Small-curvature Tunneling Correction Research Articles

The atmospheric chemical reaction of 4-tert-butylphenol initiated by OH radicals

Environmental context 4-tert-Butylphenol, an environmental endocrine disruptor, can be taken in by humans and animals resulting in reproductive and developmental problems. We report a theoretical study on the degradation mechanism of 4-tert-butylphenol in the atmosphere, and calculate the atmospheric lifetime of this chemical. The data will help our understanding of the behaviour of 4-tert-butylphenol in the environment and thereby provide valuable information about its possible effect on human health. Abstract 4-tert-Butylphenol (TBP) is a typical environmental endocrine. In this paper, the OH-initiated degradation mechanism of TBP in the atmosphere is studied at the MPWB1K/6-31+G(d,p)//MPWB1K/6-311+G(3df,2p) level of computational theory. A profile of the potential energy surface is constructed and reaction pathways are analysed. The addition reactions of TBP with OH radicals are more important than abstraction reactions in the atmosphere. In subsequent reactions, O2 and NO may play an important role in the degradation process of TBP. The rate constants are calculated using the transition state theory and a canonical variational transition with small-curvature tunnelling correction. The Arrhenius equations of rate constants in the temperature range of 200–500K are fitted. The rate constant of the degradation of the TBP at 298.15K is 3.56×10–14cm3 molecule–1s–1 and the atmospheric lifetime is 10.8 months according to the pseudo-first-order kinetics.

Mechanisms and kinetics of hydrogen abstraction of methylhydrazine and deuterated methylhydrazine with H/D atoms

The mechanisms of multi-channel reactions (1) CH3NHNH2 + H → products (R1) and (2) (CH3)2NNH2 + H → products (R2) have been investigated, and the dynamic properties are studied by using the dual-level direct dynamics method. The electronic structures and energies of the stationary points are calculated by the combination of various methods and basis sets. The rate constants of reactions are evaluated in a temperature range of 200–2,000 K by using the canonical variational transition state theory with a small-curvature tunneling correction at the MCG3-MPWPW91//MPW1K/6-311G(d,p) level. The effect of treating torsional anharmonicity with different models on the rate constants is compared. Theoretical and experimental studies indicate that substitution of H by –CH3 group on NH2NH2 leads to a significant rate coefficients increase. The kinetic isotope effects are studied for both the methylhydrazine with hydrogen/deuterium and the methylhydrazine/deuterated methylhydrazine with hydrogen atom.

Theoretical study and rate constant calculations for the reactions of SiHX3 with CF3 and CH3 radicals (X = F, Cl)

Theoretical investigations were carried out on the multi-channel reactions CF3 + SiHF3, CF3 + SiHCl3, CH3 + SiHF3, and CH3 + SiHCl3. Electronic structures were calculated at the MP2/6-311+G(d,p) level, and energetic information further refined by the MC-QCISD (single-point) method. The rate constants for major reaction channels were calculated by the canonical variational transition state theory with small-curvature tunneling correction over the temperature range of 200-1,500 K. The theoretical rate constants were in good agreement with the available experimental data and were fitted to the three parameter expression: k 1a(T) = 2.93 × 10(-26) T (4.25) exp (-318.68/T), and k 2a(T) = 3.67 × 10(-22) T (2.72) exp (-1,414.22/T), k 3a (T) = 7.00 × 10(-24) T (3.27) exp (-384.04/T), k 4a(T) = 6.35 × 10(-22) T (2.59) exp (-603.18/T) (in unit of cm(3)molecule(-1)s(-1)) are given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smaller barrier height among four channels considered.

Theoretical studies on mechanism and kinetics of the hydrogen-abstraction reaction of CF3CH2CH2OH with OH radical

The hydrogen abstraction reaction of CF3CH2CH2OH+OH has been studied theoretically by dual-level direct dynamics method. The required potential energy surface information for the kinetic calculation was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. Five stable conformers of CF3CH2CH2OH have been located. For each conformer, there are three potential H-abstraction sites (Cα, Cβ and –OH), and some of the H atoms can be abstracted by more than one abstraction channel due to the different attack orientations of the incoming OH radical. As a result, 31 distinct Habstraction channels have been identified for the reaction. The individual rate constants for each Habstraction channel were calculated by the improved canonical transition-state theory with smallcurvature tunneling correction (ICVT/SCT), and the overall rate constant was evaluated by the Boltzmann distribution function. It is shown that the calculated rate constant is in good agreement with the available experimental data at 298K, and exhibits negative temperature dependence with 200–350K. H-abstraction from the α site dominates the reaction at low temperatures, while the contributions from the β and OH abstractions should be taken into account as temperature increases. The fitted four-parameter expressions within 200–1000K for the overall rate constants as well as the rate constants from the α, β and OH abstractions were given to provide good estimation for future laboratory investigations. In addition, because of the lack of available experimental data for the product radicals involved in the reactions, their enthalpies of the formation (ΔHf,298°) were predicted via isodesmic reaction at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level.

