 10-12 exp((883 ± 65)/T)] cm3 molecule-1 s-1, respectively. A negative temperature dependency was observed for both the reactions. In addition to this, the kinetics of the studied reactions was evaluated computationally at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory in the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and interpolated single point energy (ISPE) methods. The product analysis of the reactions of VTFA and ATFA with Cl atoms in the presence of O2 was also investigated using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). The plausible oxidation mechanism of the title reactions was proposed based on the product analyses. Further, to comprehend the impact of these molecules on the troposphere, atmospheric lifetimes, lifetime-corrected radiative forcing (RF), and global warming potential (GWP) were estimated and are presented in this manuscript.

Oxidative Degradation Kinetics and Mechanism of Two Biodiesel Constituents (Ethyl-2-Methyl Propionate (E2MP) and Ethyl-2,2-Dimethyl Propionate (E22DMP)) Initiated by Cl Atoms

The oxidative reaction kinetics and mechanism of Ethyl-2-methyl propionate (E2MP) and Ethyl-2, 2-dimethyl propionate (E22DMP) initiated by Cl atoms were investigated both experimentally and computationally. Temperature dependent kinetic studies on the reaction of E2MP and E22DMP with Cl atoms were measured using relative rate technique over the temperature range of 268-363 K and at 760 Torr. The rate coefficients of both the reactions at 298 K were measured (using IUPAC recommended reference rate coefficients) to be k_(E2MP+Cl)^(298 K)= (7.19±1.60) × 10-11 cm3 molecule-1 s-1 and k_(E22DMP+Cl)^(298 K)= (5.87±1.33) × 10-11 cm3 molecule-1 s-1 respectively. In addition to complement our experimental results, the kinetics were computed at CCSD(T)/MG3S//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using Canonical Variational Transition State theory (CVT) in conjunction with Small Curvature Tunneling (SCT) and Interpolated Single Point Energy (ISPE). Furthermore, the product deg...

A theoretical insight on the kinetics for the reaction of (E)-/(Z)-CHF=CF(CF2)x=1,2CF3 with OH radicals under tropospheric conditions

Hydrofluoroolefins (HFOs) are the potential replacements to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). HFOs are well-known alternates to regular refrigerants and foam expansion agents. The aim of this work is to provide comprehensive description on the environmental acceptability and atmospheric impact of some of these HFOs. In this study, OH-radical initiated photo-degradation of (E)-/(Z)-CHF=CF(CF2)x=1,2CF3 was investigated over the temperature range of 200–400 K using CCSD(T)/aug-cc-pVTZ//M062x/6-31+G(d,p) level of theory. The reaction kinetics was computed using the canonical variational transition state theory (CVT) in conjunction with the small curvature tunneling (SCT) and interpolated single-point energy (ISPE) corrections. A negative temperature-dependent Arrhenius behavior was observed for all the HFO + OH reactions. Various fluorinated aldehydes and acids formed via the interaction of the products with O2 and NOx are also reported as part of this manuscript. Short lifespans and low global warming potentials (GWPs) furthermore characterized the importance of these HFOs as propitious replacements to high GWP CFCs.

Temperature-Dependent Kinetics of the Reaction of a Criegee Intermediate with Propionaldehyde: A Computational Investigation.

The temperature-dependent kinetics for the reaction of a Criegee intermediate (CH2OO) with propionaldehyde (CH3CH2CHO) was investigated using canonical variational transition state theory (CVT) in conjunction with the small curvature tunneling (SCT) method and the interpolated single point energy (ISPE) method at the CCSD(T)/AUG-cc-pVTZ//B3LYP/6-311G(d,p) level of theory. A rich chemistry was depicted by the title reaction, though the contributions of all of the reaction pathways were limited to atmospheric pressure conditions. The reaction of CH2OO with CH3CH2CHO was identified to proceed via the formation of secondary ozonide (SOZ), which then underwent a sequence of unimolecular isomerization and decomposition reactions to form a variety of products. The obtained rate coefficient for the formation of SOZ at 298 K was determined to be k = 2.44 × 10-12 cm3 molecule-1 s-1. At low temperature, collisionally stabilized SOZ was found to be the more stable product. Contrarily, at high temperature, SOZ degraded to HCHO, and CH3CH2COOH was found to be the major product. The complete degradation mechanism and thermochemistry for the reaction of CH2OO with CH3CH2CHO along with their rate coefficients over the temperature range of 200-1000 K are reported.

