Crossed-beam dynamics studies of the radical-radical combustion reaction O((3)P) + CH3 (methyl)

The dynamics of the radical-radical reaction O((3)P) + CH(3), a prototypical case for the reactions of atomic oxygen with alkyl radicals of great relevance in combustion chemistry, has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 55.9 kJ mol(-1). The results have been examined in the light of previous kinetic and theoretical work. From product angular and velocity distribution measurements, the dynamics of the predominant H-displacement channel leading to formaldehyde formation has been characterized. This channel has been found to proceed via the formation of an osculating complex; a significant coupling between the product centre-of-mass angular and translational energy distributions has been noted. Experimental attempts to characterize the dynamics of the channel leading to HCO + H(2) have failed and it remains unclear whether HCO is formed by the reaction and/or, if formed, a part of HCO does not dissociate quickly into CO + H.     

Theoretical study on the atmospheric reaction of CH3O2 with OH

A quantum chemical investigation on the reaction mechanism of CH₃O₂ with OH has been performed. Based on B3LYP and QCISD(T) calculations, seven possible singlet pathways and seven possible triplet pathways have been found. On the singlet potential energy surface (PES), the most favorable channel starts with a barrierless addition of O atom to CH₃O₂ leading to CH₃OOOH and then the O? O bond dissociates to give out CH₃O + HO₂. On the triplet PES, the calculations indicate that the dominant products should be ³CH₂O₂ + H₂O with an energy barrier of 29.95 kJ/mol. The results obtained in this work enrich the theoretical information of the title reaction and provide guidance for analogous atmospheric chemistry reactions. © 2015 Wiley Periodicals, Inc.     

A combined crossed-beam and theoretical investigation of radical-radical reaction dynamics of O(3P)+t-C4H9-->OH+iso-C4H8

The oxidation reaction dynamics of a saturated hydrocarbon radical t-butyl leading to the isobutene +OH (X 2Pi:v"=0, 1, 2) products in the gas phase were first investigated by applying a combination of high-resolution spectroscopy in a crossed-beam configuration and ab initio calculations. By comparing the nascent OH populations with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major direct abstraction process leading to the inversion of vibrational populations, and the minor short-lived addition-complex process for hot rotational distributions.     

Products of the gas-phase reactions of OH radicals with (C2H5O)2P(S)CH3 and (C2H5O)3PS

Products of the gas-phase reactions of OH radicals with O,O-diethyl methylphosphonothioate [(C2H5O)2P(S)CH3, DEMPT] and O,O,O-triethyl phosphorothioate [(C2H5O)3PS, TEPT] have been investigated at room temperature and atmospheric pressure of air using in situ atmospheric pressure ionization mass spectrometry (API-MS) and, for the TEPT reaction, gas chromatography and in situ Fourier transform infrared (FT-IR) spectroscopy. Combined with products quantified previously by gas chromatography, the products observed were: from the DEMPT reaction, (C2H5O)2P(O)CH3 (21+/-4% yield) and C2H5OP(S)(CH3)OH or C2H5OP(O)(CH3)SH (presumed to be C2H5OP(O)(CH3)SH by analogy with the TEPT reaction); and from the TEPT reaction, (C2H5O)3PO (54-62% yield), SO2 (67+/-10% yield), CH3CHO (22-40% yield) and, tentatively, (C2H5O)2P(O)SH. The FT-IR analyses showed that the formation yields of HCHO, CO, CO2, peroxyacetyl nitrate [CH3C(O)OONO2], organic nitrates, and acetates from the TEPT reaction were <5%, 3+/-1%, <7%, <2%, 5+/-3%, and 3+/-2%, respectively. Possible reaction mechanisms are discussed.     