Theoretical Studies on the Mechanisms and Dynamics of OH Radical with (CH3)3COOH and (CH3)2CHOOH

A dual-level direct dynamic method is employed to study the reaction mechanism of hydroxyl radical with (CH(3))(3)COOH and (CH(3))(2)CHOOH. Eight hydrogen abstraction channels are found for title reactions. The energy paths are optimized at the BH&H-HLYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies method at the CCSD(T) and QCISD(T) theories. Rate coefficients for the reactions of the OH with (CH(3))(3)COOH/(CH(3))(2)CHOOH are computed by the canonical variational transition-state theory with the small-curvature tunneling correction between 200 and 2000 K. The Arrhenius expressions k(1) (T) = 1.49 × 10(-26) T(4.71) exp(1981.92/T) and k(2) (T) = 1.58 × 10(-20) T(3.32) exp(210.59/T) over 200-2000 K are obtained.

Mechanistic study and kinetic properties of the CF3CHO + Cl reaction

Theoretical investigations have been carried out on the mechanism and kinetics for the reaction of CF3CHO + Cl using duallevel direct dynamics method. The potential energy surface information was obtained at the MC-QCISD/3//MP2/cc-pVDZ level and the kinetic calculations were done using variational transition state theory with interpolated single-point energy (VTST-ISPE) approach. The calculated results show that the reaction proceeds primarily via the H-abstraction channel, while the Cl-addition channel is unfavorable due to the higher barriers. The improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT) was used to calculate the rate constants. The theoretical rate constants at room temperature are in general agreement with the experimental values. A three-parameter rate constant expression was fitted over a wide temperature range of 200–2000 K.

Rate constants calculation of hydrogen abstraction reactions CH3CHBr + HBr and CH3CBr2 + HBr

AbstractDual‐level direct dynamics method is used to study the kinetic properties of the hydrogen abstraction reactions of CH3CHBr + HBr → CH3CH2Br + Br (R1) and CH3CBr2 + HBr → CH3CHBr2 + Br (R2). Optimized geometries and frequencies of all the stationary points and extra points along the minimum‐energy path are obtained at the MPW1K/6‐311+G(d,p), MPW1K/ma‐TZVP, and BMK/6‐311+G(d,p) levels. Two complexes with energies less than that of the reactants are located in the entrance of each reaction at the MPW1K/6‐311+G(d,p) and MPW1K/ma‐TZVP levels, respectively. The energy profiles are further refined with the interpolated single‐point energies method at the G2M(RCC5)//MPW1K/6‐311+G(d,p) level of theory. By the improved canonical variational transition‐state theory with the small‐curvature tunneling correction (SCT), the rate constants are evaluated over a wide temperature range of 200–2000 K. Our calculations have shown that the radical reactivity decreases from CH3CHBr to CH3CBr2. Finally, the total rate constants are fitted by two modified Arrhenius expression. © 2012 Wiley Periodicals, Inc.

Theoretical studies on atmospheric reactions of CH2FO2 with HO2 and HO2⋅H2O complex

The formations of CHFO, 1O2, O3 and 3O2 in CH2FO2+HO2 and CH2FO2+HO2⋅H2O reaction are studied by employing the quantum chemical calculations with B3LYP and CCSD(T) theoretical methods, canonical variational transition (CVT) state theory with small curvature tunneling (SCT) correction. The calculated results show that the formations of CHFO and 3O2 are main products in the naked reaction of CH2FO2+HO2. When water is added, the formations of CHFO and O3 are main products in CH2FO2+HO2⋅H2O reaction. Moreover, the calculated rate constants for the title reaction without and with a water molecule show that, although the single water molecule plays a positive catalytic effect on enhancing the rate for CHFO and O3 formation, in humid conditions the effective rate of CH2FO2+HO2 reaction will changes little with respect to dry conditions.

