Limiting Rate Coefficients Research Articles

Understanding the kinetics and molecular mechanism of unimolecular gas phase thermal decomposition of the α-ketoester methyl benzoylformate using RRKM and BET theories

The RRKM calculation and bonding evolution theory analysis coupled with quantum theory of atoms in molecules have been used to investigate kinetics and molecular mechanism of gas phase thermal decomposition of methyl benzoylformate. The pressure-dependent rate coefficients, by applying different collisional efficiency values, indicated that the atmospheric pressure is in high-pressure limit of fall-off curve and low-pressure limit rate coefficients are in the range of 10−13-10−12 cm3 molec−1 s−1. Temperature dependence of high-pressure limiting of rate coefficient over the temperature range 733 ± 20% K was estimated to be k∞RRKM(CBS-QB3)=2.92×1013s-1exp(227.4kJmol-1/RT) and k∞RRKM(PBE1PBE)=2.67×1013s-1exp(232.8kJmol-1/RT). Topological analysis of electron localization function and electron density at the B3LYP/6-311G(d,p) level reveal that the reaction can be occurred as going through seven turning points defined as methyl benzoylformate 8-CF†FF†TSFC†[CF†]-0: methyl benzoate + carbon monoxide. The molecular mechanism can be categorized in three fundamental sections A) heterolytic rupture of O3C8 bond and detachment of methoxy part; B) formation of O3C7 bond via donation bond formation mechanism; and C) heterolytic rupture of C7C8 bond and detachment of carbon monoxide. The electron density ρ(3,−1)(r) in the region of bond forming and breaking increases and decreases along the reaction course to reflect bond strengthening and weakening, respectively. AIM parameters revealed that so long as chemical bonds are unformed/ruptured, interactions at related BCPs are covalence in nature, otherwise the nature of chemical bonds are strong shared covalence.

Thermal dissociation of alkyl nitrites and recombination of alkyl radicals

The thermal dissociations of three alkyl nitrites (C3H7ONO, n-C4H9ONO, and i-C4H9ONO) have been studied in a diaphragmless shock tube by laser schlieren densitometry over 700 < T < 1000 K and 60 < P < 250 Torr. The nitrites act as clean, thermal sources of alkyl radicals that create the radicals at much lower temperatures than are accessible with other precursors. The decomposition of the three nitrites proceeds via a common mechanism, which is discussed. Rate coefficients for dissociation of the alkyl nitrites to an alkoxy radical and NO, k1, were obtained with an uncertainty of 30%. Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) calculations for dissociation of the nitrites were performed and provide excellent simulations of the experimental results. The high-pressure limit rate coefficients from the RRKM/ME calculations are: k1,C3H7ONO=(5.69±1.71)×1021T−1.60exp(−21458/T)s−1; k1,n−C4H9ONO=(9.91±2.97)×1021T−1.59exp(−21588/T)s−1; k1,i−C4H9ONO=(3.92±1.18)×1021T−1.58exp(−21162/T)s−1. At the low temperatures of this work, the alkyl radicals are thermally stable and thus their recombination reactions were also studied. The recombination rate coefficients for ethyl, n-propyl and i-propyl radicals (uncertainties of 50%) are in excellent agreement with literature values, particularly recent theoretical results, and extend the range of experiments to higher temperatures.

On-the-fly ab initio calculations toward accurate rate coefficients

During automatic kinetic model generation a large number of reactions need to have kinetic data assigned to them. One of the main challenges is on how to account for the large scarcity of accurate data. The ever-growing computational power and availability of high performance computing solutions allows to envisage implementing on-the-fly ab initio calculations for a large number of reaction rate coefficients. Today extensive user knowledge and involvement is required to develop fast and effective ab initio based kinetic models. The present work introduces a series of automation procedures to minimize user involvement. It allows automated quantum chemical calculations for a large number of reactions for gas-phase processes with a wide variety in chemical species. 3D coordinates are calculated for all the reactants, products and transition states and automatically submitted to quantum chemistry packages. The latter yield accurate high-pressure limit rate coefficients. All steps are automated without the need for manual interventions, even for recognizing rotational frequencies and for verifying whether the saddle point searches lead to the appropriate transition state. A good agreement to literature data is found, i.e., most of the rate coefficients are within a factor of 2 of literature data.

