Publications by authors named "Akira Matsugi"

27 Publications

  • Page 1 of 1

Potential Nonstatistical Effects on the Unimolecular Decomposition of HO.

Authors:
Akira Matsugi

J Phys Chem A 2022 Jul 29;126(27):4482-4496. Epub 2022 Jun 29.

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

An attempt is made to evaluate the nonstatistical effects in the thermal decomposition of hydrogen peroxide (HO). Previous experimental studies on this reaction reported an unusual pressure dependence of the rate constant indicating broader falloff behavior than expected from conventional theory. In this work, the possibility that the rate constant is affected by nonstatistical effects is investigated based on classical trajectory calculations on the global potential energy surfaces of HO and HO + Ar. The emphasis is on the intramolecular energy redistribution from the -rotor, that is, the external rotor for rotation around the principal axis of least moment of inertia. The calculations for the HO molecules excited above the dissociation threshold suggest that the energy redistribution from the torsion and -rotor to vibrations can be competitive with dissociation. In particular, the slow redistribution of the energy associated with the -rotor significantly affects the dissociation rate. The successive trajectory calculations for collisions of HO with Ar show that the energy associated with the -rotor can be collisionally transferred more efficiently than the vibrational energy. On the basis of these results and several assumptions, a simple model is proposed to account for the nonstatistical effects on the pressure-dependent thermal rate constants. The model predicts significant broadening of the falloff curve of the rate constants but still cannot fully explain the experimental data.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.2c03501DOI Listing
July 2022

Two-Dimensional Master Equation Modeling of Some Multichannel Unimolecular Reactions.

Authors:
Akira Matsugi

J Phys Chem A 2021 Apr 22;125(12):2532-2545. Epub 2021 Mar 22.

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Multichannel thermal decomposition reactions of -propyl radicals, 1-pentyl radicals, and toluene are investigated by solving a two-dimensional master equation formulated as a function of total energy () and angular momentum (). The primary aim of this study is to elucidate the role of angular momentum in the kinetics of multichannel unimolecular reactions. The collisional transition processes of the reactants colliding with argon are characterized based on the classical trajectory calculations and implemented in the master equation. The rate constants calculated by using the two-dimensional master equation are compared with those of one-dimensional master equations. The consequence of the explicit treatment of angular momentum depends on the dependence of the microscopic rate constants and is particularly emphasized in the thermal decomposition of toluene, for which the C-H and C-C bond fission channels are considered. The centrifugal effect is insignificant in the energetically favored C-H bond fission but is substantial in the energetically higher C-C bond fission, which causes rotational channel switching of the microscopic rate constants. The proper treatment of the -dependent channel coupling effect, weak collisional transfer of , and initial--dependent collisional energy transfer are found to be essential for predicting the branching fractions at low pressures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.1c00666DOI Listing
April 2021

Gas-phase reaction mechanism in chemical dry etching using NF and remotely discharged NH/N mixture.

RSC Adv 2020 Aug 20;10(51):30806-30814. Epub 2020 Aug 20.

Institute of Advanced Technology, ULVAC, Inc. 1220-1 Suyama Susono Shizuoka 410-1231 Japan.

Modeling of dry etching processes requires a detailed understanding of the relevant reaction mechanisms. This study aims to elucidate the gas-phase mechanism of reactions in the chemical dry etching process of SiO layers which is initiated by mixing NF gas with the discharged flow of an NH/N mixture in an etching chamber. A kinetic model describing the gas-phase reactions has been constructed based on the predictions of reaction channels and rate constants by quantum chemical and statistical reaction-rate calculations. The primary reaction pathway includes the reaction of NF with H atoms, NF + H → NF + HF, and subsequent reactions involving NF and other radicals. The reaction pathways were analyzed by kinetic simulation, and a simplified kinetic model composed of 12 reactions was developed. The surface process was also investigated based on preliminary quantum chemical calculations for ammonium fluoride clusters, which are considered to contribute to etching. The results indicate the presence of negatively charged fluorine atoms in the clusters, which are suggested to serve as etchants to remove SiO from the surface.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0ra05726fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC9056326PMC
August 2020

Modeling Collisional Transitions in Thermal Unimolecular Reactions: Successive Trajectories and Two-Dimensional Master Equation for Trifluoromethane Decomposition in an Argon Bath.

Authors:
Akira Matsugi

J Phys Chem A 2020 Aug 7;124(33):6645-6659. Epub 2020 Aug 7.

