Ab initio kinetics of gas phase decomposition reactions

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process. To overcome this ambiguity, a theoretical study of a complete picture of gas phase thermo-decomposition, including reaction energies, activation barriers, and reaction rates, is illustrated with the example of the β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) molecule by means of quantum-chemical calculations. We study three types of major decomposition reactions characteristic of nitramines: the HONO elimination, the NONO rearrangement, and the N-NO(2) homolysis. The reaction rates were determined using the conventional transition state theory for the HONO and NONO decompositions and the variational transition state theory for the N-NO(2) homolysis. Our calculations show that the HMX decomposition process is more complex than it was previously believed to be and is defined by a combination of reactions at any given temperature. At all temperatures, the direct N-NO(2) homolysis prevails with the activation barrier at 38.1 kcal/mol. The nitro-nitrite isomerization and the HONO elimination, with the activation barriers at 46.3 and 39.4 kcal/mol, respectively, are slow reactions at all temperatures. The obtained conclusions provide a consistent interpretation for the reported experimental data.     

The kinetics of the addition of alkyl radicals to carbonyl groups. I. The kinetics of the gas phase decomposition reactions of the trifluoro-t-butoxy radical

Trifluoro-t-butoxy radicals have been generated by reacting fluorine with 2-trifluoromethyl propan-2-ol: Over the temperature range 361-600 K the trifluoro-t-butoxy radical decomposes exclusively by loss of the ? CF₃ group [reaction (?2)] rather than by loss of ? CH₃ group [reaction (?1)]: The limits of detectability of the product CF₃COCH₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_?1 of ca. 75. The implications of these results in relation to the reverse radical addition reactions to the carbonyl group are discussed along with the thermochemistry of the reactions.     

The kinetics and mechanism of the shock induced gas phase decomposition of ethylsilane

The decomposition kinetics of ethylsilane under shock tube conditions (P_T ca. 3100 torr, T ? 1080–1245 K), both in the absence and presence of silylene trapping agents (butadiene and acetylene) are reported. Arrhenius parameters under maximum butadiene inhibition are: log k(C₂H₅SiH₃) = 15.14-64,769 ± 1433 cal/2.303 RT; log k(C₂H₅SiD₃) = 15.29-66,206 ± 1414/2.303 RT. The uninhibited reaction is subject to silylene induced decomposition (63% lowest T -- 24% highest T). Major reaction products are ethylene and hydrogen, consistent with two dominant primary dissociation reactions: C₂H₅SiD₃ → C₂H₅SiD + D₂, ? ? 0.66; C₂H₅SiD₃ → CH₃CH = SiD₂ + HD, ? ? 0.30. Minor products suggest several other less important primary processes: alkane elimination, ? ?0.02, and free-radical production via simple bond fission, ? ?0.02. An upper limit for the activation energy of the decomposition, C₂H₅SiH → C₂H₄ + SiH₂, of E < 30 ± 4 kcal is established, and speculations on the mechanism of this decomposition (concerted or stepwise) with conclusions in favor of the stepwise path are made. Computer modeling studies for the reaction both in the absence and presence of butadiene are shown to be in good agreement with the experimental observations.     

The kinetics of the addition of alkyl radicals to carbonyl groups. Part I. The addition of methyl radicals to hexafluoroacetone in the gas phase. The formation of the hexafluoro-t-butoxy radical

By pyrolyzing di-t-butyl peroxide over the temperature range of 405–450 K in the presence of hexafluoroacetone the kinetics of the addition reaction (1), CH₃ + (CF₃)₂CO→; (CF₃)₂C(?)CH₃, have been studied. Detailed analyses have shown that the principal product of the adduct radical, (CF₃)₂C(?)CH₃, is CF₃COCH₃ from reaction (2), (CF₃)₂C(?)CH₃ → CF₃COCH₃ + CF₃. The rate constant of the addition reaction was determined to be k₁(dm³/mol·s) = (1.1 ± 4.0) + 10⁹ exp(-(3680 ± 480)/T) over the temperature range 405–450 K, based on the value k₃ = 2.2 × 10¹⁰ dm³/mol·s for reaction (3), 2CH₃ → C₂H₆. The results are discussed in relation to existing data for radical additions to carbonyl groups.     

