Canonical Transition State Theory Research Articles

Mechanisms and Kinetics of the Synthesis of FOX-12†

The mechanisms of the synthesis of N-guanylurea-dinitramide( FOX-12) were investigated at B3 LYP / 6-311G( d,p) level. The geometries of the reactants,intermediates,transition states,and products were optimized,respectively. The relevance between every transition state and the corresponding reactant /product was confirmed via the intrinsic reaction coordinates( IRC). The single-point energies of the species at the optimized geometries were corrected at the MP2 / 6-311++G( 3df,3pd) level. It was shown that the binitroamide acid( HDN) could be obtained by both channel A1,in which —SO3was preferentially replaced by nitronium( NO+2),and channel B,in which —H was preferentially replaced by NO+2,while the channel A1 was found to be the dominant channel. Then the intermediate product of HDN could be converted to the targeted product of FOX-12 via the channel of either HDN→FOX-12 or HDN→amonium dinitramide( ADN) →FOX-12,while the latter was tend to be considered as the better approach. The rate constants were calculated at temperature ranges of 200—400 K by means of the classical transition state theory( TST) and the canonical vibration transition state theory( CVT) corrected by the small-curvature tunneling( SCT). And the three-parameter Arrhenius expressions of rate constants were also provided. It was expected that the present study may provide a theoretical basis to the research and engineering amplification of FOX-12 as well as other energetic materials.

A Computational Study of the Mechanism and Kinetics for Gas-Phase Decomposition and Reactivity of the C4F9OCH2O Radical between 200 and 400 K

The decomposition processes and reactivity of C4F90CH2O• radical formed from C4F90CH3(HFE-7100) have been studied by density function theory computational methods. All calculations were performed at B3LYP and mPW1PW91 levels of theory with the 6-311G(d,p) basis set. The calculated barrier heights were further improved by QCISD(T)/6-31G(d)//MP2/6-31G(d) methodology to obtain better rate constants. Five possible pathways were investigated: reaction with O2, reaction with OH radical, C-O bond dissociation, release of H radical and finally rearrangement of the radical and then C-O bond cleavage with energy barriers of 6.35 (6.09) [12.12], 12.85 (16.87) [7.51], 17.05 (21.77) [28.34], 20.3 (20.75) [18.13], 32.60 (31.50) [32.63] and 16.07 (18.73) [20.04] kcal mol−1, respectively (the values in the parentheses for mPW1PW91 and in the brackets for the QCISD(T) method). Rate constants were calculated by utilising canonical transition state theory in the temperature range of 200–400 K and 1 atm pressure, and Arrhenius diagrams were plotted. The results showed that H elimination and H abstraction pathways are dominant for degradation of C4F90CH2O• radical in the atmosphere. A smooth transition from the reactants to products on the corresponding potential energy surface was confirmed by intrinsic reaction coordinate calculations.

Theoretical and kinetic study of reaction C2H + C3H6 on the C5H7 potential energy surface

The reaction mechanisms for reaction C2H + C3H6 (propene) on the C5H7 potential energy surface (PES) have been investigated by using quantum chemical calculations combined with canonical transition state theory and Rice–Ramsperger–Kassel–Marcus/master equation (RRKM/ME) theory. The optimization of the geometries and the calculation of the vibrational frequencies of reactants, transition states, and products are performed at the B3LYP/CBSB7 level of theory. The composite CBS-QB3 method is applied for energy calculations. The rate constants for reactions with tight transition states are obtained by canonical transition state theory, while the rate constants for barrierless reactions at the high-pressure limit are determined by the variational transition state theory. The rate constants for pressure-dependent reactions are obtained by RRKM/ME theory. The reaction of C2H + C3H6 is initiated by the internal and terminal additions of C2H to C3H6 without an entrance barrier, and the adduct of the internal and terminal additions is 2-methyl-1-butyl-3-yne (C5H7) and 4-pentyl-1-yne (C5H7), respectively. Products vinylacetylene (C4H4) + CH3 and 2-methyl-1-buten-3-yne (C5H6) + H are favored by the internal C2H addition to C3H6, whereas products 3-penten-1-yne (C5H6) + H and 4-penten-1-yne (C5H6) + H are preferred for the terminal C2H addition. The calculated rate constants are in good agreement with those available from the literature, and they are also given in modified Arrhenius equation form, which are useful in combustion modeling of hydrocarbons.

