Thermal decomposition of ethanol. 4. Ab initio chemical kinetics for reactions of H atoms with CH3CH2O and CH3CHOH radicals

The potential energy surfaces of H-atom reactions with CH(3)CH(2)O and CH(3)CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100-3000 K and 10(-4) Torr to 10(3) atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH(3)CH(2)O + H reaction is CH(3) + CH(2)OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH(3)CH(2)OH formation by collisonal deactivation becomes dominant. For CH(3)CHOH + H, there are three major product channels; at high temperatures, CH(3)+CH(2)OH production predominates at low pressures (P < 100 Torr), while the formation of CH(3)CH(2)OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH(2)CHOH + H(2) becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH(3)CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C-C breaking agree closely with available literature data.     

A crossed molecular beam study of the O(1D) + C(3)H(8) reaction: multiple reaction pathways.

The O((1)D) + C(3)H(8) reaction has been reinvestigated using the universal crossed molecular beam method. Three reaction channels, CH(3) + C(2)H(4)OH, C(2)H(5) + CH(2)OH, and OH + C(3)H(7), have been observed. All three channels are significant in the title reaction with the C(2)H(5) formation process to be the most important, while the CH(3) formation and the OH formation channels are about equal. Product kinetic energy distributions and angular distributions have been determined for the three reaction channels observed. The oxygen-containing radicals in the CH(3) and C(2)H(5) formation pathways show forward-backward symmetric angular distribution relative to the O atom beam, while the OH product shows a clearly forward angular distribution. These results indicate that the OH formation channel seems to exhibit different dynamics from the CH(3) and C(2)H(5) channels.     

Dynamics of the O(3P) + C2H4 reaction: identification of five primary product channels (vinoxy, acetyl, methyl, methylene, and ketene) and branching ratios by the crossed molecular beam technique with soft electron ionization

The crossed molecular beam scattering technique with soft electron ionization (EI) is used to disentangle the complex dynamics of the polyatomic O(3P) + C2H4 reaction, which is of great relevance in combustion and atmospheric chemistry. Exploiting the newly developed capability of attaining universal product detection by using soft EI, at a collision energy of 54.0 kJ mol(-1), five different primary products have been identified, which correspond to the five exoergic competing channels leading to CH2CHO(vinoxy) + H, CH3CO(acetyl) + H, CH3(methyl) + HCO(formyl), CH2(methylene) + HCHO(formaldehyde), and CH2CO(ketene) + H2. From laboratory product angular and velocity distributions, center-of-mass product angular and translational energy distributions and the relative branching ratios for each channel have been obtained, affording an unprecedented characterization of this important reaction.     

A direct dynamics trajectory study of F- + CH(3)OOH reactive collisions reveals a major non-IRC reaction path

A direct dynamics simulation at the B3LYP/6-311+G(d,p) level of theory was used to study the F- + CH3OOH reaction dynamics. The simulations are in excellent agreement with a previous experimental study (J. Am. Chem. Soc. 2002, 124, 3196). Two product channels, HF + CH2O + OH- and HF + CH3OO-, are observed. The former dominates and occurs via an ECO2 mechanism in which F- attacks the CH3- group, abstracting a proton. Concertedly, a carbon-oxygen double bond is formed and OH- is eliminated. Somewhat surprisingly this is not the reaction path, predicted by the intrinsic reaction coordinate (IRC), which leads to a deep potential energy minimum for the CH2(OH)2...F- complex followed by dissociation to HF + CH2(OH)O-. None of the direct dynamics trajectories followed this path, which has an energy release of -63 kcal/mol and is considerably more exothermic than the ECO2 path whose energy release is -27 kcal/mol. Other product channels not observed, and which have a lower energy than that for the ECO2 path, are F- + CO + H2 + H2O (-43 kcal/mol), F- + CH2O + H2O (-51 kcal/mol), and F- + CH2(OH)2 (-60 kcal/mol). Formation of the CH3OOH...F- complex, with randomization of its internal energy, is important, and this complex dissociates via the ECO2 mechanism. Trajectories which form HF + CH3OO- are nonstatistical events and, for the 4 ps direct dynamics simulation, are not mediated by the CH3OOH...F- complex. Dissociation of this complex to form HF + CH3OO- may occur on longer time scales.     