Kinetics of the gas-phase reactions of the OH radical with (C2H5)3PO and (CH3O)2P(S)Cl at 296 ± 2 K

The kinetics of the gas-phase reactions of the OH radical with (C₂H₅O)₃PO and (CH₃O)₂P(S)Cl and of the reactions of NO₃ radicals and O₃ with (CH₃O)₂P(S)Cl have been studied at room temperature. Using a relative rate technique, the rate constants determined for the reactions of the OH radical with (C₂H₅O)₃PO and (CH₃O)₂P(S)Cl at 296 ± 2 K and 740 torr total pressure of air were (5.53 ± 0.35) × 10^?11 and (5.96 ± 0.38) × 10^?11 cm³ molecule^?1 s^?1, respectively. Upper limits to the rate constants for the NO₃ radical and O₃ reactions with (CH₃O)₂P(S)Cl of <3 × 10^?14 cm³ molecule^?1 s^?1 and <2 × 10^?19 cm³ molecule^?1 s^?1, respectively, were obtained. These data are compared and discussed with previous literature data for organophosphorus compounds.     

Computational study of the reaction of CH2(X3B1) with CH3OH

The reaction of triplet methylene with methanol is a key process in alcohol combustion but surprisingly this reaction has never been studied. The reaction mechanism is investigated by using various high-level ab initio methods, including the complete basis set extrapolation (CBS-QB3 and CBS-APNO), the latest Gaussian-n composite method (G4), and the Weizmann-1 method (W1U). A total of five product channels and six transition states are found. The dominant mechanism is direct hydrogen abstraction, and the major product channel is CH(3) + CH(3)O, involving a weak prereactive complex and a 7.4 kcal/mol barrier. The other hydrogen abstraction channel, CH(3) + CH(2)OH, is less important even though it is more exothermic and involves a similar barrier height. The rate coefficients are predicted in the temperature range 200-3000 K. The tunneling effect and the hindered internal rotational freedoms play a key role in the reaction. Moreover, the reaction shows significant kinetic isotope effect.     

Quasiclassical trajectory study of the O(3P) + CH4 --> OH + CH3 reaction with a specific reaction parameters semiempirical Hamiltonian

We present a theoretical study of the O(3P) + CH4 --> OH + CH3 reaction using electronic structure, kinetics, and dynamics calculations. We calculate a grid of ab initio points at the PMP2/AUG-cc-pVDZ level to characterize the potential energy surface in regions of up to 1.3 eV above reagents. This grid of ab initio points is used to derive a set of specific reaction parameters (SRP) for the MSINDO semiempirical Hamiltonian. The resulting SRP-MSINDO Hamiltonian improves the quality of the standard Hamiltonian, particularly in regions of the potential energy surface beyond the minimum-energy reaction path. Quasiclassical-trajectory calculations are used to study the reaction dynamics with the original and the improved MSINDO semiempirical Hamiltonians, and a prior surface. The SRP-MSINDO semiempirical Hamiltonian yields OH rotational distributions in agreement with experimental results, improving over the results of the other surfaces. Thermal rate constants estimated with Variational Transition State Theory using the SRP-MSINDO Hamiltonian are also in agreement with experiments. Our results indicate that reparametrized semiempirical Hamiltonians are a good alternative to generating potential energy surfaces for accurate dynamics studies of polyatomic reactions.     

Direct dynamics study on hydrogen abstraction reaction of CF3CF2CH2OH with OH radical

The dual-level direct dynamics method has been employed to investigate the H-abstraction reaction of CF(3)CF(2)CH(2)OH with OH radical, which is predicted to have two classes of possible reaction channels caused by different positions of hydrogen atom attack. The minimum-energy path is calculated at the B3LYP/6-311G(d,p) level, and the energetic information is further refined by the MC-QCISD method. To compare the structures, the other method MPW1K/6-311G(d,p) is also applied to this system. Hydrogen-bonded complexes are presented in the reactant and product sides of the three channels, indicating that each reaction may proceed via an indirect mechanism. The rate constants for each reaction channel are evaluated by canonical variational transition-state theory (CVT) with the small-curvature tunneling correction (SCT) over a wide range of temperatures from 200 to 2000 K. The calculated CVT/SCT rate constants are found to be in good agreement with the available experimental values. The result shows that the variational effect is small, and in the lower-temperature range, the SCT effect is important for each reaction. It is shown that hydrogen abstracted from the -CH(2)- position is the major channel, while H-abstraction from the -OH position may be neglected with the temperature increasing.     

