Investigation of Product Formation in the O(1D, 3P) + N2O Reactions: Comparison of Experimental and Theoretical Kinetics.

The spin-forbidden and spin-allowed reactions of the excited and ground electronic state O(1D, 3P) + N2O(X1Σ+) systems have been studied theoretically. Quantum calculations at the UCCSD(T)/CBS(T, Q, 5)//CCSD/aug-cc-pVTZ level have located two crossing points, MSX1 and MSX2, with energies of 11.2 and 22.7 kcal mol-1 above O(3P) + N2O, respectively. The second-order P-independent rate constants for the adiabatic and non-adiabatic thermal reactions predicted by adiabatic TST/VTST and non-adiabatic TST, respectively, agree closely with the available literature results. The second-order rate constant, k2a = 9.55 × 10-11 exp(-26.09 kcal mol-1/RT) cm3 molecule-1 s-1, for the O(3P) + N2O → 2NO reaction, contributed by both the dominant MSX2 and the minor TS1-a channels, is in reasonable accord with prior experiments and recommendations, covering the temperature range of 1200-4100 K. The calculated rate constant, k2b = 4.47 × 10-12 exp(-12.9 kcal mol-1/RT) cm3 molecule-1 s-1, for the O(3P) + N2O → N2 + O2(a1Δg) reaction, occurring exclusively via MSX1, is also in good agreement with the combined experimental data measured in a shock tube study at T = 1940-3340 K (ref 16) and the result measured by Fourier transform infrared spectroscopy in the temperature range of 988-1083 K (ref 17). Moreover, the spin-allowed rate constants predicted for the singlet-state reactions, k1a = (7.06-7.46) × 10-11 cm3 molecule-1 s-1 for O(1D) + N2O → 2NO and k1b = (4.36-4.66) × 10-11 cm3 molecule-1 s-1 for O(1D) + N2O → N2 + O2(a1Δg) in the temperature range of 200-350 K, agree quantitatively with the experimentally measured data, while the total rate constant k1 = k1a + k1b was also found to be in excellent accordance with many reported values.

Theoretical Investigation of the Mechanisms and Kinetics of the Bimolecular and Unimolecular Reactions Involving in the C4H6 Species.

A theoretical study of the mechanisms and kinetics for the C4H6 system was carried out using ab initio molecular orbital theory based on the CCSD(T)/CBS//B3LYP/6-311++G(3df,2p) method in conjunction with statistical theoretical variable reaction coordinate transition-state theory and RRKM/ME calculations. The calculated results indicate that buta-1,3-diene, but-1-yne, and C4H5 + H can be the major products of the C3 + C1 reaction, while CCH2 + C2H4 and C4H5 + H play an important role in the C2 + C2 reaction. In contrast, the C4H6 fragmentation giving rise to C3 + C1 and C4H5 + H becomes the key reaction paths under any temperature and pressure. The rate constants for the system have been calculated in the 300-2000 K temperature range at various pressures for which the C2 + C2 → C4H6 high-P limit rate constant, 10.24 × 1014T-0.51 cm3/mol/s, agrees well with the measured value of Hidaka et al., 9.64 × 1014T-0.5 cm3/mol/s. Also, the high-P limit rate constants of the channels but-2-yne → 2-C4H5 + H and C3 + C1 → C4H6, being 1.7 × 1014 exp(-351.5 kJ·mol-1/RT) s-1 and 5.07 × 1013 exp(0.694 kJ·mol-1/RT) cm3/mol/s, are in good agreement with the available literature data 5 × 1014 exp(-365.3 kJ·mol-1/RT) s-1 and 4.09 × 1013 exp(1.08 kJ·mol-1/RT) cm3/mol/s reported by Hidaka et al. and Knyazev and Slagle, respectively. Moreover, the 298 K/50 Torr branching ratios for the formation of buta-1,2-diene (0.43) and but-1-yne (0.57) as well as the total rate constant 5.18 × 1013 cm3/mol/s of the channels C3 + C1 → buta-1,2-diene and C3 + C1 → but-1-yne are in excellent accord with the laboratory values given by Fahr and Nayak, being 0.4, 0.6, and (9.03 ± 1.8) × 1013 cm3/mol/s, respectively. Last but not least, the rate constants and branching ratios for the C4H6 dissociation processes in the present study also agree closely with the theoretically and experimentally reported data.