Experimental and Kinetic Modeling Study of Cyclohexane Pyrolysis

The pyrolysis of undiluted cyclohexane has been studied in a continuous flow tubular reactor at temperatures from 913 to 1073 K and inlet feed flow rates in the range 288–304 g·h–1 at 0.17 MPa reactor pressure with average reactor residence time of 0.5 s calculated based on the pressure in the reactor, the temperature profile along the reactor, and the molar flow rate along the reactor estimated by the logarithmic average of the inlet and outlet molar flows. The reactions lead to conversions between 2% and 95%. Forty-nine products were identified and quantified using two-dimensional gas chromatography equipped with thermal conductivity and flame ionization detectors. The products with molecular weights between those of hydrogen and naphthalene constitute more than 99 mass % of the total products. A kinetic mechanism composed exclusively of elementary step reactions with high pressure limit rate coefficients has been generated with the automatic network generation tool “Genesys”. The kinetic parameters for the reactions originate either directly from high level ab initio calculations or from reported group additive values which were derived from ab initio calculations. The Genesys model performs well when compared to five models available in the literature, and its predictions agree well with the experimental data for 15 products without any adjustments of the kinetic parameters. Reaction path analysis shows that cyclohexane consumption is initiated by the unimolecular isomerization to 1-hexene but is overall dominated by hydrogen abstraction reactions by hydrogen atoms and methyl radicals. Dominant pathways to major products predicted with the new model are discussed and compared to other well performing models in the literature.

Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

ABSTRACTA theoretical kinetic study of the thermal decomposition of 1‐chlorohexane in gas phase between 600 and 1000 K was performed. Transition‐state theory and unimolecular reaction rate theory were combined with molecular information provided by quantum chemical calculations. Particularly, the B3LYP, BMK, M05–2X, and M06–2X formulations of the density functional theory (DFT) and the high‐level ab initio methods G3B3 and G4 were employed. The possible reaction channels for the thermal decomposition of 1‐chlorohexane were investigated, and the reaction takes place through the elimination of HCl with the formation of 1‐hexene. The derived high‐pressure limit rate coefficients are k∞(600–1000 K) = (8 ± 5) × 1013 exp[‐((56.7 ± 0.4) kcal mol−1/RT)] s−1. The pressure effect over the reaction was analyzed from the calculation of the low‐pressure limit rate coefficients and the falloff curves. In addition, the standard enthalpies of formation at 298 K of −46.9 ± 1.5 kcal mol−1 for 1‐chlorohexane and 5.8 ± 1.5 kcal mol−1 for C6H13 radical were derived from isodesmic and isogiric reactions at high levels of theory.

Pressure Dependence of Rate Coefficients for Formation and Dissociation of Pentachlorodisilane and Related Chemical Activation Reactions

Four entrance channels to the pentachlorodisilane (Si2HCl5) potential energy well, namely, SiCl2 + SiHCl3, SiCl4 + SiHCl, Cl3SiSiCl + HCl, and SiCl3 + SiHCl2, were analyzed in detail through transition state theory, Rice–Ramsperger–Kassel–Marcus (RRKM) theory, and solution of the multichannel master equation. The stationary points in the potential energy surface were optimized, and their vibrational frequencies and rotational constants calculated at the (U)B3LYP/6–31+G(d,p) level of theory; the (U)CCSD(T)/aug-cc-pVTZ level was then used for accurate estimation of activation energies. The pressure and temperature dependence of the rate coefficients of the channels related to Si2HCl5 stabilization/dissociation was determined along a wide range of conditions, for the first time. All channels showed strong pressure dependence in the four cases, at least at low-to-moderate pressure conditions. Each entrance channel leads to the formation of different products under different conditions, and the mechanism was analyzed in detail. The results indicated that at atmospheric pressure the reactions are in the falloff region, and therefore do not support the adoption of high-pressure limit rate coefficients in reaction models designed for simulation of systems at atmospheric or subatmospheric pressure conditions.

An Experimental and Master Equation Study of the Kinetics of OH/OD + SO2: The Limiting High-Pressure Rate Coefficients.