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Collisional transition processes in thermal unimolecular reactions are modeled by collision frequency, , and probability distribution function, (, ; ', '), which describes the probabilities of collisional transitions from the initial state specified by the total energy and angular momentum, (', '), to the final states, (, ). The validity of the collisional transition model, consisting of and (, ; ', '), is assessed here for the title reaction. The present model and its parameters are derived from the moments of transition probabilities calculated by classical trajectory simulations. The model explicitly accounts for coupling between the energy and angular momentum transfer and the dependence of transition probability on the initial state. The performance of the model is evaluated by comparing the rate constants calculated by solving the two-dimensional master equation with those obtained from the classical trajectory calculations of the sequence of successive collisions. The rate constants are also compared with available experimental data. The present collisional transition model is found to perform fairly well for predicting the pressure-dependent rate constants. The uncertainty in the prediction and sensitivities of the rate constants to the model parameters are discussed. A simplified version of the model is proposed, which performs as well as the full model. The simplifications and robust procedures for calculating the model parameters are described.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.0c05906DOI Listing
August 2020

A high-repetition-rate shock tube for transient absorption and laser-induced fluorescence studies of high-temperature chemical kinetics.

Authors:
Akira Matsugi

Rev Sci Instrum 2020 May;91(5):054101

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

A newly constructed high-repetition-rate shock tube designed for kinetic studies of high-temperature reactions using spectroscopic methods is described. The instrument operates at a 0.2-Hz cycle rate with a high reproducibility of reaction conditions that permits extensive signal averaging to improve the quality of kinetic trace data. The density and temperature of the gas behind the reflected shock wave are examined by probing the product formation from reference reactions. Two types of experimental techniques are implemented: transient absorption spectroscopy and time-resolved laser-induced fluorescence. Both methods are shown to be suitable for kinetic measurements of elementary reactions, as illustrated by their application in thermal decomposition reactions of the benzyl radicals and trifluoromethane.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/5.0007394DOI Listing
May 2020

Thermal Decomposition of Benzyl Radicals: Kinetics and Spectroscopy in a Shock Tube.

Authors:
Akira Matsugi

J Phys Chem A 2020 Feb 22;124(5):824-835. Epub 2020 Jan 22.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa , Tsukuba , Ibaraki 305-8569 , Japan.

Understanding the mechanism of high-temperature reactions of aromatic hydrocarbons and radicals is essential for the modeling of hydrocarbon growth processes in combustion environments. In this study, the thermal decomposition reaction of benzyl radicals was investigated using time-resolved broadband cavity-enhanced absorption spectroscopy behind reflected shock waves at a postshock pressure of 100 kPa and temperatures of 1530, 1630, and 1740 K. The transient absorption spectra during the decomposition were recorded over the spectral range of 282-410 nm. The spectra were contributed by the absorption of benzyl radicals and some transient and residual absorbing species. The temporal behavior of the absorption was analyzed using a kinetic model to determine the rate constant for benzyl decomposition. The obtained rate constants can be represented by the Arrhenius expression = 1.1 × 10 exp(-30 500 K/) s with an estimated logarithmic uncertainty of Δlog = ±0.2. Kinetic simulation of the secondary reactions indicated that fulvenallenyl radicals are potentially responsible for the transient absorption that appeared around 400 nm. This assignment is consistent with the available spectroscopic information of this radical. Possible candidates for the residual absorbing species are presented, suggesting the potential importance of -benzyne as a reactive intermediate.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.9b10705DOI Listing
February 2020

Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations.

J Phys Chem Lett 2020 Jan 16;11(1):67-74. Epub 2019 Dec 16.

National Institute for Environmental Studies , 16-2 Onogawa , Tsukuba 305-8506 , Japan.

The air-water interface plays central roles in "on-droplet" synthesis, living systems, and the atmosphere; however, what makes reactions at the interface specific is largely unknown. Here, we examined carbocationic reactions of monoterpene (CH isomer) on an acidic water microjet by using spray ionization mass spectrometry. Gaseous monoterpenes are trapped in the uppermost layers of a water surface via proton transfer and then undergo a chain-propagation reaction. The oligomerization pathway of β-pinene (), which showed prompt chain-propagation, is examined by simultaneous exposure to camphene (). ()H is the most stable isomer formed via rearrangement of ()H in the gas phase; however, no co-oligomerization was observed. This indicates that the oligomerization of ()H proceeded via ring-opening isomerization. Quantum chemical calculations for [carbocation-(HO)] complexes revealed that the ring-opened isomer is stabilized by hydrogen-π bonds. We propose that partial hydration is a key factor that makes the interfacial reaction unique.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.9b03110DOI Listing
January 2020

A simple heuristic approach to estimate the thermochemistry of condensed-phase molecules based on the polarizable continuum model.

Phys Chem Chem Phys 2019 Sep 27;21(35):18920-18929. Epub 2019 Aug 27.