Trajectory surface hopping study of the O(3P) + ethylene reaction dynamics

Spin-orbit coupling (SOC) induced intersystem crossing (ISC) has long been believed to play a crucial role in determining the product distributions in the O(3P) + C2H4 reaction. In this paper, we present the first nonadiabatic dynamics study of the title reaction at two center-of-mass collision energies: 0.56 eV, which is barely above the H-atom abstraction barrier on the triplet surface, and 3.0 eV, which is in the hyperthermal regime. The calculations were performed using a quasiclassical trajectory surface hopping (TSH) method with the potential energy surface generated on the fly at the unrestricted B3LYP/6-31G(d,p) level of theory. To simplify our calculations, nonadiabatic transitions were only considered when the singlet surface intersects the triplet surface. At the crossing points, Landau-Zener transition probabilities were computed assuming a fixed spin-orbit coupling parameter, which was taken to be 70 cm-1 in most calculations. Comparison with a recent crossed molecular beam experiment at 0.56 eV collision energy shows qualitative agreement as to the primary product branching ratios, with the CH3 + CHO and H + CH2CHO channels accounting for over 70% of total product formation. However, our direct dynamics TSH calculations overestimate ISC so that the total triplet/singlet ratio is 25:75, compared to the observed 43:57. Smaller values of SOC reduce ISC, resulting in better agreement with the experimental product relative yields; we demonstrate that these smaller SOC values are close to being consistent with estimates based on CASSCF calculations. As the collision energy increases, ISC becomes much less important and at 3.0 eV, the triplet to singlet branching ratio is 71:29. As a result, the triplet products CH2 + CH2O, H + CH2CHO and OH + C2H3 dominate over the singlet products CH3 + CHO, H2 + CH2CO, etc.     

A systematic computational study on the reactions of HO2 with RO2: The HO2 + CH3O2(CD3O2) and HO2 + CH2FO2 reactions

A systematic theoretical study of the reactions of HO2 with RO2 has been carried out. The major concern of the present work is to gain insight into the reaction mechanism and then to explain experimental observations and to predict new product channels for this class of reactions of importance in the atmosphere. In this paper, the reaction mechanisms for two reactions, namely, HO2 + CH3O2 and HO2 + CH2FO2, are reported. Both singlet and triplet potential energy surfaces are investigated. The complexity of the present system makes it impossible to use a single ab initio method to map out all the reaction paths. Various ab initio methods including MP2, CISD, QCISD(T), CCSD(T), CASSCF, and density function theory (B3LYP) have been employed with the basis sets ranging from 6-31G(d) to an extrapolated complete basis set (CBS) limit. It has been established that the CCSD(T)/cc-pVDZ//B3LYP/6-311G(d,p) scheme represents the most feasible method for our systematic study. For the HO2 + CH3O2 reaction, the production of CH3OOH is determined to be the dominant channel. For the HO2 + CH2FO2 reaction, both CH2FOOH and CHFO are major products, whereas the formation of CHFO is dominant in the overall reaction. The computational findings give a fair explanation for the experimental observation of the products.     

Quantum dynamics study on the product branching for the C(3P) + C2H2 reaction: cyclic-C3H versus linear-C3H

Reduced-dimensionality quantum reactive scattering calculations for the C(3P) + C2H2 reaction have been carried out in order to understand the product branching dynamics of cyclic-C3H + H and linear-C3H + H. Our model treats only two degrees of freedom but can explicitly describe both of the C3H isomer product channels. The lowest triplet potential energy surface has been obtained by the hybrid density-functional method at the B3LYP/6-31G(d,p) level of theory. The calculated reaction probabilities were found to be dominated by resonance consistent with the complex-formation potential, and the results show that cyclic-C3H is preferentially formed via the cyclic-C3H2 intermediate produced by insertion of C(3P) into the CC bond. We have found that the isomerization from the cyclic-C3H2 to linear-C3H2 intermediate is suppressed by a barrier separating potential wells corresponding to these two intermediates. It has also been found that the energy dependence of the calculated total reaction cross section is in good agreement with the result of crossed molecular beam experiments.     