Theoretical investigation on mechanism and kinetics of the Cl-initiated hydrogen abstraction reactions of ethyl trifluoroacetate at 298 K.

Theoretical investigations were carried out on the gas-phase reactions of CF3C(O)OCH2CH3, ethyl trifluoroacetate (ETFA) with Cl atoms by means of modern density functional theory methods. The optimized geometries, frequencies and minimum energy path were obtained with the hybrid density functional model MPWB1K using the 6-31+G(d,p) basis set. The single point energy calculations were refined further using the G2(MP2) method. Two conformers relatively close in energy were identified for ETFA; both are likely to be important in the temperature range of our study. The existence of transition states on the corresponding potential energy surface was ascertained by performing intrinsic reaction coordinate calculations. The rate constant at 298 K calculated theoretically using canonical transition state theory was found to be in good agreement with experimentally measured values. Our calculations suggest that H abstraction from the -CH2 group is kinetically and thermodynamically more favorable than abstraction from the -CH3 group. The atmospheric lifetime of ETFA with Cl atoms was determined to be 1.98 years. To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of chlorine atoms in ETFA.

Reactivity of CHI3 with OH radicals: X-abstraction reaction pathways (X = H, I), atmospheric chemistry, and nuclear safety.

The X-abstraction (X = H, I) pathways in the reaction of CHI3 with OH radical, a possible iodoform removal process relevant to the Earth's atmosphere and conditions prevailing in the case of a nuclear accident, have been studied applying highly correlated ab initio quantum chemistry methods and canonical transition-state theory to obtain reaction energy profiles and rate constants. Geometry optimizations of reactants, products, molecular complexes, and transition states determined at the MP2/cc-pVTZ level of theory have been followed by DK-CCSD(T)/ANO-RCC single-point energy calculations. Further improvement of electronic energies has been achieved by applying spin-orbit coupling, corrections toward full configuration interaction, vibration contributions, and tunneling corrections. Calculated reaction enthalpies at 0 K are -108.2 and -5.1 kJ mol(-1) for the H- and I-abstraction pathways, respectively; the strongly exothermic H-abstraction pathway is energetically favored over the modestly exothermic I-abstraction one. The overall rate constant at 298 K based on our ab initio calculations is 4.90 × 10(-11) cm(3) molecule(-1) s(-1), with the I-abstraction pathway being the major channel over the temperature range of 250-2000 K. The CHI3 atmospheric lifetime with respect to the removal reaction with OH radical is predicted to be about 6 h, very short compared to that of other halomethanes.

A theoretical investigation on kinetics, mechanism, and thermochemistry of the gas-phase reactions of methyl fluoroacetate with OH radicals and fate of alkoxy radical.

We theoretically investigated OH-initiated hydrogen abstraction reactions of methyl fluoroacetate (MFA) CH2FC(O)OCH3 at the MPWB1K level of theory in conjunction with the 6-31+G(d,p) basis set. Thermodynamic and kinetic data are computed using the comparatively accurate G2(MP2) method. Two most stable conformers of MFA are identified, and the energy difference between them is found to be only 0.32 kcal mol(-1). Both of them are considered for rate coefficient calculations, and the contribution from each of the conformers is found to be quite significant. We propose an indirect mechanism due to validation of pre- and post-reactive complexes. The rate parameters are determined using canonical transition state theory and energetics at the G2(MP2) level. The temperature dependence of the rate constant can be described by the Arrhenius expressions: k = 8.79 × 10(-13) exp[(-377.27 ± 64)/T] cm(3) molecule(-1) s(-1) over a temperature range of 250-450 K. The ΔfH°298 for CH2FC(O)OCH3, CH2FC(O)OC(•)H2, and C(•)HFC(O)OCH3 are also computed using an isodesmic procedure. The OH-driven atmospheric lifetime of MFA was estimated to be 24 days. A mechanistic study to shed light on the atmospheric degradation and the sole fate for the consumption of CH2FC(O)OCH2O(•) radical has also been reported.