Institute of Advanced Sciences, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Japan.

A simple model based on a quantum chemical approach with polarizable continuum models (PCMs) to provide reasonable translational and rotational entropies for liquid phase molecules was developed. A translational term was evaluated with free-volume compensation for the Sackur-Tetrode equation. We assumed that the free-volume corresponds to the cavity volume in the PCM. A rotational term was modeled as restricted rotation of a dipole in the electrostatic field. Entropies were assessed for twenty species in the liquid-phase using the proposed model, and the computed values were compared with experimental values. Quantum chemistry calculations were conducted at the ωB97X-D/6-311++G(d,p) level with the conductor-like PCM method. Predicted entropies were in good agreement with the experimental entropies, and the root mean square deviation was 17.2 J mol K. The standard enthalpy change of formation was then investigated for eleven specific species. The CBS-QB3//ωB97X-D method provides a reasonable standard enthalpy of formation for gasified species; however, improvement of the accuracy is required for liquid species. Finally, the dependence of the Gibbs energy on temperature was investigated for the eleven specific species. When the ideal gas treatment is used, the Gibbs energy trends for the gaseous and liquid phases are quasi-parallel for all of the species, although the Gibbs energy trends for liquids based on the proposed model intersected the gaseous trend (i.e. the intersection is the boiling point). However, the model significantly under or overestimated the experimental boiling points. The error of the boiling points was predominantly due to the inaccuracy of the enthalpy.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c9cp03226fDOI Listing
September 2019

Origin of Bath Gas Dependence in Unimolecular Reaction Rates.

Authors:
Akira Matsugi

J Phys Chem A 2019 Jan 18;123(4):764-770. Epub 2019 Jan 18.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa , Tsukuba , Ibaraki 305-8569 , Japan.

The bath gas dependence of thermal unimolecular reaction rates arises from different rates and efficiencies of collisions between reactant and third-body molecules. This study aims to unravel the mechanistic origin of this dependence based on the classical trajectories of methyl isocyanide (CHNC) colliding with 15 different bath gas molecules (CHNC, He, Ar, H, N, CO, CO, HCN, NH, CH, CHF, CF, CH, CH, and CH). The collision frequencies, energy transfer parameters, and relative third-body efficiencies are evaluated from the trajectory calculations. The relative third-body efficiencies of the studied bath gases are found to be in good agreement with available experimental data. The results indicate that differences in collision frequencies are the primary source of the bath gas dependence of the low-pressure rate constants. The nature of the long-range intermolecular interaction, particularly, its anisotropy, is suggested to play a key role in determining the collision frequency.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.8b11081DOI Listing
January 2019

Chain-propagation, chain-transfer, and hydride-abstraction by cyclic carbocations on water surfaces.

Phys Chem Chem Phys 2018 Oct;20(39):25256-25267

National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan.

Atmospheric particles contain a wide range of oligomers, but the formation mechanism and the origin of complexity are still unclear. Here, we report the direct detection of carbocationic oligomers generated from the exposure of a series of cyclic unsaturated hydrocarbon gases to acidic water microjets through interface-sensitive mass spectrometry. By changing gas concentrations, H2O (D2O) solvent, bulk pH and comparing results from experiments on acyclic, cyclic, and aromatic compounds, we elucidated three competing reaction mechanisms: chain propagation (CP), chain transfer (CT), and hydride abstraction (HA). We found that conjugative π-electron delocalization in the carbocation is the most important factor for the interfacial oligomerization processes. Our results showed that electrophilic attack on C[double bond, length as m-dash]C double bonds (CP and CT) is limited, and that on C-H single bonds (HA) is enhanced for carbocations lacking conjugation, which is not the case in bulk organic solutions. Carbocationic oligomers generated by the encounter of gaseous unsaturated hydrocarbons and acidic water surfaces potentially contribute to the molecular complexity in atmospheric particles.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8cp04993aDOI Listing
October 2018

Dissociation channels, collisional energy transfer, and multichannel coupling effects in the thermal decomposition of CHF.