Combination Reactions of Propargyl Radical with Hydroxyl Radical and the Isomerization and Dissociation of trans-Propenal.

Ab initio investigation for the ground-electronic potential energy surface (PES) of the CH2CCH + OH combination and the trans-CH2CHCHO isomerization and decomposition has been performed at the UCCSD(T)/CBS(TQ5)//M06-2X/aug-cc-pVTZ level of theory. Thermal and microcanonical rate constants, as well as branching ratios in the 300-2000 K temperature range have been predicted based on optimized structures and vibrational frequencies of species involved using statistical theoretical VRC-TST and RRKM master equation computations. The calculated results are in good agreement with the prior reported data, particularly as an accurate scaling of the energy barriers was carried out. Based on the view of PES and kinetic-predicted values, the reaction paths leading to C2H2 + CO + H2, CH3CH + CO, C2H4 + CO, C2H3 + HCO, and C3H3O + H are the prevailing product channels for the C3H3 + OH bimolecular reaction under the considered 300-2000 K temperature range. Among those products, CH3CH + CO is the most dominant one in the low-temperature condition; however, C2H2 + CO + H2 becomes the most favorable product in the high-temperature region. Alternatively, the C3H4O dissociation processes leading to C2H2 + CO + H2, C2H3 + HCO, C2H4 + CO, and CH2C + CH2O constitute the major paths, in which, C2H2 + CO + H2 is the most critical one with the ∼62% and ∼59% branching ratios at E = 148 and 182 kcal/mol, respectively. The overall second-order rate constants of the bimolecular reaction C3H3 + OH → products obtained at the pressure 760 Torr (Ar) can be illustrated by the modified Arrhenius expression of k(T) = 1.36 × 10-13T1.26 exp[(-1.12 ± 0.43 kcal mol-1)/RT] and/or k(T) = 3.77 × 1017T-7.58 exp[(-18.82 ± 0.20 kcal mol-1)/RT] cm3 molecule-1 s-1, covering the temperature range of 300-1300 and/or 1300-2000 K, respectively. The total high-pressure limit rate constant for the C3H3 + OH → CH2CCHOH barrierless processes is in good agreement with the k(T) = 8.30 × 10-10 T-0.1 cm3 molecule-1 s-1 literature data. Moreover, microcanonical rate constants for the C3H4O isomerization and dissociation are in excellent accordance with the previously predicted values given by Chin and Lee. The present study supplies a thorough insight into the mechanisms and kinetics of the C3H3 + OH combination as well as the C3H4O multistep isomerization/dissociation pathways.

Computational Study on the Mechanisms and Rate Constants for the O(3P,1D) + OCS Reactions.

The mechanisms and kinetics of O(3P,1D) + OCS(X1Σ+) reactions have been studied by the high-level G2M(CC2) and CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) methods in conjunction with the transition-state theory and variational Rice-Ramsperger-Kassel-Marcus theory calculations. The result shows that the triplet surface proceeds directly by abstraction and substitution channels to produce SO(3P) + CO(X1Σ+) and S(3P) + CO2(X1 Σg+) by passing the barriers of 7.6 and 9.1 kcal·mol-1 at the G2M(CC2)//B3LYP/6-311+G(3df) level, respectively, while two stable intermediates, LM1 (OSCO1) and LM2 (SC(O)O1), are formed barrierlessly from O(1D) + OCS(X1Σ+) in the singlet surface, which lie at -40.5 and -50.1 kcal·mol-1 relative to O(3P) + OCS(X1Σ+) reactants and decompose to CO(X1Σ+) + SO(a1Δ) and S(1D) + CO2(X1Σg+). LM1 and LM2 may also be produced by singlet-triplet surface crossings via MSX1 and MSX2; the predicted total rate constant for the O(3P) + OCS(X1Σ+) reaction including the crossings, 9.2 × 10-11 exp(-5.18 kcal·mol-1/RT) cm3 molecule-1 s-1, is in good agreement with available experimental data. The branching ratio of the CO2 product channel, 0.22-0.32, between 1200 and 1600 K, is also in excellent agreement with the value of 0.2-0.3 measured by Isshiki et al. (J. Phys. Chem. A. 2003, 107, 2464).

Infrared Emission from Photodissociation of Methyl Formate [HC(O)OCH3] at 248 and 193 nm: Absence of Roaming Signature.

