Mechanism and kinetics of the reaction of the 2‐propargyl radical with ammonia

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 k_a1 = 7.5 × 10^-23 T^3.04 exp[(-40.63 ± 0.29 kJ·mol^-1)/RT] and k_b2 = 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 k_total = 7.71 × 10^-17 T^1.20 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 T^0.33 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.

Gas Phase Reaction of Isocyanic Acid: Kinetics, Mechanisms, and Formation of Isopropyl Aminocarbonyl

Isocyanic acid, HNCO, mainly emitted by combustion processes, is doubted to be detrimental to human health if its concentration surpasses ∼1 ppbv. Very little information has been found regarding the HNCO loss in the gas phase. This study aims to close this knowledge gap by performing a theoretical kinetic study on the reaction of HNCO with the propargyl radical. The potential energy surface of the HNCO + C₃H₃ reaction was characterized utilizing high-level CCSD(T)/CBS(TQ5)//B3LYP/6-311++G(3df,2p) quantum-chemical approaches, followed by TST and RRKM/ME kinetic computations. The obtained results reveal that the reaction can proceed via H-abstraction, leading to the C₃H₄ + NCO bimolecular products with energy barriers of 23-25 kcal/mol, and/or addition, resulting in C₄H₄NO intermediates with 23-26 kcal/mol barrier heights. The C₄H₄NO adducts when formed can decompose to products and/or return to HNCO + C₃H₃ in which the reverse decompositions are found to be dominant with a branching ratio that accounts for nearly 100% at 300 K and 760 Torr. The calculated P-independent rate coefficients indicate that at low temperatures, the H-abstraction channels are insignificant. However, at high temperatures (T > 1500 K), the H-abstraction path leading to H₃CCCH + NCO prevails with a branching ratio of ∼50-53% in the descending 1800-1500 K temperature range at 760 Torr, while the H-abstraction leading to H₂CCCH₂ + NCO is favorable at T > 1800 K, with the yield reaching above 50% at 760 Torr. In contrast to the H-abstraction rate constants, the calculated values for the additions and the C₄H₄NO decompositions show a positive pressure dependence. Both the total rate constants for the reactions HNCO + C₃H₃ → products and C₄H₄NO → products, which are, respectively, k _{_total_bimo}(T) = 3.53 × 10^-23 T ^3.27 exp[(-21.35 ± 0.06 kcal/mol)/RT] cm³ molecule^-1 s^-1 and k _{_total_uni}(T) = 1.13 × 10²⁵ T ^-4.02 exp[(-11.77 ± 0.16 kcal/mol)/RT] s^-1, increase with the increasing temperature in the 300-2000 K range at 760 Torr. The rate constant of HNCO + C₃H₃ → products is about 8 orders of magnitude smaller than the value of HCHO + C₃H₃ → products, showing that HCHO is more reactive toward the C₃H₃ free radicals than HNCO. The computed heats of formation for several species agree well with the available literature data with the deviation less than 1.0 kcal/mol, indicating that the methods used in this study are extremely reliable. With the given results, it is vigorously suggested that the predicted rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling HNCO-related systems under atmospheric and combustion conditions.