Insights into the unusual barrierless reaction between two closed shell molecules, (CH3)2S + F2, and its H2S + F2 analogue: role of recoupled pair bonding

Early flowtube studies showed that (CH(3))(2)S (DMS) reacted very rapidly with F(2); hydrogen sulfide (H(2)S), however, did not. Recent crossed molecular beam studies found no barrier to the reaction between DMS and F(2) to form CH(2)S(F)CH(3) + HF. At higher collision energies, a second product channel yielding (CH(3))(2)S-F + F was identified. Both reaction channels proceed through an intermediate with an unusual (CH(3))(2)S-F-F bond structure. Curiously, these experimental studies have found no evidence of direct F(2) addition to DMS, resulting in (CH(3))(2)SF(2), despite the fact that the isomer in which both fluorines occupy axial positions is the lowest energy product. We have characterized both reactions, H(2)S + F(2) and DMS + F(2), with high-level ab initio and generalized valence bond calculations. We found that recoupled pair bonding accounts for the structure and stability of the intermediates present in both reactions. Further, all sulfur products possess recoupled pair bonds with CH(2)S(F)CH(3) having an unusual recoupled pair bond dyad involving π bonding. In addition to explaining why DMS reacts readily with F(2) while H(2)S does not, we have studied the pathways for direct F(2) addition to both sulfide species and found that (for (CH(3))(2)S + F(2)) the CH(2)S(F)CH(3) + HF channel dominates the potential energy surface, effectively blocking access to F(2) addition. In the H(2)S + F(2) system, the energy of the transition state for formation of H(2)SF(2) lies very close to the H(2)SF + F asymptote, making the potential pathway a roaming atom mechanism.     

Theoretical dynamic studies on the reactions of CH3C(O)CH3-nCl(n) (n = 0-3) with the chlorine atom

The theoretical investigations were performed on the reaction mechanisms for the title reactions CH(3)C(O)CH(3) + Cl --> products (R1), CH(3)C(O)CH(2)Cl + Cl --> products (R2), CH(3)C(O)CHCl(2) + Cl --> products (R3), and CH(3)C(O)CCl(3) + Cl --> products (R4) by ab initio direct dynamics approach. Two different reaction channels have been found: abstract of the H atom from methyl (--CH(3)) group or chloromethyl (--CH(3-n)Cl(n)) group of chloroacetone and addition of a Cl atom to the carbon atom of the carbonyl group of chloroacetone followed by methyl or chloromethyl eliminations. Because of the higher potential energy barrier, the contribution of addition-elimination reaction pathway to the total rate constants is very small and thus this pathway is insignificant in atmospheric conditions. The rate constants for the H-abstraction reaction channels are evaluated by using canonical variational transition state theory incorporating with the small-curvature tunneling correction. Theoretical overall rate constants are in good agreement with the available experimental values and decrease in the order of k(1) > k(2) > k(3) > k(4). The results indicate that for halogenated acetones the substitution of halogen atom (F or Cl) leads to the decrease in the C--H bond reactivity and more decrease of reactivity is caused by F-substitution.     

Variational transition-state theory study of the dimethyl sulfoxide (DMSO) and OH reaction

The mechanism for the atmospheric oxidation of DMSO has been studied. For the first time, all the possible channels in the DMSO + OH reaction are studied together theoretically, and their corresponding rate constants have been evaluated under the variational transition-state formalism. Three different channels have been characterized: an addition-elimination process to form MSIA (CH3SOOH) and CH3, a H-abstraction pathway to give CH3SOCH2 and H2O, and a nonkinetically relevant S(N)2-type reaction to form methanol and CH3SO. In agreement with previous experimental and theoretical works, the main product in the DMSO + OH reaction turns out to be the MSIA, with a branching ratio at 298.15 K around 97%. The effects of pressure in the global rate constant have also been analyzed.     