Authors:
Akira Matsugi

Phys Chem Chem Phys 2018 Jun;20(22):15128-15138

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

The thermal unimolecular decomposition of CH3F has been studied with the aim of elucidating the multichannel character of the reaction. Experimentally, the temporal profiles of HF were recorded following the decomposition of CH3F in a shock tube. The profiles indicated that the yield of HF is close to unity at a pressure of ∼100 kPa (Ar bath) over the studied temperature range 1888-2279 K. The reaction channels were explored using quantum chemical calculations, which suggested that the decomposition of CH3F proceeds through direct C-H bond fission (CH3F → CH2F + H) or HF elimination (CH3F → 1CH2 + HF) reactions on the singlet potential energy surface. The rate constants were calculated by multichannel master equation analysis based on statistical reaction rate theory and classical trajectory calculations of the collisional energy transfer process. The analysis indicated that the two decomposition channels are competitive at the high-pressure limit but the 1CH2 + HF channel is dominant under the experimental conditions due to the multichannel coupling effect. The collision model dependency of the predicted rate constants and branching fractions has also been investigated, highlighting the importance of selecting the appropriate model for the collision frequency and energy transfer probability function.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8cp02126kDOI Listing
June 2018

Controlling factors of oligomerization at the water surface: why is isoprene such a unique VOC?

Phys Chem Chem Phys 2018 Jun;20(22):15400-15410

Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan.

Recent studies have shown that atmospheric particles are sufficiently acidic to enhance the uptake of unsaturated volatile organic compounds (VOCs) by triggering acid-catalyzed oligomerization. Controlling factors of oligomerization at the aqueous surfaces, however, remain to be elucidated. Herein, isoprene (2-methyl-1,3-butadiene, ISO), 1,3-butadiene (1,3-b), 1,4-pentadiene (1,4-p), 1-pentene (1-p), and 2-pentene (2-p) vapors are exposed to an acidic water microjet (1 ≤ pH ≤ 5), where cationic products are generated on its surface within ∼10 μs and directly detected using surface-sensitive mass spectrometry. We found that carbocations form at the air-water interface in all the cases, whereas the extent of oligomerization largely depends on the structure in the following order: ISO ≫ 1,3-b > 1,4-p ≫ 1-p ≈ 2-p. Importantly, the cationic oligomerization of ISO yields a protonated decamer ((ISO)10H+, a C50 species of m/z 681.6), while the pentenes 1-p/2-p remain as protonated monomers. We suggest that ISO oligomerization is uniquely facilitated by (1) the resonance stabilization of (ISO)H+ through the formation of a tertiary carbocation with a conjugated C[double bond, length as m-dash]C bond pair, and (2) π-electron enrichment induced by the neighboring methyl group. Experiments in D2O and D2O : H2O mixtures revealed that ISO oligomerization on the acidic water surface proceeds via two competitive mechanisms: chain-propagation and proton-exchange reactions. Furthermore, we found that ISO carbocations undergo addition to relatively inert 1-p, generating hitherto uncharacterized co-oligomers.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8cp01551aDOI Listing
June 2018

Collision Frequency for Energy Transfer in Unimolecular Reactions.

Authors:
Akira Matsugi

J Phys Chem A 2018 Mar 13;122(8):1972-1985. Epub 2018 Feb 13.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Pressure dependence of unimolecular reaction rates is governed by the energy transfer in collisions of reactants with bath gas molecules. Pressure-dependent rate constants can be theoretically determined by solving master equations for unimolecular reactions. In general, master equation formulations describe energy transfer processes using a collision frequency and a probability distribution model of the energy transferred per collision. The present study proposes a novel method for determining the collision frequency from the results of classical trajectory calculations. Classical trajectories for collisions of several polyatomic molecules (ethane, methane, tetrafluoromethane, and cyclohexane) with monatomic colliders (Ar, Kr, and Xe) were calculated on potential energy surfaces described by the third-order density-functional tight-binding method in combination with simple pairwise interaction potentials. Low-order (including non-integer-order) moments of the energy transferred in deactivating collisions were extracted from the trajectories and compared with those derived using some probability distribution models. The comparison demonstrates the inadequacy of the conventional Lennard-Jones collision model for representing the collision frequency and suggests a robust method for evaluating the collision frequency that is consistent with a given probability distribution model, such as the exponential-down model. The resulting collision frequencies for the exponential-down model are substantially higher than the Lennard-Jones collision frequencies and are close to the (hypothetical) capture rate constants for dispersion interactions. The practical adequacy of the exponential-down model is also briefly discussed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.8b00444DOI Listing
March 2018

Thermal Decomposition of 2,3,3,3- and trans-1,3,3,3-Tetrafluoropropenes.

J Phys Chem A 2017 Jul 27;121(26):4881-4890. Epub 2017 Jun 27.

Department of Materials and Life Sciences, Sophia University , 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan.