Theoretical study of the reaction mechanism and kinetics of low-molecular-weight atmospheric aldehydes (C1–C4) with NO2

Accurate description of atmospheric reactions of a series of low-molecular-weight (LMW) aldehydes (C1–C4) with NO2 has been modeled using a direct dynamic approach. The profiles of the potential energy surface were constructed at the BMC-CCSD//MPWB1K/6-311G(d,p) level of theory, and two different pathways have been found: H-abstraction and NO2-addition. The modeling results found that the contribution of NO2-addition reaction pathway to the total rate constant is very small and thus this kind of pathway is insignificant in atmospheric conditions. The predicted H-abstraction products are mainly reactive acyl radical and nitrous acid (HONO) which is very mutagenic and carcinogenic pollutant as well as the precursor of acid deposition. The rate constants of both pathways were also deduced by using canonical variational transition state theory incorporating with the small curvature tunneling correction within 200–360 and 360–2000 K. Theoretical overall rate constants are in good agreement with the available experimental values, whose increase in the order of kformaldehyde < kacetaldehyde < kpropanal < kbutanal, implying that relative long-chain LMW aldehydes are more reactive toward NO2 than those short-chain LMW aldehydes in the atmospheric condition. At 298 K, the total rate constants of LMW aldehydes (C1–C4) with NO2 are obtained as 1.65 × 10−25, 1.43 × 10−24, 3.39 × 10−24 and 1.83 × 10−23 cm3 molecule−1 s−1, respectively.

Theoretical study for the CH3OCF2CF2OCHO + Cl reaction

The reaction of CH3OCF2CF2OCHO with Cl atom has been investigated theoretically by direct dynamics method. The BB1K hybrid functional in conjunction with the 6-31 + G(d,p) basis set has been used to optimize the geometries for the stationary points and explore the potential energy surface of the reaction. Four rotation conformers (RC1-4) of CH3OCF2CF2OCHO are identified, and they are all considered in the kinetic calculation. For each conformer, there are two kinds of H-abstraction channels and one displacement channel, and the latter one should be negligible due to involving much higher energy barrier than the former two. The individual rate constants for each H-abstraction channel are evaluated by the improved canonical variational transition-state theory with a small-curvature tunneling correction. The overall rate constant is evaluated by the Boltzmann distribution function, and a fitted four-parameter rate constant expression is obtained over a wide temperature range of 200–2,000 K. The agreement between the calculated and available experimental value at 296 K is good. The contribution of each conformer to the title reaction is discussed with respect to the temperature. In addition, because of the lack of available experimental data for the species involved in the reactions, the enthalpies of the formation (ΔHf,298°) for the reactant and its product radicals are predicted via isodesmic reaction at the BB1K/6-31 + G(d,p) level.

Kinetics of levoglucosan and formaldehyde formation during cellulose pyrolysis process

The mechanisms and kinetics studies of the formation of levoglucosan and formaldehyde from anhydroglucose radical have been carried out theoretically in this paper. The geometries and frequencies of all the stationary points are calculated at the B3LYP/6-31+G(D,P) level based on quantum mechanics, Six elementary reactions are found, and three global reactions are involved. The variational transition-state rate constants for the elementary reactions are calculated within 450–1500K. The global rate constants for every pathway are evaluated from the sum of the individual elementary reaction rate constants. The first-order Arrhenius expressions for these six elementary reactions and the three pathways are suggested. By comparing with the experimental data, computational methods without tunneling correction give good description for Path1 (the formation of levoglucosan); while methods with tunneling correction (zero-curvature tunneling and small-curvature tunneling correction) give good results for Path2 (the first possibility for the formation of formaldehyde), all the test methods give similar results for Path3 (the second possibility for the formation of formaldehyde), all the modeling results for Path3 are in good agreement with the experimental data, verifying that it is the most possible way for the formation of formaldehyde during cellulose pyrolysis.