Photo Oxidation Reaction Kinetics of Ethyl Propionate with Cl Atom and Formation of Propionic Acid

Temperature-dependent rate coefficients for the photo oxidation reaction of ethyl propionate with Cl atom were investigated experimentally using the relative rate technique. Gas chromatography with flame ionization detector (GC-FID), gas chromatography-mass spectrometry (GC-MS), and GC-infrared spectroscopy (GC-IR) were used to follow the concentrations and identification of reactants and products. The kinetics of ethyl propionate with Cl atoms was investigated over the temperature range of 263-363 K at atmospheric pressure, relative to C2H6 and C2H4. Theoretical calculations were also performed at CCSD(T)/6-311++G(d,p)//BHandHLYP/6-311G(d,p) level of theory, and the rate coefficients for H-abstraction reactions were calculated using canonical variational transition state theory (CVT) with interpolated single point energies (ISPE) method over the temperature range of 200-800 K. The temperature-dependent rate coefficients for the reaction of ethyl propionate with Cl atom were obtained both experimentally as well as theoretically and are kExpt( T) = [(6.88 ± 1.65) × 10-24] T4.5 exp[(1108 ± 87)/ T] cm3 molecule-1 s-1 and kTheory( T) = (6.73 × 10-19) T2.74 exp[(571)/ T] cm3 molecule-1 s-1, respectively. On the basis of product analysis on the title reaction and the computational studies, we have proposed the atmospheric degradation mechanism and various pathways for Cl atom-initiated photo oxidation of EP. Propionic acid is identified as the major product in the degradation of ethyl propionate on reaction with Cl atom. The thermochemistry, branching ratios, and cumulative lifetime of ethyl propionate are calculated and presented in this Article.

Cl-initiated photo-oxidation reactions of methyl propionate in atmospheric condition.

Cl-initiated photo-oxidation reaction of methyl propionate was investigated experimentally using relative rate method. Gas chromatography/mass spectrometry (GC-MS) and GC/infrared spectroscopy (GC-IR) were used as analytical tools to follow the concentrations of reactants and products during reaction. The gas-phase kinetics of methyl propionate with Cl atoms was measured over the temperature range of 263-363K at 760Torr in N2 atmosphere using C2H6 and C2H4 as reference compounds. The temperature-dependent rate coefficient for the reaction of methyl propionate with Cl atom was obtained as k(T) = [(3.25 ± 1.23) × 10-16] T2 exp [- (33 ± 4)/T] cm3 molecule-1s-1. Theoretical calculations were also performed at CCSD(T)/cc-pVDZ//B3LYP/6-31G(d,p) level of theory, and the rate coefficients for H abstraction reactions were evaluated using canonical variational transition state theory (CVT/SCT) with interpolated single point energy (ISPE) method over the temperature range of 200-400K. The rate coefficients over the studied temperature range yielded the Arrhenius expression k(T) = (7.22 × 10-16) T1.5 exp (466/T) cm3 molecule-1s-1. The reaction mechanism based on product analysis, thermochemistry, branching ratios, atmospheric implications, degradation pathways, and cumulative lifetime of methyl propionate is also presented in this manuscript. Graphical abstract ᅟ.