The thermal decomposition reactions of 2,3,3,3- and trans-1,3,3,3-tetrafluoropropenes (TFPs) have been studied both experimentally and computationally to elucidate their kinetics and mechanism. The experiments were performed by observing the temporal profiles of HF produced in the decomposition of the tetrafluoropropenes behind shock waves at temperatures of 1540-1952 K (for 2,3,3,3-TFP) or 1525-1823 K (for trans-1,3,3,3-TFP) and pressure of 100-200 kPa in Ar bath. The reaction pathways responsible for the profiles were explored based on quantum chemical calculations. The decomposition of 2,3,3,3-TFP was predicted to proceed predominantly via direct 1,2-HF elimination to yield CHCCF, while trans-1,3,3,3-TFP was found to decompose to HF and a variety of isomeric CHF products including CHCCF, CFCCHF, CCHCF, and CFCHCF. The CHF isomers can subsequently decompose to either CF + CHCF or CFCC + HF products. Multichannel RRKM/master equation calculations were performed for the identified decomposition channels. The observed formation rates and apparent yields of HF are shown to be consistent with the computational predictions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.7b04086DOI Listing
July 2017

Thermal Decomposition of Nitromethane and Reaction between CH and NO.

J Phys Chem A 2017 Jun 26;121(22):4218-4224. Epub 2017 May 26.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

The thermal decomposition of gaseous nitromethane and the subsequent bimolecular reaction between CH and NO have been experimentally studied using time-resolved cavity-enhanced absorption spectroscopy behind reflected shock waves in the temperature range 1336-1827 K and at a pressure of 100 kPa. Temporal evolution of NO was observed following the pyrolysis of nitromethane (diluted to 80-140 ppm in argon) by monitoring the absorption around 400 nm. The primary objectives of the current work were to evaluate the rate constant for the CH + NO reaction (k) and to examine the contribution of the roaming isomerization pathway in nitromethane decomposition. The resultant rate constant can be expressed as k = (9.3 ± 1.8) × 10(T/K) cm molecule s, which is in reasonable agreement with available literature data. The decomposition of nitromethane was found to predominantly proceed with the C-N bond fission process with the branching fraction of 0.97 ± 0.06. Therefore, the upper limit of the branching fraction for the roaming pathway was evaluated to be 0.09.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.7b03715DOI Listing
June 2017

Time-Resolved Broadband Cavity-Enhanced Absorption Spectroscopy behind Shock Waves.

J Phys Chem A 2016 Apr 28;120(13):2070-7. Epub 2016 Mar 28.

Department of Materials and Life Sciences, Sophia University , 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan.

A fast and sensitive broadband absorption technique for measurements of high-temperature chemical kinetics and spectroscopy has been developed by applying broadband cavity-enhanced absorption spectroscopy (BBCEAS) in a shock tube. The developed method has effective absorption path lengths of 60-200 cm, or cavity enhancement factors of 12-40, over a wavelength range of 280-420 nm, and is capable of simultaneously recording absorption time profiles over an ∼32 nm spectral bandpass in a single experiment with temporal and spectral resolutions of 5 μs and 2 nm, respectively. The accuracy of the kinetic and spectroscopic measurements was examined by investigating high-temperature reactions and absorption spectra of formaldehyde behind reflected shock waves using 1,3,5-trioxane as a precursor. The rate constants obtained for the thermal decomposition reactions of 1,3,5-trioxane (to three formaldehyde molecules) and formaldehyde (to HCO + H) agreed well with the literature data. High-temperature absorption cross sections of formaldehyde between 280 and 410 nm have been determined at the post-reflected-shock temperatures of 955, 1265, and 1708 K. The results demonstrate the applicability of the BBCEAS technique to time- and wavelength-resolved sensitive absorption measurements at high temperatures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.6b01069DOI Listing
April 2016

Dissociation of 1,1,1-trifluoroethane is an intrinsic RRKM process: classical trajectories and successful master equation modeling.

Authors:
Akira Matsugi

J Phys Chem A 2015 Mar 18;119(10):1846-58. Epub 2015 Feb 18.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Rate constants for thermal decomposition of 1,1,1-trifluoroethane (CH3CF3) in the high-temperature falloff region were previously reported to have an unusual pressure dependence that could not be explained by Rice-Ramsperger-Kassel-Marcus (RRKM) theory in combination with unimolecular master equation analysis. This study investigates the dynamics of the CH3CF3 dissociation and the energy transfer of CH3CF3 in collisions with Ar and Kr by classical trajectory calculations on a global potential energy surface constructed from a large number of quantum chemical calculations. The simulations showed that the ensemble-averaged CH3CF3 populations decay with single exponential profiles that have rate constants close to those predicted by RRKM theory, indicating that the microcanonical ensemble is maintained during decomposition. The trajectory calculation also indicated that a significant portion of the HF product is formed in its vibrationally excited state. Such observation motivated this study to correct some of the reported rate constants for the CH3CF3 decomposition. With the correction applied, the experimental rate constants were well reproduced by the RRKM/master equation calculation using the collisional energy transfer parameters that were also obtained from trajectory calculations. Overall, the title reaction is demonstrated to be another successful example of RRKM/master equation modeling.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.5b00796DOI Listing
March 2015

Thermal decomposition of 1,1,1-trifluoroethane revisited.