Dual-Level Direct Dynamics Studies on the Hydrogen Abstraction Reactions of CH2CH3 - n Xn+HBr (X=Cl, Br and n=1, 2)

The reactions of the HBr molecule with CH2CH2Cl (reaction R1), CH2CHCl2 (R2), CH2CH2Br (R3) and CH2CHBr2 (R4) are investigated by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points were calculated at the MPW1K/6–311+G(d,p) and BMK/6–311+G(d,p) levels. To refine the reaction enthalpy and energy barrier height of each reaction, single-point energy calculations were carried out by the G2M(RCC5) method based on the geometries optimized at the above-mentioned two levels. Using the canonical variational transition state theory or the canonical variational transition state theory with the small-curvature tunneling correction, the rate constants of HBr with CH2CH2Cl (R1), CH2CHCl2 (R2), CH2CH2Br (R3), and CH2CHBr2 (R4) were calculated over a wide temperature range of 200–2000 K at the G2M(RCC5)//MPW1K/6–311+G(d,p) level. The effect of chlorine or bromine substitution on the ethyl radical reactivity is discussed. Finally, the total rate constants are fitted by two models, i.e. three-parameter and four-parameter expressions.

Theoretical study and rate constants calculation for the reactions X + CF3CH2OCF3 (X = F, Cl, Br)

The multiple-channel reactions X + CF3CH2OCF3 (X = F, Cl, Br) are theoretically investigated. The minimum energy paths (MEP) are calculated at the MP2/6-31 + G(d,p) level, and energetic information is further refined by the MC-QCISD (single-point) method. The rate constants for major reaction channels are calculated by canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction over the temperature range 200–2000 K. The theoretical three-parameter expressions for the three channels k1a(T) = 1.24 x 10(-15T1.24)exp(-304.81/T), k2a(T) = 7.27 x 10(-15T0.37)exp(-630.69/T), and k3a(T) = 2.84 x 10(-19T2.51)exp(-2725.17/T) cm3 molecule(-1) s(-1) are given. Our calculations indicate that hydrogen abstraction channel is only feasible channel due to the smaller barrier height among five channels considered.

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.

Theoretical study on the reaction CX3 + SiH(CH3)3 (X = H, F)

Theoretical investigations are carried out on the multiple-channel reactions, CH(3) + SiH(CH(3))(3) → products and CF(3) + SiH(CH(3))(3) → products. The minimum energy paths (MEP) are calculated at the MP2/6-311 + G(d,p) level, and energetic information is further refined by the MC-QCISD (single point) method. The rate constants for major reaction channels are calculated by the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction over the temperature range 200-1500 K. The theoretical rate constants are in good agreement with the available experimental data and are found to be k(1a)(T) = 1.93 × 10(-24) T(3.15) exp(-1214.59/T) and k(2a)(T) = 1.33 × 10(-25) T(4.13) exp(-397.94/T) (in unit of cm(3) molecule(-1) s(-1)). Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel due to the smaller barrier height among five channels considered.

AB INITIO CALCULATIONS AND KINETICS STUDY FOR THE DIRECT HYDROGEN ABSTRACTION REACTION OF HYDROGEN SELENIDE WITH HYDROGEN ATOM

We present a direct ab initio dynamics study of thermal rate constants of the hydrogen abstraction reaction of H 2 Se + H → SeH + H 2. The QCISD and CCSD(T) methods were employed to optimize the geometries of stationary points and to calculate the harmonic vibrational frequencies. The split-valence 6-311 + G(d,p) and correlation-consistent cc-pVTZ basis sets big enough to describe the geometries and vibrational frequencies of the species involving in the title reaction. The energies obtained at the QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) level of theory is able to compare to those calculated at the CCSD(T)/6-311 ++ G(3df,3pd)//CCSD(T)/6-311 + G(d,p) level of theory. The energies of all the selected points along the minimum energy path (MEP) were refined at the QCISD(T)/6-311 ++ G(3df,3pd) level of theory. The barriers were obtained at the both QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) and CCSD(T)/6-311 ++ G(3df,3pd)//CCSD(T)/6-311 + G(d,p) levels of theory are in good agreement with experimental one. The forward rate constants were evaluated with both canonical variational transition state theory with small curvature tunneling correction (CVT/SCT) and improved canonical variational transition state theory with small curvature tunneling correction (ICVT/SCT) in the temperature range of 200–2500 K. The calculated forward rate constants of the reaction at the QCISD(T)/6-311 ++ G(3df,3pd)//QCISD/6-311 + G(d,p) level of theory are in good agreement with available experimental data.