Thermochemical kinetics of nitrogen compounds. V. The unimolecular thermal decomposition of diallylamine in the gas phase

The kinetics of the thermal decomposition of diallylamine to propylene and prop-2-enaldimine have been studied in the gas phase in presence of an excess of methylamine over the temperature range of 532.7 to 615.6°K, using a static reaction system. Methylamine reacted with the unstable primary product prop-2-enaldimine, forming the thermally stable N-methyl prop-2-enaldimine. First-order rate constants, based on the internal standard technique, fit the Arrhenius relationship log k(s^?1) = (11.04 ± 0.13) ? (37.11 ± 0.33 kcal/mole)/2.303 RT. They were independent on the initial total pressure (46–340 torr), the initial pressure of diallylamine (9.2–65 torr), or methylamine as well as the conversion attained. Despite an apparent surface sensitivity, the reaction is essentially homogeneous in nature as demonstrated by experiments carried out in a packed reaction vessel. The observed activation parameters for the title reaction together with those observed earlier for triallylamine and allylcyclohexylamine are consistent with the proposed concerted reaction mechanism involving a cyclic 6-center transition state. The observed substituent effects suggest a nonsynchronous mode of bond breaking and bond formation.     

Low temperature kinetics of unstable radical reactions

Recent advances in Earth and satellite based observations of molecules in interstellar environments and in planetary atmospheres have highlighted the lack of information regarding many important gas-phase formation mechanisms involving neutral species at low temperatures. Whilst significant progress has been made towards a better understanding of radical-molecule reactions in these regions, the inherent difficulties involved in the investigation of reactions between two unstable radical species have hindered progress in this area. This perspective article provides a brief review of the most common techniques applied to study radical-radical reactions below room temperature, before outlining the developments in our laboratory that have allowed us to extend such measurements to temperatures relevant to astrochemical environments. These developments will be discussed with particular emphasis on our recent investigations of the reactions of atomic nitrogen with diatomic radicals.     

The two-parameter taft equation in kinetics of free radical reactions

A new method was developed for the calculation of the resonance substituent constants of the two-parameter Taft equation log k_sub/k₀=ρ*σ*+r*σ^r. It is based on a relationship between the spin density in free radicals and the rate constants of radical substitution reactions of CH₃. Possibilities and limitations of the application of this correlation equation to the investigation of substitution and addition radical reactions are discussed.     

The kinetics of hydroxyl radical reactions with cyclopropane and cyclobutane

A laser flash photolysis/resonance fluorescence investigation has been carried out to study the kinetics of the overall reactions OH + cyclopropane (1) and OH + cyclobutane (2) in the temperature range 298–490 K and at 298 K, respectively. The following kinetic parameters have been determined: k₁ =(3.9 ±0.6) 10⁻¹²exp- (2.2 ± 0.1)kcal mol⁻¹/RT molecule⁻¹cm³s⁻¹, k₂(298 K) = (17.5 ± 1.5)10⁻¹³molecule⁻¹ cm³s⁻¹.     

The preexponential factor of reactions of radical decomposition of nitro compounds

Conclusions An analysis of the experimental data on the decomposition of aliphatic nitro compounds reveals fine effects of the influence of the structure of the molecule on the preexponential factor of the reaction rate constant. The value of the latter depends on the contribution of three factors, manifested in the formation of the activated complex: liberation of the internal rotation relative to the dissociating C-N bond, a decrease in the frequencies of the deformational vibrations associated with this bond, and rotation of the alkyl groups next to the reaction center.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 811–816, April, 1972.     