Computational study of the reaction of the methylsulfonyl radical,CH3S(O)2, with NO2

The mechanism of the reaction between the methylsulfonyl radical, CH₃S(O)₂, and NO₂ is examined using density functional theory and ab initio calculations. Two stable association intermediates, CH₃SNO₂ and CH₃S(O)ONO, may be formed through the attack of the nitrogen or the oxygen atom of NO₂ radical to the S atom. Interisomerization and decomposition of these intermediates are investigated using high level energy methods and specifically, CCSD(T), CBS‐QB3, and G3//B3LYP. The computational investigation indicates that the lowest energy reaction pathway leads to the products CH₃S(O)₃ + NO, through the decomposition of the most stable association adduct CH₃S(O)ONO. This result fully supports the relevant assumption of Ray et al. (Ray et al., J. Phys. Chem. 1996, 100, 8895], on which the experimental evaluation of the rate constant was based, namely that CH₃S(O)₃ + NO are the most probable products of the reaction CH₃S(O)₂ + NO₂. © 2014 Wiley Periodicals, Inc.     

Theoretical study and rate constant calculation for reaction of CF(3)CH(2)OH with OH

The reaction mechanism of CF(3)CH(2)OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF(3)CH(2)OH, CF(3)CHOH, and CF(3)CH(2)O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200-2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250-430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel.     

A study of the radical-radical reaction dynamics of O(3P)+t-C4H9 --> OH+iso-C4H8

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with t-butyl radicals (t-C4H9) in the gas phase were investigated using high-resolution laser spectroscopy in a crossed-beam configuration, together with ab initio theoretical calculations. The radical reactants, O(3P) and t-C4H9, were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of the precursor, azo-t-butane, respectively. A new exothermic channel, O(3P)+t-C4H9 --> OH+iso-C4H8, was identified and the nascent rovibrational distributions of the OH (X 2Pi: upsilon" = 0,1,2) products were examined. The population analyses for the two spin-orbit states of F1(2Pi32) and F2(2Pi12) showed that the upsilon" = 0 level is described by a bimodal feature composed of low- and high-N" rotational components, whereas the upsilon" = 1 and 2 levels exhibit unimodal distributions. No noticeable spin-orbit or Lambda-doublet propensities were observed in any vibrational state. The partitioning ratio of the vibrational populations (Pupsilon") with respect to the low-N" components of the upsilon" = 0 level was estimated to be P0:P1:P2 = 1:1.17+/-0.24:1.40+/-0.11, indicating that the nascent internal distributions are highly excited. On the basis of the comparison of the experimental results with the statistical theory, the reaction mechanism at the molecular level can be described in terms of two competing dynamic pathways: the major, direct abstraction process leading to the inversion of the vibrational populations, and the minor, short-lived addition-complex process responsible for the hot rotational distributions. After considering the reaction exothermicity, the barrier height, and the number of intermediates along the addition reaction pathways on the lowest doublet potential energy surface, the formation of CH3COCH3(acetone)+CH3 was predicted to be dominant in the addition mechanism.     

Ab initio investigations of the radical-radical reaction of O(3P) + C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O((3)P)] with a propargyl (C(3)H(3)) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O((3)P) to C(3)H(3) on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C(2)H(3)+CO, C(3)H(2)O+H, C(3)H(2)+OH, C(2)H(2)+CHO, C(2)H(2)O+CH, C(2)HO+CH(2), and CH(2)O+C(2)H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C(3)H(2)+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.     