Theoretical investigation on the atmospheric fate of CF3C(O)OCH 2O radical: alpha-ester rearrangement vs oxidation at 298 K.

A theoretical study on the mechanism of the thermal decomposition of CF(3)C(O)OCH(2)O radical is presented for the first time. Geometry optimization and frequency calculations were performed at the MPWB1K/6-31 + G(d, p) level of theory and energetic information further refined by calculating the energy of the species using G2(MP2) theory. Three plausible decomposition pathways including α-ester rearrangement, reaction with O(2) and thermal decomposition (C-O bond scission) were considered in detail. Our results reveal that reaction with O(2) is the dominant path for the decomposition of CF(3)C(O)OCH(2)O radical in the atmosphere, involving the lowest energy barrier, which is in accord with experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid TFA makes a negligible contribution to decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways were evaluated using canonical transition state theory (CTST) at 298 K.

Mechanism and kinetics of low-temperature oxidation of a biodiesel surrogate−methyl acetate radicals with molecular oxygen

Accurate description of reactions between methyl acetate (MA) radicals and molecular oxygen is an essential prerequisite for understanding as well as modeling low-temperature oxidation and/or ignition of MA, a small biodiesel surrogate, because their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The accurate composite CBS-QB3 level of theory was used to explore potential energy surfaces for MA radicals + O2 system. Using the electronic structure calculation results under the framework of canonical statistical mechanics and transition state theory, thermodynamic properties of all species as well as high-pressure rate constants of all reaction channels were derived with explicit corrections for tunneling and hindered internal rotations. Our calculated results are in good agreement with a limited number of scattered data in the literature. Furthermore, pressure- and temperature-dependent rate constants were then computed using the Quantum Rice–Ramsperger–Kassel and the modified strong collision theories. This procedure resulted in a thermodynamically consistent detailed kinetic mechanism for low-temperature oxidation of the title fuel. We also demonstrated that even the detailed mechanism consists of several reactions of different reaction types, only the addition of the reactants and the re-dissociation of the initially formed adducts are important for low-temperature combustion at engine-liked conditions.

Theoretical investigation on unimolecular decomposition of malonic acid: a potential sink for ketene

DFT and ab initio calculations are performed to study the unimolecular decmposition pathways of malonic acid. The reaction is explored along three plausible decomposition pathways. The calculated results shows that decarboxylation is a two step process with an energy barrier of 32.16 kcal mol−1 at the CCSD(T)/6-311++G(d,p) level which is in reasonable agreement with available data. In contrast to the usual interpretations, novel features of this study are dehydration and in situ decarboxylation–dehydration pathways that involve energy barriers of 67.13 and 44.75 kcal mol−1, respectively at the CCSD(T)/6-311++G(d,p) level leading to the formation of carbon-suboxide and ketene along with conventional H2O and CO2. The thermal rate constants for the above decomposition pathways are also evaluated using Canonical Transition State Theory at 298 K.

Theoretical studies on kinetics, mechanism and thermochemistry of gas-phase reactions of CF3CHFCF2OCF3 with OH radicals and Cl atoms and fate of alkoxy radical at 298 K

Detailed theoretical investigation has been performed on the mechanism, kinetics and thermochemistry of the gas phase reactions of CF3CHFCF2OCF3 with OH radicals and Cl atoms using M06-2X/6-31+G(d,p) level of theory. The energetic calculations were refined by using aug-cc-pVTZ basis set. Our calculations reveal that OH-initiated hydrogen abstraction is thermodynamically and kinetically more facile than that from the Cl-initiated hydrogen abstraction. Using group-balanced isodesmic reactions, the standard enthalpies of formation (ΔHf°) for species and bond dissociation energies for CH bond are also reported. The ΔHf° for CF3CHFCF2OCF3 and •CF(CF3)CF2OCF3 species calculated from M06-2X/6-31+G(d,p) results are −523.15 and −471.65kcalmol−1, respectively. The rate constants evaluated by canonical transition state theory (CTST) are found to be 1.22×10−15 and 3.40×10−17cm3molecule−1s−1 which are in a reasonable agreement with the available experimental data of (1.43±0.28)×10−15 and (4.09±0.42)×10−17cm3molecule−1s−1, respectively, for OH and Cl-initiated hydrogen abstraction reactions. The atmospheric life time of CF3CHFCF2OCF3 is estimated to be 38.5 years. The atmospheric fate of the alkoxy radical, CF3CF(O•)CF2OCF3 is also investigated for the first time using same level of theory.