MECHANISM AND KINETICS OF THE CH3CH2C(O)OCH2CH3 + OH REACTION: A THEORETICAL STUDY

The hydrogen abstraction reaction of CH 3 CH 2 C(O)OCH 2 CH 3 + OH has been studied theoretically by dual-level direct dynamics method. Six H-abstraction channels were found for this reaction. The required potential energy surface information for the kinetic calculations was obtained at the MCG3-MPWB//M06-2X/aug-cc-pVDZ level. The rate constants were calculated by the improved canonical variational transition-state theory with small-curvature tunneling correction (ICVT/SCT) approach in the temperature range of 200–2000 K. It is shown that the "methylene H-abstraction" from the alkoxy end of the ester CH 3 CH 2 C(O)OCH 2 CH 3 is the dominant channel at lower temperature (&lt; 400 K), while the other channels from the acetyl end should be taken into account as the temperature increases and become the competitive ones at higher temperature. The calculated global rate constants are in good agreement with the experimental ones in the measured temperature range and exhibit a negative temperature dependence below 500 K. A four-parameter rate constant expression was fitted from the calculated kinetic data between 200–2000 K.

Hydrogen abstraction reactions of OH radicals with CH3CH2CH2Cl and CH3CHClCH3: A mechanistic and kinetic study

The hydrogen abstraction reactions of OH radicals with CH₃CH₂CH₂Cl and CH₃CHClCH₃ (R2) have been investigated theoretically by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the B3LYP/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the BMC-CCSD method. Using canonical variational transition-state theory (CVT) with the small-curvature tunneling correction, the rate constants are evaluated over a wide temperature range of 200-2000 K at the BMC-CCSD//B3LYP/6-311G(d,p) level. For the reaction channels with the negative barrier heights, the rate constants are calculated by using the CVT. The calculated total rate constants are consistent with available experimental data. The results show that at lower temperatures, the tunneling correction has an important contribution in the calculation of rate constants for all the reaction channels with the positive barrier heights, while the variational effect is found negligible for some reaction channels. For reactions OH radicals with CH₃CH₂CH₂Cl (R1) and CH₃CHClCH₃ (R2), the channels of H-abstraction from -CH₂ - and -CHCl groups are the major reaction channels, respectively, at lower temperatures. With temperature increasing, contributions from other channels should be taken into account. Finally, the total rate constants are fitted by two models, i.e., three-parameter and four-parameter expressions. The enthalpies of formation of the species CH₃CHClCH₂, CH₃CHCH₂Cl, and CH₃CH₂CH₂Cl are evaluated by isodesmic reactions.

Theoretical study on the reactions of trimethylsilane with chlorine and bromine atoms

Theoretical studies are carried out on the multi-channel reactions of SiH(CH3)3 with Cl (reaction 1, R1) and Br atoms (R2) by direct dynamics method. The minimum energy path is calculated at the MP2/6-31+G(d,p) level, and energetic information is further refined by the MC-QCISD (single-point) method. The rate constants for individual reaction channels, R1a, R1b-in, R1b-out, R1c, R1d, R2a, R2b-in, R2b-out, R2c, and R2d, are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200–1,500 K. The theoretical three-parameter expressions k 1 (T) = 6.30 × 10−15 T 1.36exp(704.94/T) and k 2 (T) = 9.41 × 10−26 T 4.89exp(−1,033.80/T) cm3 molecule−1 s−1 are given. Our calculations indicate that reaction channels R1c and R2c are the major channel.

HOSO+X(X=F,Cl,Br)的反应机理及电子密度拓扑分析

The reaction mechanisms of the reactions between HOSO and X(X=F, Cl, and Br) were investigated at the CCSD(T)/6-311++G(d, p)//MP2/6-311++G (d, p) level. The geometries of the reactants, transition states and the products were optimized at the MP2/ 6-311++G (d, p) level. The intrinsic reaction coordinates were traced according to Fukuis theory and the relationships between the transition states, the reactants and the products confirmed. The single point energies of the species were corrected at the CCSD(T)/ 6-311++G(d, p) level. Reaction rate constants were calculated over a temperature range of 200-3000 K using classical transition state theory and canonical variational transition state theory combined with a small-curvature tunneling correction. The results show that the HOSO+X(X= F, Cl and Br) reaction could occur in the singlet and the triplet reaction channels. The singlet reaction channels are dominant and HX + SO₂ are the main products. The formation and breaking of the chemical bond in the main reaction channel was analyzed by topological analysis of the electron density.