Theoretical study on the methyl radical with chlorinated methyl radicals CH(3-n)Cln (n = 1, 2, 3) and CCl2

Radical-radical reactions involving chlorinated methyl radicals are particularly important in the mechanism of combustion of chlorinated hydrocarbons. Yet, they are usually difficult to study experimentally. In this paper, four chloride-related radical-radical reactions, i.e., CH3+CH(3-n)Cln (n = 1, 2, 3) and CH3+CCl2, are theoretically studied for the first time by means of the Gaussian-3//B3LYP potential energy surface survey combined with the master equation study over a wide range of temperatures and pressures. Our calculated results show that the three CH3+CH(3-n)Cln reactions can barrierlessly generate the former two kinetically allowed products P1 H(2)C=C(H)(3-n)Cl(n-1)+HCl and P2 CH3CH(3-n)Cl(n-1)+Cl with the very high predominance of P1 over P2. For the CH3 reaction with the biradical CCl2, which inevitably takes place during the CH3+CCl3 reaction and yet has never been studied experimentally or theoretically, H(2)C=CCl2+H and H(2)C=C(H)Cl+Cl are predicted to be the respective major and minor products. The results are compared with the recent laser photolysis/photoionization mass spectroscopy study on the CH3+CH(3-n)Cln (n = 1, 2, 3) reactions. The predicted rate constants and product branching ratios of the CH3+CCl2 reaction await future experimental verification.     

New mechanistic insights to the O(3P) + propene reaction from multiplexed photoionization mass spectrometry

The reaction of O((3)P) with propene (C(3)H(6)) has been examined using tunable vacuum ultraviolet radiation and time-resolved multiplexed photoionization mass spectrometry at 4 Torr and 298 K. The temporal and isomeric resolution of these experiments allow the separation of primary from secondary reaction products and determination of branching ratios of 1.00, 0.91 ± 0.30, and 0.05 ± 0.04 for the primary product channels CH(3) + CH(2)CHO, C(2)H(5) + HCO, and H(2) + CH(3)CHCO, respectively. The H + CH(3)CHCHO product channel was not observable for technical reasons in these experiments, so literature values for the branching fraction of this channel were used to convert the measured product branching ratios to branching fractions. The results of the present study, in combination with past experimental and theoretical studies of O((3)P) + C(3)H(6), identify important pathways leading to products on the C(3)H(6)O potential energy surface (PES). The present results suggest that up to 40% of the total product yield may require intersystem crossing from the initial triplet C(3)H(6)O PES to the lower-lying singlet PES.     

A theoretical study of the reaction of O(3P) with isobutene

The reaction of O((3)P) with isobutene ((CH(3))(2)C=CH(2)) is investigated using the unrestricted second-order M?ller-Plesset perturbation (UMP2) and complete basis set CBS-4M level methods. The minimum energy crossing point (MECP) between the singlet and triplet potential energy surfaces is located using the Newton-Lagrange method, and it is shown that the MECP plays a key role. The calculational results indicate that the site selectivity of the addition of O((3)P) to either carbon atom of the double bond of isobutene is weak, and the major product channels are CH(2)C(O)CH(3) + CH(3,) cis-/trans-CH(3)CHCHO + CH(3), (CH(3))(2)CCO + H(2), and CH(3)C(CH(2))(2) + OH, among which (CH(3))(2)CCO + H(2) is predicted to be the energetically most favorable one. The complex multichannel reaction mechanisms are revealed, and the observations in several recent experiments could be rationalized on the basis of the present calculations. The formation mechanisms of butenols are also discussed.     