Following photodissociation at 248 nm of gaseous methyl formate (HC(O)OCH3, 0.73 Torr) and Ar (0.14 Torr), temporally resolved vibration-rotational emission spectra of highly internally excited CO (ν ≤ 11, J ≤ 27) in the 1850-2250 cm-1 region were recorded with a step-scan Fourier-transform spectrometer. The vibration-rotational distribution of CO is almost Boltzmann, with a nascent average rotational energy (ER0) of 3 ± 1 kJ mol-1 and a vibrational energy (EV0) of 76 ± 9 kJ mol-1. With 3 Torr of Ar added to the system, the average vibrational energy was decreased to EV0 = 61 ± 7 kJ mol-1. We observed no distinct evidence of a bimodal rotational distribution for ν = 1 and 2, as reported previously [Lombardi et al., J. Phys. Chem. A 2016, 129, 5155], as evidence of a roaming mechanism. The vibrational distribution with a temperature of ∼13000 ± 1000 K, however, agrees satisfactorily with trajectory calculations of these authors, who took into account conical intersections from the S1 state. Highly internally excited CH3OH that is expected to be produced from a roaming mechanism was unobserved. Following photodissociation at 193 nm of gaseous HC(O)OCH3 (0.42 Torr) and Ar (0.09 Torr), vibration-rotational emission spectra of CO (ν ≤ 4, J ≤ 38) and CO2 (with two components of varied internal distributions) were observed, indicating that new channels are open. Quantum-chemical calculations, computed at varied levels of theory, on the ground electronic potential-energy schemes provide a possible explanation for some of our observations. At 193 nm, the CO2 was produced from secondary dissociation of the products HC(O)O and CH3OCO, and CO was produced primarily from secondary dissociation of the product HCO produced on the S1 surface or the decomposition to CH3OH + CO on the S0 surface.

Kinetics and mechanism of the gas-phase reaction of the hydroxyl radical with meta-aminotoluene compound.

CONTEXT: An ab initio investigation into the potential energy landscape of the meta-aminotoluene + •OH reaction has been conducted in this study. The calculated results reveal that the reaction channel leading to the product (NHC6H4CH3 + H2O) prevails under the 300-1700 K temperature range, while the reaction path forming the product (NH2C6H4CH2 + H2O) dominates in the higher-temperature region (T ≥ 1800 K). Within the specified temperature range, the product branching ratio for the former declines from 48 to 30%, while the latter shows an increase, reaching 29%. The overall second-order rate constants of the titled reaction obtained at the pressure 760 Torr (N2) can be illustrated by the modified Arrhenius expression of ktotal = 1.46 × 10-13 T0.58 exp[(-0.759 kcal.mol-1)/RT] cm3 molecule-1 s-1 and ktotal = 1.86 × 10-22 T3.24 exp[(-5.086 kcal.mol-1)/RT] cm3 molecule-1 s-1, covering the temperature range of T = 300-600 K and T > 600 K, respectively. The total rate constant at the ambient conditions in this work, 1.43 × 10-11 cm3 molecule-1 s-1, has been found to be roughly one order of magnitude lower than the available experimental data, ~ 1.2 × 10-10 cm3 molecule-1 s-1, measured by Atkinson et al., Rinke et al., and Witte et al., or the theoretical value, 4.4 × 10-10 cm3 molecule-1 s-1, and calculated by Abdel-Rahman and co-workers for the aniline + •OH reaction. METHODS: The structures of reactants, transition states, intermediate states, and products of the meta-aminotoluene + •OH reaction are calculated with the aug-cc-pVTZ basis set and the methods DFT/B3LYP and CCSD(T). The rate constants and branching ratios in the 300-2000 K temperature range are calculated with the statistical theoretical TST and RRKM master equation computations including tunneling corrections, with potential energy surface constructed by the CCSD(T)//B3LYP/aug-cc-pVTZ approach.

Mechanistic and kinetic insights of the formation of allene and propyne from the C3H3 reaction with water.