J Phys Chem A 2014 Dec 5;118(50):11688-95. Epub 2014 Dec 5.

National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

1,1,1-Trifluoroethane (CH3CF3) has been frequently used as a chemical thermometer or an internal standard in shock tube studies to determine relative rates of chemical reactions. The rate constants for the thermal decomposition of CH3CF3 were recently reported to have anomalous pressure dependence in the high-temperature falloff region. In the present study, the kinetics of the CH3CF3 decomposition were reinvestigated using shock tube/laser absorption (ST/LA) spectroscopy and single-pulse shock tube (SPST) methods over the temperature range 1163-1831 K at pressures from 95 to 290 kPa. The present rate constants are 2-3 times smaller than those reported in previous single-pulse experiments performed at near high-pressure limit conditions. The recommended rate constant expression, k = 5.71 × 10(46)T(-9.341) exp(-47073 K/T) s(-1), was obtained over the temperature range 1000-1600 K with uncertainties of ±40% at temperatures below 1300 K and ±32% at 1600 K. The rate constants at the high-temperature region showed clear falloff behavior and were in good agreement with recent high-temperature experiments. The falloff rate constants could not be reproduced by a standard RRKM/master-equation model; this study provides additional evidence for the unusual pressure dependence previously reported for this reaction. Additionally, the rate constants for the decomposition of 1,1-difluoroethylene (CH2CF2) were determined over the temperature range 1650-2059 K at pressures of 100 and 205 kPa, and were reproduced by the RRKM/master-equation calculation with an average downward energy transfer of 900 cm(-1).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp510227kDOI Listing
December 2014

Mode selective dynamics and kinetics of the H2 + F2 → H + HF + F reaction.

Phys Chem Chem Phys 2014 Nov;16(41):22517-26

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

The reaction between vibrationally excited H2 and F2 had previously been suggested to be a critical chain-branching step in the combustion of mixtures containing H2 and F2. In the present study, the vibrational state specific dynamics and kinetics for the reaction H2 + F2 → H + HF + F were investigated by quasiclassical trajectory (QCT) and quantum mechanical (QM) reactive scattering calculations on an accurate potential energy surface that was constructed based on a large number of quantum chemical calculations at the MRCI-F12(CV)+Q/cc-pCVTZ-F12 level. The reaction probabilities for in collinear configurations were obtained from the QCT and QM calculations, and the state specific rate constants were evaluated by the full-dimensional QCT calculations. Both the collinear and full-dimensional results demonstrated that can be significantly promoted by vibrational excitation of F2, whereas excitation of H2 vibration has a smaller effect on the reactivity. This indicates that the rate constants for the presumed chain-branching reaction, H2(ν = 1) + F2 → H + HF + F, used in the previous kinetic modeling study of H2-F2 combustion were overestimated. The mode-selective reactivity observed for was interpreted in terms of the coupling between the vibrational modes of the reactants and the reaction coordinate motion.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c4cp03362kDOI Listing
November 2014

Shock tube study on the thermal decomposition of fluoroethane using infrared laser absorption detection of hydrogen fluoride.

J Phys Chem A 2014 Aug 11;118(34):6832-7. Epub 2014 Aug 11.

Research Institute of Science for Safety and Sustainability, National Institute of Advanced Industrial Science and Technology , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Motivated by recent shock tube studies on the thermal unimolecular decomposition of fluoroethanes, in which unusual trends have been reported for collisional energy-transfer parameters, the rate constants for the thermal decomposition of fluoroethane were investigated using a shock tube/laser absorption spectroscopy technique. The rate constants were measured behind reflected shock waves by monitoring the formation of HF by IR absorption at the R(1) transition in the fundamental vibrational band near 2476 nm using a distributed-feedback diode laser. The peak absorption cross sections of this absorption line have also been determined and parametrized using the Rautian-Sobel'man line shape function. The rate constant measurements covered a wide temperature range of 1018-1710 K at pressures from 100 to 290 kPa, and the derived rate constants were successfully reproduced by the master equation calculation with an average downward energy transfer, ⟨ΔEdown⟩, of 400 cm(-1).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp5066709DOI Listing
August 2014

Roaming Dissociation of Ethyl Radicals.

Authors:
Akira Matsugi

J Phys Chem Lett 2013 Dec 2;4(24):4237-40. Epub 2013 Dec 2.

