Akira Miyoshi

To extend the rule‐based approach for hydrogen abstraction reactions from oxygenated compounds, a systematic investigation was performed to examine the reactivity of gas‐phase hydrogen abstraction reactions from alkyl groups (methyl and ethyl groups) bound to oxygen atoms in five types of oxygenated compounds (alcohols, ethers, formate esters, acetate esters, and carbonate esters) by H atoms and HO₂ radicals comprehensively considering rotational conformers. Quantum chemical calculations were conducted at the CBS‐QB3 level for stationary points. Rate constants were determined employing conventional transition state theory (TST). For hydrogen abstraction reactions by H, the rotational conformer distribution partition function was employed to approximate partition functions, owing to the similarity in vibrational energy‐level structures among conformers. In hydrogen abstraction reactions by HO₂, the vibrational structures of transition‐state (TS) conformers varied significantly due to the hydrogen bonding, leading to an inappropriate evaluation of rate constants when using the lowest‐energy conformer as a representative. Therefore, the rate constants were calculated by the multi‐structural TST. It was revealed that the differences in functional groups containing O atoms mainly affect the bond dissociation energies of the C–H bonds and the activation energies of hydrogen abstraction reactions only when the C atoms are adjacent to the O atoms. Additionally, it was found that hydrogen bonds formed in the TSs show minor effect on rate parameters for the overall rate constants, apart from the reduction of the pre‐exponential factors for the H‐abstraction reactions from the methylene position of ethyl groups. The comparison with the rate constants from previous studies showed reasonable results, indicating that the rate constants in this study, which thoroughly consider rotational conformers, can be the current best estimates.

Context. Cyanamide (NH₂CN) and its tautomer carbodiimide (NHCHN) are believed to have been key precursors of purines and pyrimidines during abiogenesis on primitive Earth. The detection of guanine and cytosine in meteorites and comets provides evidence of their nonterrestrial formation. Although NH₂CN has been found in several molecular clouds, NHCHN has only been detected in Sgr B2(N). Their possible molecular formation mechanisms in the gas phase and therefore their respective molecular precursors remain an open subject of investigation. Aims. The main objective of this paper is to determine which reactions can produce NH₂CN and HNCNH in the amounts observed under the astrophysical conditions of Sgr B2(N). The determination of their most likely precursors could serve to provide new insights into possible routes to purine and pyrimidine synthesis, and by extension to nucleosides, under the astrophysical conditions of dense molecular clouds. Methods. Initially, we proposed 120 reaction mechanisms, 60 being dedicated to NH₂CN formation and the remaining 60 to HNCNH. These mechanisms were constructed using 25 chemical species that were identified in outer space. We calculated the molecular energies of reactants and products at the CCSD(T)-F12/cc-pVTZ-F12 and MP2/aug-cc-pVDZ levels of theory, and defined the values of thermodynamic functions using the Maxwell-Boltzmann statistical quantum theory. Via an extensive literature review on the abundances of reactants and products in Sgr B2(N), in addition to a detailed kinetic study for a range of 20–300 K, we identify the most likely reaction mechanisms for both cyanamides of those proposed previously and presently. Results. From the 120 analyzed reactions, only nine for NH₂CN and four for HNCNH could thermodynamically account for their synthesis in Sgr B2(N). The kinetic portion of our study shows that Ra60 (CH₃NH₂ +·CN → NH₂CN +·CH₃), with a modified Arrhenius expression of k_T = 1.22 × 10⁻⁹ (T/300)^−0.038 exp⁻ (−147.34/T) cm³ mol⁻¹ s⁻¹, is the most efficient reaction at low temperatures (2 → NH₂CN +³NH) appears to be the most likely candidate, showing a modified Arrhenius constant of k_T = 2.51 × 10⁻¹¹ (T/300)^−32.18 exp⁻ (−1.332/T) cm³ mol⁻¹ s⁻¹. In the case of carbodiimide production, Rb18 (·H₂NC +·NH₂ → HNCNH +·H) is the most efficient reaction, fitting a rate constant of k_T = 4.70 × 10⁻¹³ (300/T)^−3.24 exp⁻ (36.28/T) cm³ mol⁻¹ s⁻¹ in Sgr B2(N). Conclusions. The detected gas-phase abundances of cyanamide (NH₂CN) in Sgr B2(N) can be explained as: Ra60 (·CN +·CH₃NH₂) from 20 to 60 K; Ra5: (·CN +·NH₂) from 60 to 120 K; and Ra37-2 (·HNCN +·NH₂) from 120 to 300 K. The carbodiimide (HNCNH) synthesis could proceed via Rb18 (·H₂NC +·NH₂). Moreover, the presence of·HNCN and·H₂NC in Sgr B2(N) are predicted here, making them viable candidates for future astronomical observations. The foreseen column density for the cyanomidil radical is ~10¹⁶ cm² s⁻¹ at 150 K or higher, while for amino methylidine, the value is a few 10¹³ cm² s⁻¹ at 100 K.