Ab initio direct dynamics studies on the reactions of chlorine atom with CH3-nFnCH2OH (n = 1-3)

The hydrogen abstraction reactions of Cl atom with a series of fluorinated alcohols, i.e., CH(3-n)F(n)CH(2)OH + Cl (n = 1-3) (R1-R3) have been studied systematically by ab initio direct dynamics method and the canonical variational transition state theory (CVT). The potential energy surface information is calculated at the MP2/6-311G(d,p) level. Energies along the minimum energy paths are improved by a series of single-point calculations at the higher modified GAUSSIAN-2 (G2M) level of theory. Theoretical analysis shows that three kinds of hydrogen atoms can be abstracted from the reactants CH(2)FCH(2)OH and CHF(2)CH(2)OH, and for CF(3)CH(2)OH, two possible pathways are found. The rate constants for each reaction channel are evaluated by CVT with the small-curvature tunneling correction (SCT) over a wide range of temperature from 200 to 2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values for the reactions CHF(2)CH(2)OH + Cl and CF(3)CH(2)OH + Cl. However, for the reaction CH(2)FCH(2)OH + Cl, there is negative temperature dependence below 500 K, which is different from the experimental fitted. It is shown that in the low temperature ranges, the three reactions all proceed predominantly via H-abstraction from the methylene positions, and with the increase of the temperature the H-abstraction channels from the fluorinated-methyl positions should be taken into account, while the H-abstraction channels from the hydroxyl groups are negligible over the whole temperature ranges. Also, the reactivity decreases substantially with fluorine substitution at the methyl position of alcohol.     

Potential energy surface and product branching ratios for the reaction of dicarbon, C(2)(X(1)Sigma(g)(+)), with methylacetylene, CH(3)CCH(X(1)A(1)): an ab initio/RRKM study

Ab initio calculations of the potential energy surface for the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction have been carried at the G2M level of theory. The calculations show that the dicarbon molecule in the ground singlet electronic state can add to methylacetylene without a barrier producing a three-member or a four-member ring intermediate, which can rapidly rearrange to the most stable H(3)CCCCCH isomer on the C(5)H(4) singlet surface. This isomer can then lose a hydrogen atom (H) or molecular hydrogen (H(2)) from the CH(3) group with the formation of H(2)CCCCCH and HCCCCCH, respectively. Alternatively, H atom migrations and three-member-ring closure/opening rearrangements followed by H and H(2) losses can lead to other isomers of the C(5)H(3) and C(5)H(2) species. According to the calculated energetics, the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH reaction is likely to be a major source of the C(5)H(3) radicals (in particular, the most stable H(2)CCCCCH and HCCCHCCH isomers, which are relevant to the formation of benzene through the reactions with CH(3)). Among heavy-fragment product channels, only C(3)H(3) + C(2)H and c-C(3)H(2) + C(2)H(2) might compete with C(5)H(3) + H and C(5)H(2) + H(2). RRKM calculations of reaction rate constants and product branching ratios depending on the reactive collision energy showed that the major reaction products are expected to be H(2)CCCCCH + H (64-66%) and HCCCHCCH + H (34-30%), with minor contributions from HCCCCCH + H(2) (1-2%), HCCCHCC + H(2) (up to 1%), C(3)H(3) + C(2)H (up to 1%), and c-C(3)H(2) + C(2)H(2) (up to 0.1%) if the energy randomization is complete. The calculations also indicate that the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction can proceed by direct H-abstraction of a methyl hydrogen to form C(3)H(3) + C(2)H almost without a barrier.     