The kinetics of the reaction OH/OD + SO2 were studied using a laser flash photolysis/laser-induced fluorescence technique. Evidence for two-photon photolysis of SO2 at 248 nm is presented and quantified, and which appears to have been evident to some extent in most previous photolysis studies, potentially leading to values for the rate coefficient k1 that are too large. The kinetics of the reaction OH(v = 0) + SO2 (T = 295 K, p = 25-300 torr) were measured under conditions where SO2 photolysis was taken into account. These results, together with literature data, were modeled using a master equation analysis. This analysis highlighted problems with the literature data: the rate coefficients derived from flash photolysis data were generally too high and from the flow tube data too low. Our best estimate of the high-pressure limiting rate coefficient k1∞ was obtained from selected data and gives a value of (7.8 ± 2.2) × 10-13 cm3 molecule-1 s-1, which is lower than that recommended in the literature. A parametrized form of k1([N2],T) is provided. The OD(v = 0) + SO2 (T = 295 K, p = 25-300 torr) data are reported for the first time, and master equation analysis reinforces our assignment of k1∞.

Theoretical Study of the Ti–Cl Bond Cleavage Reaction in TiCl4

Abstract In this work the kinetics of the TiCl4 ⇌ TiCl3 + Cl reaction is studied theoretically. A variable-reaction coordinate transition-state theory (VRC-TST) is used to calculate the high-pressure limit rate coefficients. The interaction energy surface for the VRC-TST step is sampled directly at the CASPT2(6e,4o)/cc-pVDZ level of theory including an approximate treatment of the spin-orbit coupling. The pressure-dependence of the reaction in an argon bath gas is explored using the master equation in conjunction with the optimised VRC-TST transition-state number of states. The collisional energy transfer parameters for the TiCl4–Ar system are estimated via a “one-dimensional minimisation” method and classical trajectories. The Ti–Cl bond dissociation energy is computed using a complete basis set extrapolation technique with cc-pVQZ and cc-pV5Z basis sets. Good quantitative agreement between the estimated rate constants and available literature data is observed. However, the fall-off behaviour of the model results is not seen in the current experimental data. Sensitivity analysis shows that the fall-off effect is insensitive to the choice of model parameters and methods. More experimental work and development of higher-level theoretical methods are needed to further investigate this discrepancy.

Role of the Recombination Channel in the Reaction between the HO and HO2 Radicals.

The kinetics of the gas phase recombination reaction HO + HO2 + He → HOOOH + He has been studied between 200 and 600 K by using the SACM/CT model and the unimolecular rate theory. The molecular properties of HOOOH were derived at the CCSD(T)/aug-cc-pVTZ ab initio level of theory, while relevant potential energy features of the reaction were calculated at the CCSD(T)/aug-cc-pVTZ//CCSD(T)/aug-cc-pVDZ level. The resulting high and low pressure limit rate coefficients are k∞ = 3.55 × 10-12 (T/300)0.20 cm3 molecule-1 s-1 and k0 = [He] 1.55 × 10-31 (T/300)-3.2 cm3 molecule-1 s-1. The rate coefficients calculated over the 6 × 10-4 - 400 bar range are smaller at least in a factor of about 60 than the consensus value determined for the main reaction channel HO + HO2 → H2O + O2, indicating that the recombination pathway is irrelevant.

Thermal Decomposition of 2-Pentanol: A Shock Tube Study and RRKM Calculations.

A single pulse shock tube was used to study the thermal decomposition of 2-pentanol in the temperatures between 1110 and 1325 K. Three major decomposition products are methane, ethylene, and propylene. The minor products detected in lower concentration are ethane, acetylene, acetaldehyde, 1-pentene, and 2-pentene. The rate coefficient for the overall decomposition of 2-pentanol was found to be ktotalexp(1110-1325 K) = (4.01 ± 0.51) × 109 exp(-(36.2 ± 4.7)/RT) s-1, where the activation energies are given in kcal mol-1. To simulate reactant and product distribution over the experimentally studied temperatures between 1110 and 1325 K, a reaction scheme was constructed with 34 species and 39 reactions. In addition to this, the temperature and pressure dependent rate coefficients were computed for various unimolecular dissociation pathways using RRKM theory. The high pressure limit rate coefficient for overall decomposition of 2-pentanol was obtained to be ktotaltheory(500-2500 K) = (9.67 ± 1.11) × 1014 exp(-(67.7 ± 2.9)/RT) s-1. The calculated high pressure rate coefficients and experimentally measured rate constants are in good agreement with each other. The reaction is primarily governed by the unimolecular elimination of water.