The unimolecular elimination kinetics of benzaldoxime in the gas phase

The kinetics of the gas‐phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C–400°C and 56–140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was found to be homogeneous, unimolecular, and tend to obey a first‐order rate law. The observed rate coefficient is represented by the following Arrhenius equation: According to kinetic and thermodynamic parameters, the reaction proceeds through a concerted, semi‐polar, four‐membered cyclic transition state type of mechanism. © 2007 Wiley Periodicals, Inc. 39: 145–147, 2007     

The thermochemical kinetics of retro- “ene” reactions of molecules with the general structure (allyl) XYH in the gas phase. IX. The thermal unimolecular decomposition of ethyallylether in the gas phase

The thermal decomposition of ethylallylether (EAE) has been studied in the gas phase over the temperature range of 560–648°K. Propylene and acetaldehyde are the only reaction products observed. The reaction is apparently homogeneous in nature and independent of the pressure of EAE and of added foreign gases. The experimetally determined first-order rate constants, using the internal standard technique, fit the Arrhenius relationship log k(s^?1) = 11.84 ± 0.29 ? (43.57 ± 0.77 kcal/mole)/2.303RT. Independently the same rate constants are obtained, based on the amounts of products formed. The observed activation parameters are in general agreement with expectations based on the concept of a 6-center 1,5-H-shift retro-“ene” reaction mechanism, and they agree with previous results obtained for the similar reactions involving alkylallylamines and olefins.     

Laboratory studies of photochemistry and gas phase radical reaction kinetics relevant to planetary atmospheres

This review seeks to bring together a selection of recent laboratory work on gas phase photochemistry, kinetics and reaction dynamics of radical species relevant to the understanding of planetary atmospheres other than that of Earth. A majority of work focuses on the rich organic chemistry associated with photochemically initiated reactions in the upper atmospheres of the giant planets. Reactions relevant to Titan, the largest moon of Saturn and with a nitrogen/methane dominated atmosphere, have also received much focus due to potential to explain the chemistry of Earth's prebiotic atmosphere. Analogies are drawn between the approaches of terrestrial and non-terrestrial atmospheric chemistry.     

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

The UV absorption spectrum of trimethylsilyl radical in the gas phase

The UV absorption spectrum of trimethylsilyl radical was observed at 256 nm for the first time by photolysing allyltrimethylsilane and hexamethyldisilane with an ArF excimer laser. A bimolecular rate constant for recombination of trimethylsilyl radical of (2.5±0.5×10⁻¹¹ molecule⁻¹ cm³ s⁻¹ was measured.     

The elimination kinetics of secondary Alkyl Bromides in the gas phase

Elimination kinetics of 2-bromohexane and 2-bromo-4-methylpentane in the gas phase were examined over the temperature range of 310–360°C and pressure range of 46–213 torr. The reactionsin seasoned, static reaction vessels, and in the presence of the free radical inhibitor cyclohexene, are homogeneous, unimolecular, and follow first order rate laws. The overall rate coefficients are described by the following Arrhenius equations: For 2-bromohexane, log??₁(s^?1) = (13.08 ± 0.70) ? (185.7 ± 8.2) kJ mol^?1 (2.303RT)^?1; for 2-bromo-4-methylpentane, log??₁(s^?1) = (13.08 ± 0.33) ? (183.4 ± 3.8) kJ mol^?1 (2.303RT)^?1. The electron releasing effect of alkyl groups influences the overall elimination rates. The olefin products isomerize in the presence of HBr gas until an equilibrium mixture is reached.     

A flash photolysis investigation of the gas phase uv absorption spectrum and self-reaction kinetics of the neopentylperoxy radical

The ultraviolet absorption spectrum of the neopentylperoxy radical, (CH₃)₃CCH₂O₂ (or C₅H₁₁O₂), and the kinetics of its self-reaction have been studied in the gas phase using a flash photolysis technique. The room temperature absorption cross-section at 250 nm was determined to be and was used to normalize the radical absorption spectrum between 210 and 300 nm. Detailed modeling of the self-reaction system was used to interpret the transient absorption kinetic decay curves over the temperature range 228–380 K, at total pressures between 25 and 100 torr. The results are discussed in relation to previous measurements of alkylperoxy radical spectra and kinetics.