Ab Initio Study on the Mechanism of Cycloaddition Reaction between Silylene Carbene （H_2Si=C：） and Acetone

The mechanism of cycloaddition reaction between singlet silylene carbene and acetone has been investigated with CCSD(T)//MP2/6-31G method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. One consists of two steps: (1) the two reactants (R1, R2) firstly form a four-membered ring intermediate (INT4) through a barrier-free exothermic reaction of 585.9 kJ/mol; (2) Then intermediate (INT4) isomerizes to CH3-transfer product (P4.1) via a transition state (TS4.1) with energy barrier of 5.3 kJ/mol. The other is as follows: on the basis of intermediate (INT4) created between R1 and R2, intermediate (INT4) further reacts with acetone (R2) to form the intermediate (INT5) through a barrier-free exothermic reaction of 166.3 kJ/mol; Then, intermediate (INT5) isomerizes to a silicic bis-heterocyclic product (P5) via a transition state (TS5), for which the barrier is 54.9 kJ/mol. The presented rule of this reaction: the [2+2] cycloaddition effect between the π orbital of silylene carbene and the π orbital of π-bonded compounds leads to the formation of a four-membered ring intermediate (INT4); The unsaturated property of C atom from carbene in the four-membered ring intermediate (INT4) results in the generation of CH3-transfer product (P4.1) and silicic bis-heterocyclic compound (P5).     

CH3CH2+O(3P)反应机理的理论研究

The reaction for CH3CH2+O(3P) was studied by ab initio method. The geometries of the reactants, intermediates, transition states and products were optimized at MP2/6-311+G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single-point calculations for all the stationary points were carried out at the QCISD(T)/6-311+G(d,p) level using the MP2/6-311+G(d,p) optimized geometries. The results of the theoretical study indicate that the major products are the CH2O＋CH3, CH3CHO+H and CH2CH2+OH in the reaction. For the products CH2O＋CH3 and CH3CHO+H, the major production channels are A1: (R)→IM1→TS3→(A) and B1: (R)→IM1→TS4→(B), respectively. The majority of the products CH2CH2+OH are formed via the direct abstraction channels C1 and C2: (R)→TS1(TS2)→(C). In addition, the results suggest that the barrier heights to form the CO reaction channels are very high, so the CO is not a major product in the reaction.     

Unimolecular dissociation of the CH3OCO radical: an intermediate in the CH3O + CO reaction

This work investigates the unimolecular dissociation of the methoxycarbonyl, CH(3)OCO, radical. Photolysis of methyl chloroformate at 193 nm produces nascent CH(3)OCO radicals with a distribution of internal energies, determined by the velocities of the momentum-matched Cl atoms, that spans the theoretically predicted barriers to the CH(3)O + CO and CH(3) + CO(2) product channels. Both electronic ground- and excited-state radicals undergo competitive dissociation to both product channels. The experimental product branching to CH(3) + CO(2) from the ground-state radical, about 70%, is orders of magnitude larger than Rice-Ramsperger-Kassel-Marcus (RRKM)-predicted branching, suggesting that previously calculated barriers to the CH(3)OCO --> CH(3) + CO(2) reaction are dramatically in error. Our electronic structure calculations reveal that the cis conformer of the transition state leading to the CH(3) + CO(2) product channel has a much lower barrier than the trans transition state. RRKM calculations using this cis transition state give product branching in agreement with the experimental branching. The data also suggest that our experiments produce a low-lying excited state of the CH(3)OCO radical and give an upper limit to its adiabatic excitation energy of 55 kcal/mol.     

A systematic computational study of the reactions of HO2 with RO2: the HO2 + CH2ClO2, CHCl2O2, and CCl3O2 reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.     

Experimental and theoretical studies of the reactions Y (a2D) + H2CO and Y (a2D) + CH3CHO

The reactions of ground state Y (a(2)D) with H(2)CO and CH(3)CHO were studied at a range of collision energies in crossed molecular beams. For reaction with H(2)CO, three product channels were observed: formation of YH(2) + CO, YCO + H(2), and YHCO + H. Reaction with CH(3)CHO led to three analogous product channels involving formation of HYCH(3) + CO, YCH(2)CO + H(2), and YCH(3)CO + H. The calculated CCSD(T) energetics and DFT geometries for key intermediates in both reactions, together with RRKM theory, are used to calculate a priori the branching ratios between various product channels. These calculated values are compared to those obtained experimentally.