Recombination and dissociation of 2-methyl allyl radicals: Experiment and theory

The recombination and dissociation of the resonantly stabilized 2-methylallyl radical has been studied in a diaphragmless shock tube by laser schlieren densitometry (LS) over temperatures of 700–1350K and pressures of 60–260Torr. Both 2,5-dimethyl-1,5-hexadiene and the new low temperature precursor 3-methylbut-3-enyl nitrite were used to generate 2-methylallyl radicals under these conditions. Rate coefficients were obtained for dissociation of the precursors, recombination of 2-methylallyl, and dissociation of 2-methylallyl by simulation of the LS profiles. The experiments are complemented by a priori theoretical calculations for both the recombination and dissociation of 2-methylallyl. The experimental results and theoretical predictions are in excellent agreement with one another. The calculated high pressure limit rate coefficient for recombination of 2-methylallyl is log(k1)=14.737−0.641logT+251.39/(2.303×T) and that for dissociation of 2-methylallyl is log(k3)=11.100−1.2295logT−28545/(2.303×T). The uncertainties in k1 and k3 are estimated as factors of 1.5. Rate coefficients are provided over a broad range of pressures for chemical kinetic modeling.

The Effect of Alcohol and Carbonyl Functional Groups on the Competition between Unimolecular Decomposition and Isomerization in C4 and C5 Alkoxy Radicals

ABSTRACTPresented here are computed rates for the thermal unimolecular decomposition of a variety of alkoxy radicals with four‐ and five‐carbon length backbones. Three classes of molecules are examined: alkoxy radicals with saturated hydrocarbon backbones, those with alcohol functional groups, and those with carbonyl functional groups. The chosen species represent many of those found during the combustion of fossil fuels as well as bio‐derived alternatives. Density functional theory calculations were benchmarked against higher level coupled cluster calculations and used to explore the potential energy surfaces of these systems. Transition state theory was used to calculate high‐pressure limit rate coefficients of all radical intermediates in the regimes relevant to atmospheric chemistry and combustion. We show that the assumption that alkoxy radicals quickly decompose via β‐scission to aldehydes and other radicals is not valid for some of the alkoxy radicals investigated in this work. We further illustrate how intra‐H migrations in larger alkoxy radicals with carbonyl and alcohol functional groups can dominate unimolecular decomposition under combustion and atmospheric relevant conditions. Finally, we discuss why carbonyl groups can increase or decrease intra‐H migration barriers depending on their location relative to the transferring H‐atom.

A theoretical study of cyclic ether formation reactions

Cyclisation reactions of hydroperoxyl–alkyl radicals forming cyclic ethers and hydroxyl radicals play an important role in low temperature oxidation chemistry. These reactions contribute to the competition between radical chain propagation and chain branching reaction pathways which dominate the reactivity of alkanes at temperatures where negative temperature coefficient (NTC) behaviour is often observed. This work is motivated by previous experimental and modelling evidences that current literature rate coefficients for these reactions are in need of refinement and/or re-determination. In light of this, the current study presents quantum-chemically-derived high-pressure limit rate coefficients for all cyclisation reactions leading to cyclic ether formation in alkanes ranging in size from C2 to C5. Ro-vibrational properties of each stationary point were determined at the M06-2X/6-311++G(d,p) level of theory. Coupled cluster (CCSD(T)) and Møller–Plesset perturbation theory (MP2) methods were employed with various basis sets and complete basis set extrapolation techniques to compute the energies of the resulting geometries. These methods, combined with canonical transition state theory, have been used to determine 43 rate coefficients, with enough structural diversity within the reactions to allow for their application to larger species for which the use of the levels of theory employed herein would be computationally prohibitive. The validity of an alternative, and computationally less expensive, technique to approximate the complete basis set limit energies is also discussed, together with implications of this work for combustion modelling.

High-Temperature Experimental and Theoretical Study of the Unimolecular Dissociation of 1,3,5-Trioxane.