Theoretical study on the gas-phase reactions of ethyl butyrate with OH radicals at 298 K

Detailed theoretical investigation has been carried out on the mechanism, kinetics, and thermochem- istry of the gas-phase reactions of CH3CH2CH2C(O) OCH2CH3 with OH radicals using a modern DFT func- tional. Reaction profiles are modeled with the formation of pre- and post-reactive complexes. Energetic calculations are made at M06-2X/6-31?G(d,p) level of theory. Intrinsic reaction coordinate calculation has also been performed to confirm the smooth transition from the reactant to product through the respective transition state. It has been estab- lished that the reaction proceeded via H-atom abstraction from the -CH2 position of ethyl butyrate. The calculated rate constant using canonical transition state theory is found to be in a reasonable agreement with the experi- mental data. Using group-balanced isodesmic reactions, the standard heats of formation of ethyl butyrate and radicals generated by H-atom abstraction are also reported for the first time. The branching ratios of the different reaction channels are also determined. The estimated atmospheric life time of ethyl butyrate is found to be 2.37 days.

Computational Study on OH and Cl Initiated Oxidation of 2,2,2-Trifluoroethyl Trifluoroacetate (CF3C(O)OCH2CF3)

Hydrofluoroethers (HFEs) are developed as a suitable for the replacement of environmentally hazardous CFCs and are termed as third generation refrigerants. One of the major products of decomposition of HFEs in the atmosphere is a fluoroester. The present study relates to the OH and Cl initiated oxidation of CF3C(O)OCH2CF3 formed from the oxidation of HFE-356mff. The latter is used as a solvent in the industry and reaches the atmosphere without any degradation. Kinetics of the titled molecule has been studied at MPWB1K/631+G(d,p) level of theory. Single point energy calculations have been made at G2(MP2) level of theory and barrier heights are determined. The rate constants are calculated using canonical transition state theory. Tunnelling correction are made using one-dimensional Eckart potential barrier. The rate constant calculated during the present study are compared with the experimental values determined using relative rate method and FTIR detection technique.

Pathways, kinetics and thermochemistry of methyl-ester peroxy radical decomposition in the low-temperature oxidation of methyl butanoate: A computational study of a biodiesel fuel surrogate

A chemical kinetic submechanism proposed here is an essential prerequisite to model autoignition of methyl esters at low temperature region (less than ∼900K), where reactions of methyl-ester peroxy radicals (ROO) and hydroperoxy methyl-ester radicals (QOOH) are crucial and relatively unexplored. The potential energy surfaces of the methyl butanoate peroxy radicals+O2 systems are computed by the G3MP2B3 composite approach. 114 pathways are identified leading to the formation of key radicals in the ignition kernel such as OH and HO2.Particular attention is focused on: (1) intramolecular H-migration of ROO, (2) unimolecular dissociations of ROO and QOOH and (3) reactions of ROO+HO2. Using the canonical transition state theory, the high-pressure limit rate constants for reactions in the kinetic submechanism are calculated. Standard enthalpy of formation, entropy and heat capacities are evaluated for intermediates and products formed during combustion by means of the standard statistical mechanics formulae. The agreement and disagreement between our calculated kinetic parameters and previous estimates offer further insight into the uncertainty associated with theoretical estimation. We perform the branching ratio analysis for the competing channels between the reverse dissociation of ROO (ROO→R+O2) and explored unimolecular reactions decomposing ROO. Additionally, we also quantify the similarity and dissimilarity between the rate constants determined here and those previously calculated for normal alkanes. Finally, the effect of transition state size on the rate constants for the isomerizations of methyl-ester peroxy radicals is systematically analyzed.