Molecular and crystal structures of Ph2P(O)(CH2)2OH and Ph2P(O)CH2(C6H6)OH

The molecular and crystal structures of Ph₂P(O)(CH₂)₂OH and Ph₂P(O)CH₂(C₆H₆)OH have been determined. For the first compound the space group is with unit cell dimensions a=10.505(2), b=13.720(2), c=14.782(3) Å; =72.58(6), =76.95(6), =72.49(6)° for Z=6 (Syntex diffractometer,MoK radiation, 2996 reflections, R=3.2%). The second compound crystallizes in the space group P2₁2₁2₁ with unit cell dimensions a=9.371(3), b=9.014(3), c=18.461(5) Å for Z=4 (DAR-UM diffractometer,CuK radiation, 909 reflections, R=4.9%). In Ph₂P(O)(CH₂)₂OH, three independent molecules differing in structural details are linked by the P=O...O hydrogen bonds (O...H is 1.84, 1.80, and 1.86 Å), to form a chain. In Ph₂P(O)CH₂(C₆H₆)OH, the molecules are joined by pairs of the P=O...H–O bonds (O...H is 1.81 Å) to form 16-membered dimeric associates.Institute of Chemical Physics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 3, pp. 109–118, May–June 1993.Translated by T. Yudanova     

An experimental and theoretical study on the kinetics of the reaction between 4‐hydroxy‐3‐hexanone CH3CH2C(O)CH(OH)CH2CH3 and OH radicals

Experimental and theoretical rate coefficients are determined for the first time for the reaction of 4‐hydroxy‐3‐hexanone (CH₃CH₂C(O)CH(OH)CH₂CH₃) with OH radicals as a function of temperature. Experimental studies were carried out using two techniques. Absolute rate coefficients were measured using a cryogenically cooled cell coupled to the pulsed laser photolysis‐laser‐induced fluorescence technique with temperature and pressure ranges of 280‐365 K and 5‐80 Torr, respectively. Relative values of the studied reaction were measured under atmospheric pressure in the range of 298‐354 K by using a simulation chamber coupled to a FT‐IR spectrometer. In addition, the reaction of 4H3H with OH radicals was studied theoretically by using the density functional theory method over the range of 278‐350 K. Results show that H‐atom abstraction occurs more favorably from the C–H bound adjacent to the hydroxyl group with small barrier height. Theoretical rate coefficients are in good agreement with the experimental data. A slight negative temperature dependence was observed in both theoretical and experimental works. Overall, the results are deliberated in terms of structure–reactivity relationship and atmospheric implications.     

Product branching from the CH2CH2OH radical intermediate of the OH + ethene reaction

Using a crossed laser-molecular beam scattering apparatus and tunable photoionization detection, these experiments determine the branching to the product channels accessible from the 2-hydroxyethyl radical, the first radical intermediate in the addition reaction of OH with ethene. Photodissociation of 2-bromoethanol at 193 nm forms 2-hydroxyethyl radicals with a range of vibrational energies, which was characterized in our first study of this system ( J. Phys. Chem. A 2010 , 114 , 4934 ). In this second study, we measure the relative signal intensities of ethene (at m/e = 28), vinyl (at m/e = 27), ethenol (at m/e = 44), formaldehyde (at m/e = 30), and acetaldehyde (at m/e = 44) products and correct for the photoionization cross sections and kinematic factors to determine a 0.765:0.145:0.026:0.063:<0.01 branching to the OH + C(2)H(4), H(2)O + C(2)H(3), CH(2)CHOH + H, H(2)CO + CH(3), and CH(3)CHO + H product asymptotes. The detection of the H(2)O + vinyl product channel is surprising when starting from the CH(2)CH(2)OH radical adduct; prior studies had assumed that the H(2)O + vinyl products were solely from the direct abstraction channel in the bimolecular collision of OH and ethene. We suggest that these products may result from a frustrated dissociation of the CH(2)CH(2)OH radical to OH + ethene in which the C-O bond begins to stretch, but the leaving OH moiety abstracts an H atom to form H(2)O + vinyl. We compare our experimental branching ratio to that predicted from statistical microcanonical rate constants averaged over the vibrational energy distribution of our CH(2)CH(2)OH radicals. The comparison suggests that a statistical prediction using 1-D Eckart tunneling underestimates the rate constants for the branching to the product channels of OH + ethene, and that the mechanism for the branching to the H(2)O + vinyl channel is not adequately treated in such theories.     