Kinetic parameters for the reaction of OH radical with cis-CHFCHCHF2, trans-CHFCHCHF2, CF2CHCHF2 and CF2CCHF: Hybrid meta DFT and CVT/SCT/ISPE calculations

A dual-level direct dynamic method is employed to estimate the addition and hydrogen abstraction rate coefficients for the reactions of cis-CHFCHCHF2, trans-CHFCHCHF2, CF2CHCHF2 and CF2CCHF with hydroxyl radicals (OH) using variational transition state theory (VTST) with interpolated single-point energies (ISPE) at CCSD(T)/cc-pVTZ//M06-2X/6-31+G(d,p) level of theory. The rate coefficients of cis-CHFCHCHF2, trans-CHFCHCHF2, CF2CHCHF2 and CF2CCHF + OH reactions were computed using Canonical Variational Transition state Theory (CVT) with Small Curvature Tunneling (SCT) in the temperature range of 200–400 K. It was found that the contribution of abstraction reactions toward the overall reaction to be insignificant, and hence negligible in comparison to the addition reactions in the studied temperature range of 200–400 K. The total rate coefficient for cis-CHFCHCHF2, trans-CHFCHCHF2, CF2CHCHF2 and CF2CCHF + OH reactions were calculated to be 6.83 × 10−13, 3.17 × 10−12, 5.43 × 10−13 and 2.22 × 10−13 cm3 molecule−1 s−1, respectively at 298 K. The atmospheric life times of cis-CHFCHCHF2, trans-CHFCHCHF2CHF, CF2CHCHF2 and CF2CCHF are estimated to be 16, 3.4, 21 and 57 days respectively at 277 K. The global warming potentials (GWPs) of title molecules at the time horizons of 20, 100 and 500 years were computed using the rate coefficients and radiative forcing values obtained in this study. Since these title molecules show significantly lower GWPs than many hydrofluorocarbons (HFCs) and chlorofluorocarbons (CFCs), it is conclude that these hydrofluoro-olefines (HFOs) may be possible replacements to HFCs in many industrial applications.

Theoretical studies on mechanism and kinetics of the hydrogen-abstraction reaction of CF2HCOOCH3 with OH radicals

The hydrogen abstraction reaction of CF2HCOOCH3+OH has been studied theoretically by a dual-level direct dynamics method. The geometries and frequencies of all stationary points are optimized at the M06-2X/aug-cc-pVDZ level, and the energy profile is refined with interpolated single-point energy (ISPE) by the MCG3-MPWB method. Two stable conformers of CF2HCOOCH3 are located, and for each conformer, the possible H-abstraction channels from –CH3 and –CF2H groups are taken into account. The rate constants are calculated using the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT) and are fitted by a three-parameter Arrhenius equation over a wide temperature range. Due to the lack of the kinetic data of these reactions, the present theoretical results are expected to be useful and reasonable to estimate the kinetic data of the reaction over a wide temperature range where no experimental value is available.

Mechanism, kinetics and atmospheric fate of CF3CH[dbnd]CH2, CF3CF[dbnd]CH2, and CF3CF[dbnd]CF2 by its reaction with OH-radicals: CVT/SCT/ISPE and hybrid meta-DFT methods

The dual level direct dynamic study is carried out for the reactions of CF3CHCH2, CF3CFCH2 and CF3CFCF2 with hydroxyl radicals. The dynamic calculations are performed using the variational transition state theory (VTST) with interpolated single-point energies (ISPE) method at M06-2X/MG3S//M06-2X/6-31+G(d,p) level of theory. All the possible reactions such as abstraction and addition–elimination pathways are explored for the title reactions. The temperature dependent rate coefficients using canonical variational transition state theory with small curvature tunneling for the reaction of OH radicals with test molecules over the temperature range of 200–3000K are computed. The predicted rate coefficients (in 10−12cm3molecule−1s−1) using CVT/SCT/ISPE methodology for the reaction of CF3CHCH2, CF3CFCH2 and CF3CFCF2 with OH radicals are 1.48, 1.02 and 1.77, respectively, are in good agreement with reported ones at 298K. The atmospheric lifetimes for the test molecules CF3CHCH2, CF3CFCH2 and CF3CFCF2 are calculated at 277K to be 8, 11 and 6 days, respectively. Global warming potentials are also reported for the different time horizon of 20, 100 and 500 years.