Unimolecular dissociation of 1,3,5-trioxane was investigated experimentally and theoretically over a wide range of conditions. Experiments were performed behind reflected shock waves over the temperature range of 775-1082 K and pressures near 900 Torr using a high-repetition rate time of flight mass spectrometer (TOF-MS) coupled to a shock tube (ST). Reaction products were identified directly, and it was found that formaldehyde is the sole product of 1,3,5-trioxane dissociation. Reaction rate coefficients were extracted by the best fit to the experimentally measured concentration-time histories. Additionally, high-level quantum chemical and RRKM calculations were employed to study the falloff behavior of 1,3,5-trioxane dissociation. Molecular geometries and frequencies of all species were obtained at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and MP2/aug-cc-pVDZ levels of theory, whereas the single-point energies of the stationary points were calculated using coupled cluster with single and double excitations including the perturbative treatment of triple excitation (CCSD(T)) level of theory. It was found that the dissociation occurs via a concerted mechanism requiring an energy barrier of 48.3 kcal/mol to be overcome. The new experimental data and theoretical calculations serve as a validation and extension of kinetic data published earlier by other groups. Calculated values for the pressure limiting rate coefficient can be expressed as log10 k∞ (s(-1)) = [15.84 - (49.54 (kcal/mol)/2.3RT)] (500-1400 K).

Ultracold chemistry with alkali-metal–rare-earth molecules

A first principles study of the dynamics of $^6$Li($^{2}$S) + $^6$Li$^{174}$Yb($^2\Sigma^+$)$ \to ^6$Li$_2(^1\Sigma^+$) + $^{174}$Yb($^1$S) reaction is presented at cold and ultracold temperatures. The computations involve determination and analytic fitting of a three-dimensional potential energy surface for the Li$_2$Yb system and quantum dynamics calculations of varying complexities, ranging from exact quantum dynamics within the close-coupling scheme, to statistical quantum treatment, and universal models. It is demonstrated that the two simplified methods yield zero-temperature limiting reaction rate coefficients in reasonable agreement with the full close-coupling calculations. The effect of the three-body term in the interaction potential is explored by comparing quantum dynamics results from a pairwise potential that neglects the three-body term to that derived from the full interaction potential. Inclusion of the three-body term in the close-coupling calculations was found to reduce the limiting rate coefficients by a factor of two. The reaction exoergicity populates vibrational levels as high as $v=19$ of the $^6$Li$_2$ molecule in the limit of zero collision energy. Product vibrational distributions from the close-coupling calculations reveal sensitivity to inclusion of three-body forces in the interaction potential. Overall, the results indicate that a simplified model based on the long-range potential is able to yield reliable values of the total reaction rate coefficient in the ultracold limit but a more rigorous approach based on statistical quantum or quantum close-coupling methods is desirable when product rovibrational distribution is required.

Theoretical kinetic study of the unimolecular decomposition of 2-bromopropene

The kinetics of the gas phase thermal decomposition of 2-bromopropene at 600–1400K has been studied by using the unimolecular rate theory combined with different formulations of the density functional theory and high level ab initio composite methods. This hydrogen bromide elimination reaction presents two dissociation channels, one forming propyne and another forming allene. High-pressure limit rate coefficients of (6.2±1.2)×1014exp [−(64.5±2kcalmol−1)/RT] and (1.1±0.1)×1014exp [−(63.6±2kcalmol−1)/RT]s−1 were obtained for these reaction pathways. The present results allow to elucidate reported contradictory experimental data.

Collision Efficiency of Water in the Unimolecular Reaction CH4 (+H2O) ⇆ CH3 + H (+H2O): One-Dimensional and Two-Dimensional Solutions of the Low-Pressure-Limit Master Equation