CONTEXT: Allene (H2C = C = CH2) and propyne (CH3-C≡CH) are important compounds in the combustion chemistry. They can be created from the reaction of proparyl radicals with water. In this study, therefore, a computational study into the C3H3 + H2O potential energy landscape has been carefully conducted. The computed results indicate that the reaction paths forming the products (allene: CH2CCH2 + •OH) and (propyne: HCCCH3 + •OH) prevail under the 300-2000 K temperature range, where the latter is much more predominant compared to the former. However, these two products are not easily formed under ambient conditions due to the high energy barriers. In the 300 - 2000 K temperature range, the branching ratio for the propyne + •OH product declines from 100 to 86%, whereas the allene + •OH product shows an increase, reaching 14% at 2000 K. The overall bimolecular rate constant of the title reaction can be presented by the modified Arrhenius expression of ktotal = 1.94 × 10-12 T0.14 exp[(-30.55 kcal.mol-1)/RT] cm3 molecule-1 s-1. The total rate constant at the ambient conditions in this work, 2.37 × 10-34 cm3 molecule-1 s-1, was found to be over five orders of magnitude lower than the total rate constant of the C3H3 + NH3 reaction, 7.98 × 10-29 cm3 molecule-1 s-1, calculated by Hue et al. (Int. J. Chem. Kinet. 2020, 4(2), 84-91). The results in this study contribute to elucidating the mechanism of allene and propylene formation from the C3H3 + H2O reaction, and they can be used for modeling C3H3-related systems under atmospheric and combustion conditions. METHODS: All the geometric structures of the C3H3 + H2O system were optimized by the B3LYP method in conjunction with the 6-311 + + G(3df,2p) basis set. Single-point energies of these species were calculated at the CCSD(T)/6-311 + + G(3df,2p) level of theory. The CCSD(T)/CBS level has also been used to compute single-point energies for the two major reaction channels (C3H3 + H2O → allene + •OH and C3H3 + H2O → propyne + •OH). Rate constants and branching ratios of the key reaction channels were calculated in the 300-2000 K temperature interval using the Chemrate software based on the transition state theory (TST) with Eckart tunneling corrections.

Mechanistic and Kinetic Approach on the Propargyl Radical (C3H3) with the Criegee Intermediate (CH2OO).

The detailed reaction mechanism and kinetics of the C3H3 + CH2OO system have been thoroughly investigated. The CBS-QB3 method in conjunction with the ME/vRRKM theory has been applied to figure out the potential energy surface and rate constants for the C3H3 + CH2OO system. The C3H3 + CH2OO reaction leading to the CH2-[cyc-CCHCHOO] + H product dominates compared to the others. Rate constants of the reaction are dependent on temperatures (300-2000 K) and pressures (1-76,000 Torr), for which the rate constant of the channel C3H3 + CH2OO → CH2-[cyc-CCHCHOO] + H decreases at low pressures (1-76 Torr), but it increases with rising temperature if the pressure P ≥ 760 Torr. Rate constants of the three reaction channels C3H3 + CH2OO → CHCCH2CHO + OH, C3H3 + CH2OO → OCHCHCHCHO + H, and C3H3 + CH2OO → CHCHCHO + CH2O fluctuate with temperatures. The branching ratio of the C3H3 + CH2OO → CH2-[cyc-CCHCHOO] + H channel is the highest, accounting for 51-98.7% in the temperature range of 300-2000 K and 760 Torr pressure, while those of the channels forming the products PR10 (OCHCHCHCHO + H) and PR11 (CHCHCHO + CH2O) are the lowest, less than 0.1%, indicating that the contribution of these two reaction paths to the title reaction is insignificant. The proposed temperature- and pressure-dependent rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling CH2OO-related systems under atmospheric and combustion conditions.

Reactions of Methyl Radicals with Aniline Acting as Hydrogen Donors and as Methyl Radical Acceptors.

The present investigation theoretically reports the comprehensive kinetic mechanism of the reaction between aniline and the methyl radical over a wide range of temperatures (300-2000 K) and pressures (76-76,000 Torr). The potential energy surface of the C6H5NH2 + CH3 reaction has been established at the CCSD(T)//M06-2X/6-311++G(3df,2p) level of theory. The conventional transition-state theory (TST) was utilized to calculate rate constants for the elementary reaction channels, while the stochastic RRKM-based master equation framework was applied for the T- and P-dependent rate-coefficient calculation of multiwell reaction paths. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH2 group of aniline (to form P1 (C6H5NH + CH4)) has been found to compete with the CH3-addition on the C atom at the ortho site of aniline (to form IS2) with the atmospheric rate expressions (in cm3 molecule-1 s-1) as ka1 = 7.5 × 10-23 T3.04 exp[(-40.63 ± 0.29 kJ·mol-1)/RT] and kb2 = 2.29 × 10-3 T-3.19 exp[(-56.94 ± 1.17 kJ·mol-1)/RT] for T = 300-2000 K and P = 760 Torr. Even though rate constants of several reaction channels decrease with increasing pressures, the total rate constant ktotal = 7.71 × 10-17 T1.20 exp[(-40.96 ± 2.18 kJ·mol-1)/RT] of the title reaction still increases as the pressure increases in the range of 76-76,000 Torr. The calculated enthalpy changes for some species are in good agreement with the available experimental data within their uncertainties (the maximum deviation between theory and experiment is ∼11 kJ·mol-1). The T1 diagnostic and spin contamination analysis for all species involved have also been observed. This work provides sound quality rate coefficients for the title reaction, which will be valuable for the development of detailed combustion reaction mechanisms for hydrocarbon fuels.

Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen.

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C3H3O2 potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice-Ramsperger-Kassel-Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH3CO + CO and CH2CO + HCO products. The branching ratios of CH3CO + CO and CH2CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300-2000 K, whereas CCCHO + H2O (0-10%) and CHCCO + H2O (0-17%) are significant minor products. The desirable products OH and H2O have been found for the first time. The individual rate constant of the C3H3 + O2 → CH2CO + HCO channel, 4.8 × 10-14 exp[(-2.92 kcal·mol-1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10-14 T0.33 exp[(-2.8 ± 0.03 kcal·mol-1)/(RT)], of the C3H3 + O2 reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7-5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875-883] with the uncertainty being less than 7.8%. At T < 500 K, the C3H3 + O2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O2 ⇌ C3H3O2. The calculated equilibrium constants, 2.60 × 10-16-8.52 × 10-16 cm3·molecule-1, were found to be in good agreement with the experimental data, being 2.48 × 10-16-8.36 × 10-16 cm3·molecule-1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.

Correction to Temperature and Pressure-Dependent Rate Constants for the Reaction of Propargyl Radical with Molecular Oxygen.

[This corrects the article DOI: 10.1021/acsomega.2c04316.].

Theoretical Study of the Kinetics of the Gas-Phase Reaction between Phenyl and Amino Radicals.

The potential energy surface (PES) of the C6H5 + NH2 reaction has been investigated by using ab initio CCSD(T)//B3LYP/6-311++G(3df,2p) calculations. The conventional transition-state theory (TST) and the variable reaction coordinate-TST (VRC-TST) have been used to predict the rate constants for the channels possessing tight and barrierless transition states, respectively. The Rice-Ramsperger-Kassel-Marcus/Master equation (RRKM/ME) theory has been utilized to determine the pressure-dependent rate constants for these reactions. The PES shows that the reaction begins with an exothermic barrierless addition of NH2 to C6H5 producing the vital intermediate state, namely, aniline (C6H5NH2, IS1). Once IS1 is generated, it can further isomerize to various intermediate states, which can give rise to different products, including PR4 (4,5,6-trihydro-1-amino phenyl + H2), PR5 (3,4,5,6-tetrahydro phenyl + NH3), PR6 (2,3,5,6-tetrahydro-1-imidogen phenyl + H2), PR9 (3,4,5,6-tetrahydro-1-imidogen phenyl + H2), and PR10 (2,5,6-trihydro-1-amino phenyl + H2), of which the most stable product, PR5, was formed by the most favorable channel going through the two advantageous transition states T1/11 (-28.9 kcal/mol) and T11P5 (-21.5 kcal/mol). The calculated rate constants for the low-energy channel, 1.37 × 10-9 and 2.16 × 10-11 cm3 molecule-1 s-1 at T = 300, P = 1 Torr and T = 2000 K, P = 760 Torr, respectively, show that the title reaction is almost pressure- and temperature-dependent. The negative temperature-dependent rate coefficients can be expressed in the modified Arrhenius form of k 1 = 8.54 × 1013 T -7.20 exp (-7.07 kcal·mol-1/RT) and k 2 = 2.42 × 1015 T -7.61 exp (-7.75 kcal·mol-1/RT) at 1 and 10 Torr, respectively, and in the temperature range of 300-2000 K. The forward and reverse rate coefficients as well as the high-pressure equilibrium constants of the C6H5 + NH2 ↔ IS1 process were also predicted; their values revealed that its kinetics do not depend on pressure at low temperature but strongly depend on pressure at high temperature. Moreover, the predicted formation enthalpies of reactants and the enthalpy changes of some channels are in good agreement with the experimental results.