Role of Quantum Mechanical Tunneling on the γ-Effect of Silicon on Carbenes in 3-Trimethylsilylcyclobutylidene

Quantum mechanical tunneling (QMT) is increasingly being realized as an important phenomenon that can enhance the rate of reactions even at room temperature. Recently, the ability of a trimethylsilane (TMS) group to activate 1,3-H shift to a carbene from a γ-position has been demonstrated. Direct dynamical calculations (using canonical varitational transition state theory) inclusive of small curvature tunneling (CVT-SCT) show that QMT plays a decisive role in such 1,3-hydrogen migration in both the presence and absence of TMS. The presence of a TMS group reduces the activation energy of 1,3-H shift reaction via 1,3-equatorial interaction of the TMS group with the carbene. Tunneling across the smaller barrier enhances the overall forward rate of the reaction. The Arrhenius plot for the reaction shows substantial curvature in comparison to the CVT mechanism at room temperature. Arrhenius plots for the kinetic isotope effects (KIEs) for the γ-deuterated and per deuterated 3-trimethylsilylcyclobutylidene also show strong deviations from the classical over the barrier mechanism. The magnitude of the KIE is suggestive of QMT from the vibrational excited states of the carbenes.

A computational perspective on mechanism and kinetics of the reactions of CF3C(O)OCH2CF3 with OH radicals and Cl atoms at 298 K

Theoretical investigations are carried out on the gas-phase reactions of CF3C(O)OCH2CF3, 2,2,2-trifluoroethyl trifluoroacetate (TFETFA) with OH radicals and Cl atoms by means of modern DFT methods. Two conformers relatively close in energy have been identified for TFETFA, both of them are likely to be important in the temperature range of our study. Reaction profiles are modeled including the formation of two pre-reactive and post-reactive complexes. The optimized geometries, frequencies and minimum energy path are obtained at hybrid density functional model, M06-2X level using 6-31+G(d,p) basis set. The single point energy calculation was further refined by using 6-311++G(d,p) and Aug-cc-pVTZ basis sets. The existence of transition states on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate (IRC) calculation. Theoretically calculated rate constants at 298K and using the canonical transition state theory (CTST) are found to be in good agreement with the experimentally measured ones. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CF3C(O)OCH2CF3 and radical generated by hydrogen abstraction, CF3C(O)OCHCF3, are also reported for the first time. The estimated atmospheric lifetime of TFETFA is expected to be 0.15 years.

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

Due to the rapidly growing interest in the use of biomass derived furanic compounds as potential platform chemicals and fossil fuel replacements, there is a simultaneous need to understand the pyrolysis and combustion properties of such molecules. To this end, the potential energy surfaces for the pyrolysis relevant reactions of the biofuel candidate 2-methylfuran have been characterized using quantum chemical methods (CBS-QB3, CBS-APNO and G3). Canonical transition state theory is employed to determine the high-pressure limiting kinetics, k(T), of elementary reactions. Rice-Ramsperger-Kassel-Marcus theory with an energy grained master equation is used to compute pressure-dependent rate constants, k(T,p), and product branching fractions for the multiple-well, multiple-channel reaction pathways which typify the pyrolysis reactions of the title species. The unimolecular decomposition of 2-methylfuran is shown to proceed via hydrogen atom transfer reactions through singlet carbene intermediates which readily undergo ring opening to form collisionally stabilised acyclic C5H6O isomers before further decomposition to C1-C4 species. Rate constants for abstraction by the hydrogen atom and methyl radical are reported, with abstraction from the alkyl side chain calculated to dominate. The fate of the primary abstraction product, 2-furanylmethyl radical, is shown to be thermal decomposition to the n-butadienyl radical and carbon monoxide through a series of ring opening and hydrogen atom transfer reactions. The dominant bimolecular products of hydrogen atom addition reactions are found to be furan and methyl radical, 1-butene-1-yl radical and carbon monoxide and vinyl ketene and methyl radical. A kinetic mechanism is assembled with computer simulations in good agreement with shock tube speciation profiles taken from the literature. The kinetic mechanism developed herein can be used in future chemical kinetic modelling studies on the pyrolysis and oxidation of 2-methylfuran, or the larger molecular structures for which it is a known pyrolysis/combustion intermediate (e.g. cellulose, coals, 2,5-dimethylfuran).