National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Previous studies on the photodissociation of C2H5 reported rate constants for H-atom formation several orders of magnitude smaller than that predicted by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. This Letter provides a potential explanation for this anomaly, based on direct trajectory calculations of C2H5 dissociation. The trajectories reveal the existence of a roaming dissociation channel that leads to the formation of C2H3 and H2. This channel is found to proceed over the ridge between the transition state of H-atom elimination and that of bimolecular H-abstraction. The formed C2H3 radical can subsequently dissociate to C2H2 and a H atom; this secondary dissociation is suggested to be a potential reason for the unexpectedly slow H-atom formation observed in the photodissociation experiments.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jz4024018DOI Listing
December 2013

Chain reaction mechanism in hydrogen/fluorine combustion.

J Phys Chem A 2013 Dec 9;117(51):14042-7. Epub 2013 Dec 9.

Research Institute of Science for Safety and Sustainability and ‡Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

Vibrationally excited species have been considered to play significant roles in H2/F2 reaction systems. In the present study, in order to obtain further understanding of the chain reaction mechanism in the combustion of mixtures containing H2 and F2, burning velocities for H2/F2/O2/N2 flames were measured and compared to that obtained from kinetic simulations using a detailed kinetic model, which involves the vibrationally excited species, HF(ν) and H2(ν), and the chain-branching reactions, HF(ν > 2) + F2 = HF + F + F (R1) and H2(ν = 1) + F2 = HF + H + F (R2). The results indicated that reaction R1 is not responsible for chain branching, whereas reaction R2 plays a dominant role in the chain reaction mechanism. The kinetic model reproduced the experimental burning velocities with the presumed rate constant of k2 = 6.6 × 10(-10) exp(-59 kJ mol(-1)/RT) cm(3) s(-1) for R2. The suggested chain-branching reaction was also investigated by quantum chemical calculations at the MRCI-F12+CV+Q/cc-pCVQZ-F12 level of theory.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp410597nDOI Listing
December 2013

Reactions of o-benzyne with propargyl and benzyl radicals: potential sources of polycyclic aromatic hydrocarbons in combustion.

Phys Chem Chem Phys 2012 Jul 8;14(27):9722-8. Epub 2012 Jun 8.

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

The kinetics and mechanisms of the reactions of o-benzyne with propargyl and benzyl radicals have been investigated computationally. The possible reaction pathways have been explored by quantum chemical calculations at the M06-2X/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level and the mechanisms have been investigated by the Rice-Ramsperger-Kassel-Marcus theory/master-equation calculations. It was found that the o-benzyne associates with the propargyl and benzyl radicals without pronounced barriers and the activated adducts easily isomerize to five-membered ring species. Indenyl radical and fluorene + H were predicted to be dominantly produced by the reactions of o-benzyne with propargyl and benzyl radicals, respectively, with the rate constants close to the high-pressure limits at temperatures below 2000 K. The related reactions on the two potential energy surfaces, namely, the reaction between fulvenallenyl radical and acetylene and the decomposition reactions of indenyl and α-phenylbenzyl radicals were also investigated. The high reactivity of o-benzyne toward the resonance stabilized radicals suggested a potential role of o-benzyne as a precursor of polycyclic aromatic hydrocarbons in combustion.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c2cp41002hDOI Listing
July 2012

Kinetics and mechanisms of the allyl + allyl and allyl + propargyl recombination reactions.

J Phys Chem A 2011 Jul 2;115(26):7610-24. Epub 2011 Jun 2.

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

The kinetics and mechanisms of the self-reaction of allyl radicals and the cross-reaction between allyl and propargyl radicals were studied both experimentally and theoretically. The experiments were carried out over the temperature range 295-800 K and the pressure range 20-200 Torr (maintained by He or N(2)). The allyl and propargyl radicals were generated by the pulsed laser photolysis of respective precursors, 1,5-hexadiene and propargyl chloride, and were probed by using a cavity ring-down spectroscopy technique. The temperature-dependent absorption cross sections of the radicals were measured relative to that of the HCO radical. The rate constants have been determined to be k(C(3)H(5) + C(3)H(5)) = 1.40 × 10(-8)T(-0.933) exp(-225/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.088) and k(C(3)H(5) + C(3)H(3)) = 1.71 × 10(-7)T(-1.182) exp(-255/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.069) with 2σ uncertainty limits. The potential energy surfaces for both reactions were calculated with the CBS-QB3 and CASPT2 quantum chemical methods, and the product channels have been investigated by the steady-state master equation analyses based on the Rice-Ramsperger-Kassel-Marcus theory. The results indicated that the reaction between allyl and propargyl radicals produces five-membered ring compounds in combustion conditions, while the formations of the cyclic species are unlikely in the self-reaction of allyl radicals. The temperature- and pressure-dependent rate constant expressions for the important reaction pathways are presented for kinetic modeling.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp203520jDOI Listing
July 2011

Deuterium kinetic isotope effects on the gas-phase reactions of C2H with H2(D2) and CH4(CD4).