In this study, reaction characteristics of C₂H₄/O₂ and C₃H₆/O₂ on practical Pt/Al₂O₃ monolith catalyst are investigated, and a simplified microkinetic surface reaction mechanism is proposed by taking the observed surface species and their occupied active surface sites into account. Importance of more than one active PGM surface site occupations at the adsorption step of C₂H₄ or C₃H₆ and multiple-sited stable intermediate CH_x species are considered. Gas phase experimental results show that the light-off temperature as well as complete conversion temperature of both C₂H₄/O₂ and C₃H₆/O₂ mixture shift to lower side with the increase in oxygen concentration. The surface species analyses show the presence of stable intermediate CH_x species and linearly adsorbed CO on the platinum surface. Numerically simulated light-off performance with the proposed reaction mechanism shows good agreement with the experimental results of C₂H₄/O₂ and C₃H₆/O₂. Moreover, simulation results also agree well with the additional experimental results with multiple gas compositions and validate the proposed detailed reaction mechanism.

It is well known that the gas-phase autoignition phenomenon often involves branched chain reactions as well as the acceleration of reactions by thermal feedback. Despite the huge combustion kinetic mechanisms of large hydrocarbons found in practical fuels, chain reactions in the early stages of alkane autoignition exhibit simple kinetics since the pseudo-first-order assumption and the linear approximation are valid. In this study, this simple picture of autoignition will be presented starting from the H₂–O₂ system and then extending to practical fuel–air mixtures. The present interpretation gives the theoretical rationale for the Livengood–Wu integral which is known as an empirical method to predict the timing of knock in spark-ignition engines.

Zero-dimensional knock simulations have been performed by using the model proposed by Noda et al. [SAE technical paper 2004-01-0618] implemented in CHEMKIN-PRO software. A detailed chemical kinetic mechanism for fivecomponent gasoline surrogate was used with composition corresponding to JIS 2nd-grade gasoline (RON=90.8 and MON=82.9). An interesting effect of the cool flames appearing before the compression by flame propagation on the knocking was observed, which was suggested to play a key role in the recent development of SI engines. Present study implies that the modeling with kinetic mechanisms can be a powerful tool for technology breakthrough of next generation combustion engines.

The mechanism of the thermal unimolecular decomposition of C₃H₆ (propene) is studied both theoretically and experimentally. The potential energy surfaces for possible reaction pathways are investigated by CBS-QB3 level of quantum chemical calculations, and RRKM/master-equation calculation is performed for the main channels. The time evolutions of H atoms are observed experimentally by using a highly sensitive detection technique (ARAS, detection limit ≈ 10¹¹ atoms cm^–3) behind reflected shock waves (0.5–1.0 ppm C₃H₆ diluted in Ar, 1450–1710 K at 2.0 atm). The objective of this study is to examine the main product channels by combining the experimental and theoretical investigations on the yield and the rates of H atom production. Present quantum chemical calculations identify reactions (1a–1d) as the candidates of product channels: C₃H₆ → aC₃H₅ (allyl radical) + H (1a), C₃H₆ → CH₃ + C₂H₃ (vinyl radical) (1b), C₃H₆ → CH₄ + :CCH₂ (singlet vinyldene radical) (1c), and C₃H₆ → C₃H₄ (allene) + H₂ (1d). The RRKM calculations reveal the branching fractions for (1a), (1b), and (1c) to be approximately 0.8, 0.2, and 0.01, respectively. Reaction (1d) and other product channels are negligible (< 0.1 %), and the pressure dependence of the branching fraction is small under the present experimental conditions. The experimental yield of H atoms (1.7–2.0) is consistent with the theoretical branching fractions considering the H-atom production from the rapid subsequent thermal decomposition of a C₃H₅ and C₂H₃. From the observed time profiles of H atoms, the rate of overall thermal decomposition of C₃H₆ can be evaluated as Ln(k₁/s^–1) = (38.05 ± 1.18) – (48.91 ± 1.85) × 10³ K/T, which is in excellent agreement with the theoretical prediction.