Theoretical study on the kinetics of OH radical reactions with CH3OOH and CH3CH2OOH

The mechanisms and kinetics studies of the OH radical with alkyl hydroperoxides CH(3)OOH and CH(3)CH(2)OOH reactions have been carried out theoretically. The geometries and frequencies of all the stationary points are calculated at the UBHandHLYP/6-311G(d,p) level, and the energy profiles are further refined by interpolated single-point energies method at the MC-QCISD level of theory. For two reactions, five H-abstraction channels are found and five products (CH(3)OO, CH(2)OOH, CH(3)CH(2)OO, CH(2)CH(2)OOH, and CH(3)CHOOH) are produced during the above processes. The rate constants for the CH(3)OOH/CH(3)CH(2)OOH + OH reactions are corrected by canonical variational transition state theory within 250-1500 K, and the small-curvature tunneling is included. The total rate constants are evaluated from the sum of the individual rate constants and the branching ratios are in good agreement with the experimental data. The Arrhenius expressions for the reactions are obtained.     

Investigation of the radical product channel of the CH(3)C(O)CH(2)O(2) + HO(2) reaction in the gas phase

The reaction of CH(3)C(O)CH(2)O(2) with HO(2) has been studied at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/acetone/methanol/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)CH(2)O, OH and O(2) () was investigated in experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The observed prompt formation of phenol under conditions when CH(3)C(O)CH(2)O(2) reacts mainly with HO(2) indicates that this reaction proceeds partially by channel , which forms OH both directly and indirectly, by virtue of secondary generation of CH(3)C(O)O(2) (from CH(3)C(O)CH(2)O) and its reaction with HO(2) (). The secondary generation of OH radicals was confirmed by the observed formation of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel ). A number of delayed sources of OH also contribute to the observed phenol formation, such that full characterisation of the system required simulations using a detailed chemical mechanism. The dependence of the phenol and CH(3)C(O)OOH yields on the initial peroxy radical precursor reagent concentration ratio, [methanol](0)/[acetone](0), were well described by the mechanism, consistent with a small but significant fraction of the reaction of CH(3)C(O)CH(2)O(2) with HO(2) proceeding via channel . This allowed a branching ratio of k(3b)/k(3) = 0.15 +/- 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction.     

Kinetics of the multichannel reaction of methanethiyl radical (CH3S*) with 3O2

The CH3S* + O2 reaction system is considered an important process in atmospheric chemistry and in combustion as a pathway for the exothermic conversion of methane-thiyl radical, CH3S*. Several density functional and ab initio computational methods are used in this study to determine thermochemical parameters, reaction paths, and kinetic barriers in the CH3S* + O2 reaction system. The data are also used to evaluate feasibility of the DFT methods for higher molecular weight oxy-sulfur hydrocarbons, where sulfur presents added complexity from its many valence states. The methods include: B3LYP/6-311++G(d,p), B3LYP/6-311++G(3df,2p), CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p), B3P86/6-311G(2d,2p)//B3P86/6-31G(d), B3PW91/6-311++G(3df,2p), G3MP2, and CBS-QB3. The well depth for the CH3S* + 3O2 reaction to the syn-CH3SOO* adduct is found to be 9.7 kcal/mol. Low barrier exit channels from the syn-CH3SOO* adduct include: CH2S + HO2, (TS6, E(a) is 12.5 kcal/mol), CH3 + SO2 via CH3SO2 (TS2', E(a) is 17.8) and CH3SO + O (TS17, E(a) is 24.7) where the activation energy is relative to the syn-CH3SOO* stabilized adduct. The transition state (TS5) for formation of the CH3SOO adduct from CH3S* + O2 and the reverse dissociation of CH3SOO to CH3S* + O2 is relatively tight compared to typical association and simple bond dissociation reactions; this is a result of the very weak interaction. Reverse reaction is the dominant dissociation path due to enthalpy and entropy considerations. The rate constants from the chemical activation reaction and from the stabilized adduct to these products are estimated as functions of temperature and pressure. Our forward rate constant and CH3S loss profile are in agreement with the experiments under similar conditions. Of the methods above, the G3MP2 and CBS-QB3 composite methods are recommended for thermochemical determinations on these carbon-sulfur-oxygen systems, when they are feasible.