Theoretical study on the reaction of atomic oxygen with unsymmetrical dimethylhydrazine

A dual-level direct dynamics method is employed to study the reaction mechanism of (CH3)2NNH2 (unsymmetrical dimethylhydrazine) with the oxygen (O) atom. The geometries and frequencies of all the stationary points are optimized at the MPW1K/6-311G (d, p) level, and the energy profiles are further refined by the interpolated single-point energies (ISPE) method at the BMC-CCSD level of theory. The rate constants of the O atom with (CH3)2NNH2 are evaluated over a wide temperature range of 200–2000 K by using the canonical variational transition-state theory (CVT) with the small curvature tunneling correction (SCT). The agreement between the theoretical and experimental rate constants is good around room temperature. The channels of H abstraction from the -NH2 position favor temperatures below 1200 K. With increasing temperature, contributions from other channels should be taken into account. The reactivity of N2H4, CH3NHNH2, and (CH3)2NNH2 toward atomic O is compared to explore the methylation effect.

Dual-level direct dynamics study on the hydrogen abstraction reaction of fluorine atom with 1,1-difluoro-1-chloroethane

The kinetic properties of the reaction of F atoms with CH2H′CF2Cl are investigated by a dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are obtained at the MPW1K/6–311+G(d,p) level of theory. Two complexes with energy less than that of the reactants are located in the two reactant paths, respectively. The energy profiles of two reactions are refined with the interpolated single-point energies (ISPE) method at the G3(MP2)/MPW1K level. The rate constants are evaluated using the canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide range of temperature 200–2000 K. Agreement between the calculated CVT/SCT rate constant and the experimental value is good at 295 K. Our calculations show that the reaction path CH2H′CF2Cl + F → CH2CF2Cl + H′F (Ra) is the major reaction path below 400 K. Moreover, the contribution of CH2H′CF2Cl + F → CHH′CF2Cl + HF (Rb) to the whole reaction increases with the temperature increasing and exceeds path Ra to be the major reaction path.

Direct dynamics studies for the reactions of CF3CHFCF3 and CF3CF2CHF2 with H atoms

The rate constants of the hydrogen abstraction reactions of CF3CHFCF3+H (R1) and CF3CF2CHF2+H (R2) have been calculated by means of the dual-level direct dynamics method. Optimized geometries and frequencies of stationary points and extra points along the minimum-energy path (MEP) are obtained at the MPW1K/6-311+G(d,p) level, and the classical energetic information is further corrected with the interpolated single-point energy (ISPE) approach by the G3(MP2) level of theory. Using the canonical variational transition state theory (CVT) with small-curvature tunneling corrections (SCT), the rate constants are evaluated over a wide temperature range of 200–2000K. The calculated CVT/SCT rate constants are in good agreement with available experimental values. It is found that the variational effect is very small and almost negligible over the whole temperature region. However, the small-curvature tunneling correction plays an important role in the lower temperature range. Furthermore, the heats of formation of species CF3CF2CHF2 (SC1 or SC2) and CF3CF2CF2 are studied using isodesmic reactions to further elucidate the thermodynamic properties.

Dual-level direct dynamics studies on the hydrogen abstraction reactions of fluorine atoms with CF3CH2X(X=F, Cl)

The kinetic properties of the hydrogen abstraction reactions of CF3CH2F + F → CF3CHF + HF (R1) and CF3CH2Cl + F → CF3CHCl + HF (R2) have been studied by dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) were obtained at the B3LYP/6-311 + G(2d,2p) level. Two complexes with energies less than that of the reactants were located in the reactant side of each reaction. The energy profiles were further refined with the interpolated single-point energies (ISPE) method at the G3(MP2) level of theory. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants were evaluated over a wide temperature range of 200–2,000 K. Our calculations have shown that C–H bond activity decreases when one hydrogen atom of CF3CH3 is substituted by a fluorine atom, than when substituted with a chlorine atom. This is in good agreement with the experimental results.