Phys Chem Chem Phys 2011 Mar 14;13(9):4022-31. Epub 2011 Jan 14.

Department of Chemical Systems Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Kinetics of the ethynyl (C(2)H) radical reactions with H(2), D(2), CH(4) and CD(4) was studied over the temperature range of 295-396 K by a pulsed laser photolysis/chemiluminescence technique. The C(2)H radicals were generated by ArF excimer-laser photolysis of C(2)H(2) or CF(3)C(2)H and were monitored by the chemiluminescence of CH(A(2)Δ) produced by their reaction with O(2) or O((3)P). The measured absolute rate constants for H(2) and CH(4) agreed well with the available literature data. The primary kinetic isotope effects (KIEs) were determined to be k(H(2))/k(D(2)) = 2.48 ± 0.14 and k(CH(4))/k(CD(4)) = 2.45 ± 0.16 at room temperature. Both of the KIEs increased as the temperature was lowered. The KIEs were analyzed by using the variational transition state theory with semiclassical small-curvature tunneling corrections. With anharmonic corrections on the loose transitional vibrational modes of the transition states, the theoretical predictions satisfactorily reproduced the experimental KIEs for both C(2)H + H(2)(D(2)) and C(2)H + CH(4)(CD(4)) reactions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c0cp02056gDOI Listing
March 2011

Rate constants and kinetic isotope effects on the reaction of C2(X(1)Sigma(g)+) with CH4 and CD4.

J Phys Chem A 2010 Apr;114(13):4580-5

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Rate constants and kinetic isotope effect (KIE) for the reaction of singlet dicarbon, C(2)(X(1)Sigma(g)(+)), with CH(4) and CD(4) have been measured over the temperature range 294-376 K by using the pulsed laser photolysis/laser-induced fluorescence technique. C(2)(X(1)Sigma(g)(+)) were generated by multiphoton laser decomposition of C(2)Cl(4) at 248 nm and its decay trace was monitored on the (0,0) band of the Mulliken system at 231.2 nm. Measured rate constants showed slightly positive temperature dependence, whereas the KIE [= k(CH(4))/k(CD(4))] was almost independent of temperature and the value of which was 2.1 +/- 0.2 as a simple average of the values of KIE at different temperatures. Quantum chemical calculation with CASPT2 method indicated that the reaction proceeds via a direct hydrogen abstraction mechanism to form C(2)H and CH(3) radicals. Variational transition-state theory calculations were performed employing a dual-level method. Anharmonic effects along transitional modes were included in the calculation, and a comparison of the rate constants with and without anharmonic corrections demonstrated the importance of anharmonicity. The calculated rate constants and KIE showed good agreement with the experiments except for the temperature dependence of the KIE. A possible cause of the discrepancy was discussed in terms of the long-range interaction between the reactants.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp1012494DOI Listing
April 2010

Mechanism and kinetic isotope effect of the reaction of C2(X1Sigma(g)+) radicals with H2 and D2.

J Phys Chem A 2009 Aug;113(31):8963-70

Department of Chemical System Engineering, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

The rate constants for the reactions of C2(X1Sigma(g)+) with H2 and D2 have been investigated experimentally and theoretically to assess the statistical theory of the reaction and to reveal the mechanism of the reaction. The ground-state C2 radicals were generated by multiphoton laser-photolysis of C2Cl4 at 248 nm and were probed by a laser-induced fluorescence method using the Mulliken system (D1Sigma(u)+-X1Sigma(g)+). Rate constants have been measured to be k[C2(X)+H2] = 5.6 x 10(-11) exp[-9.1 (kJ mol(-1))/RT] and k[C2(X)+D2] = 3.2 x 10(-11) exp[-9.9 (kJ mol(-1))/RT] cm3 molecule(-1) s(-1) in the temperature range 293-395 K and at total pressure around 10 Torr (He buffer). Quantum chemical calculations at the MRCI level revealed that the reaction predominantly proceeds via a collinear direct-abstraction transition state. The measured rate constants as well as the kinetic isotope effect were well reproduced by the transition-state theory based on the MRCISD+Q/aug-cc-pV5Z calculations, provided that the anharmonic bending vibrations of the transition states were properly treated. The effect of the Davidson correction was found to be significant for the potential energy surface around the early transition state.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp904165sDOI Listing
August 2009
-->