Theoretical investigation on the atmospheric fate of the CF3C(O)OCH(O)CF3 radical: alpha-ester rearrangement vs. oxidation

A detailed quantum chemical study is performed on the unimolecular decomposition reaction of the alkoxy radical, CF3C(O)OCH(O)CF3 produced from CF3C(O)OCH2CF3, trifluoroethyl trofluoroacetate (TFETFA) at the MPWB1K and M06-2X level of theories using the 6-31+G(d,p) basis set. Five plausible decomposition pathways including reaction with O2, α-ester rearrangement and thermal decomposition (C–C, C–H and C–O bonds scission) have been considered in detail. Out of the five prominent decomposition channels, our results reveal that reaction with O2 is the dominant path for the decomposition of the CF3C(O)OCH(O)CF3 radical in the atmosphere involving the lowest energy barrier which is in accordance with recent experimental findings. Our theoretical results also suggest that α-ester rearrangement leading to the formation of trifluoroacetic acid (TFA) has a negligible contribution for decomposition of the title alkoxy radical. The thermal rate constants for the above decomposition pathways are evaluated using Canonical Transition State Theory (CTST) at 298 K.

Computational study on the kinetics of OH initiated oxidation of methyl difluoroacetate (CF2HCOOCH3)

The kinetics of hydrogen atom abstraction reactions of methyl difluoroacetate (CF2HCOOCH3) by OH radical has been studied by quantum mechanical method. The geometry optimisation and frequency calculation of the titled compound was performed with density functional theory using hybrid meta density functional MPWB1K with 6-31+G(d,p) basis set. Transition states have been determined and intrinsic reaction coordinate (IRC) calculation has been performed to ascertain that the transition from reactants to products was smooth through the corresponding transition state. Energy values are refined by making single point energy calculation at G3B3 level of theory and an energy level diagram is constructed. The standard enthalpies of formation of reactants and other species formed during the reaction were calculated using isodesmic method. The rate constants are calculated using canonical transition state theory and the overall rate constant is determined to be 1.35×10−13 cm3 molecule−1 s−1 at 298 K and 1 atmospheric pressure. Tunnelling has been taken into account in the determination of the rate constant because it plays a critical role at low temperature especially when transfer of hydrogen takes place. The calculated value is found to be in good agreement with the experimentally determined value of 1.48×10−13 cm3 molecule−1 s−1.

Computational studies on the thermal decomposition of the CH2FOCHFO radical

Theoretical studies have been carried out on the kinetics and thermochemistry of the thermal decomposition of the CH2FOCHFO radical formed during the photo-oxidation of CH2FOCH2F (HFE-152E) using the dual-level method of obtaining the optimised structure at DFT(M06-2X)/6-311++G(d,p) followed by a single-point energy calculation at the G3 level of theory. The rate constant for different reaction channels involved during the decomposition processes of CH2FOCHFO is evaluated at 298 K and 1 atm using canonical transition-state theory. The results point out that the C–H bond scission is the dominant path involving an energy barrier of 9.5 kcal mol−1 determined at the G3 level of theory. A potential energy diagram is constructed and the results are compared with the data available from the literature for a structurally similar molecule.

A computational perspective on the kinetics and thermochemistry of the gas phase reactions of 1, 1-dichlorodimethylether (DCDME) with OH radical at 298 K

Kinetics and thermochemistry of the gas phase reactions between CH3OCHCl2 (DCDME) and OH radical are investigated theoretically. The geometries and all the stationary points on the potential energy surface are calculated at BHandHLYP/6-311G(d,p) method. The energy information is further refined at CCSD(T)/6-311G(d,p) level of theory. Reaction profiles are modelled including the formation of two pre-reactive and post-complexes. The rate constants, which are evaluated by Canonical Transition State Theory (CTST) including tunnelling correction at 298 K, are in very good agreement with the available experimental data. The percentage contributions of both reaction channels are also reported at 298 K. The hydrogen abstraction reaction from the –CHCl2 group is found to be dominant leading to the formation of CH3OCCl2 + H2O. Using group-balanced isodesmic reactions, the standard enthalpies of formation for CH3OCHCl2, CH3OCCl2 and CH2OCHCl2 are also reported.