The low-pressure-limit unimolecular decomposition of methane, CH4 (+M) ⇆ CH3 + H (+M), is characterized via low-order moments of the total energy, E, and angular momentum, J, transferred due to collisions. The low-order moments are calculated using ensembles of classical trajectories, with new direct dynamics results for M = H2O and new results for M = O2 compared with previous results for several typical atomic (M = He, Ne, Ar, Kr) and diatomic (M = H2 and N2) bath gases and one polyatomic bath gas, M = CH4. The calculated moments are used to parametrize three different models of the energy transfer function, from which low-pressure-limit rate coefficients for dissociation, k0, are calculated. Both one-dimensional and two-dimensional collisional energy transfer models are considered. The collision efficiency for M = H2O relative to the other bath gases (defined as the ratio of low-pressure limit rate coefficients) is found to depend on temperature, with, e.g., k0(H2O)/k0(Ar) = 7 at 2000 K but only 3 at 300 K. We also consider the rotational collision efficiency of the various baths. Water is the only bath gas found to fully equilibrate rotations, and only at temperatures below 1000 K. At elevated temperatures, the kinetic effect of "weak-collider-in-J" collisions is found to be small. At room temperature, however, the use of an explicitly two-dimensional master equation model that includes weak-collider-in-J effects predicts smaller rate coefficients by 50% relative to the use of a statistical model for rotations. The accuracies of several methods for predicting relative collision efficiencies that do not require solving the master equation and that are based on the calculated low-order moments are tested. Troe's weak collider efficiency, βc, includes the effect of saturation of collision outcomes above threshold and accurately predicts the relative collision efficiencies of the nine baths. Finally, a brief discussion is presented of mechanistic details of the energy transfer process, as inferred from the trajectories.

Quantum Chemical and Kinetics Study of the Thermal Gas Phase Decomposition of 2-Chloropropene

A detailed theoretical study of the kinetics of the thermal decomposition of 2-chloropropene over the 600-1400 K temperature range has been done. The reaction takes place through the elimination of HCl with the concomitant formation of propyne or allene products. Relevant molecular properties of the reactant and transition states were calculated for each reaction channel at 14 levels of theory. From information provided by the BMK, MPWB1K, BB1K, M05-2X, and M06-2X functionals, specific for chemical kinetics studies, high-pressure limit rate coefficients of (5.8 ± 1.0) × 10(14) exp[-(67.8 ± 0.4 kcal mol(-1))/RT] s(-1) and (1.1 ± 0.2) × 10(14) exp[-(66.8 ± 0.5 kcal mol(-1))/RT] s(-1) were obtained for the propyne and allene channels, respectively. The pressure effect over the reaction was analyzed through the calculation of the low-pressure limit rate coefficients and falloff curves. An analysis of the branching ratio between the two channels as a function of pressure and temperature, based on these results and on computed specific rate coefficients, show that the propyne forming channel is predominant.

Rate Constant and Branching Fraction for the NH2 + NO2 Reaction

The NH2 + NO2 reaction has been studied experimentally and theoretically. On the basis of laser photolysis/LIF experiments, the total rate constant was determined over the temperature range 295-625 K as k1,exp(T) = 9.5 × 10(-7)(T/K)(-2.05) exp(-404 K/T) cm(3) molecule(-1) s(-1). This value is in the upper range of data reported for this temperature range. The reactions on the NH2 + NO2 potential energy surface were studied using high level ab initio transition state theory (TST) based master equation methods, yielding a rate constant of k1,theory(T) = 7.5 × 10(-12)(T/K)(-0.172) exp(687 K/T) cm(3) molecule(-1) s(-1), in good agreement with the experimental value in the overlapping temperature range. The two entrance channel adducts H2NNO2 and H2NONO lead to formation of N2O + H2O (R1a) and H2NO + NO (R1b), respectively. The pathways through H2NNO2 and H2NONO are essentially unconnected, even though roaming may facilitate a small flux between the adducts. High- and low-pressure limit rate coefficients for the various product channels of NH2 + NO2 are determined from the ab initio TST-based master equation calculations for the temperature range 300-2000 K. The theoretical predictions are in good agreement with the measured overall rate constant but tend to overestimate the branching ratio defined as β = k1a/(k1a + k1b) at lower temperatures. Modest adjustments of the attractive potentials for the reaction yield values of k1a = 4.3 × 10(-6)(T/K)(-2.191) exp(-229 K/T) cm(3) molecule(-1) s(-1) and k1b = 1.5 × 10(-12)(T/K)(0.032) exp(761 K/T) cm(3) molecule(-1) s(-1), in good agreement with experiment, and we recommend these rate coefficients for use in modeling.

Ab Initio Kinetics for Thermal Decomposition of CH3N•NH2, cis-CH3NHN•H, trans-CH3NHN•H, and C•H2NNH2 Radicals

The thermal decomposition of the CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(•)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(•)H and trans-CH(3)NHN(•)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The β-scission of CH(3) from the cis-CH(3)NHN(•)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the β-scission of CH(3) from the trans-CH(3)NHN(•)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(•)NH(2) radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.