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.     

Crossed-beam radical-radical reaction dynamics of O(3P)+C3H3-->H(2S)+C3H2O

The radical-radical oxidation reaction, O(3P)+C3H3 (propargyl)-->H(2S)+C3H2O (propynal), was investigated using vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed-beam configuration, together with ab initio and statistical calculations. The barrierless addition of O(3P) to C3H3 is calculated to form energy-rich addition complexes on the lowest doublet potential energy surface, which subsequently undergo direct decomposition steps leading to the major reaction products, H+C3H(2)O (propynal). According to the nascent H-atom Doppler-profile analysis, the average translational energy of the products and the fraction of the average transitional energy to the total available energy were determined to be 5.09+/-0.36 kcal/mol and 0.077, respectively. On the basis of a comparison with statistical prior calculations, the reaction mechanism and the significant internal excitation of the polyatomic propynal product can be rationalized in terms of the formation of highly activated, short-lived addition-complex intermediates and the adiabaticity of the excess available energy along the reaction coordinate.     

Product spin-orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.     

Unimolecular dissociation of the propargyl radical intermediate of the CH+C2H2 and C+C2H3 reactions

This paper examines the unimolecular dissociation of propargyl (HCCCH2) radicals over a range of internal energies to probe the CH+HCCH and C+C2H3 bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by 157 nm photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and H2 elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 (+5-10) kcal/mol as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical's dissociation and also lower than calculated overall reaction enthalpies. Products from both H2+HCCC and H+C3H2 channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C-H fission channel were also observed in the photolysis of propargyl chloride.     

Determination of the rate constant for the OH(X2Pi) + OH(X()Pi) --> O(3P) + H2O reaction over the temperature range 293-373 K

The rate constant for the reaction OH(X2Pi) + OH(X2Pi) --> O(3P) + H2O has been measured over the temperature range 293-373 K and pressure range 2.6-7.8 Torr in both Ne and Ar bath gases. The OH radical was created by 193 nm laser photolysis of N2O to produce O(1D) atoms that reacted rapidly with H2O to produce the OH radical. The OH radical was detected by quantitative time-resolved near-infrared absorption spectroscopy using Lambda-doublet resolved rotational transitions of the first overtone of OH(2,0) near 1.47 microm. The temporal concentration profiles of OH were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of residuals between the experimental profiles and the model calculations. At 293 K the rate constant for the title reaction was found to be (2.7 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), where the uncertainty includes an estimate of both random and systematic errors at the 95% confidence level. The rate constant was measured at 347 and 373 K and found to decrease with increasing temperature.     

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(2P1/2):Cl(2P3/2) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.     

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.     

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

This study uses computational chemistry and statistical reaction rate theory to investigate the chemically activated reaction of diacetylene (butadiyne, C(4)H(2)) with the propargyl radical (C˙H(2)CCH) and the reaction of acetylene (C(2)H(2)) with the i-C(5)H(3) (CH(2)CCCC˙H) and n-C(5)H(3) (CHCC˙HCCH) radicals. A detailed G3SX-level C(7)H(5) energy surface demonstrates that the C(3)H(3) + C(4)H(2) and C(5)H(3) + C(2)H(2) addition reactions proceed with moderate barriers, on the order of 10 to 15 kcal mol(-1), and form activated open-chain C(7)H(5) species that can isomerize to the fulvenallenyl radical with the highest barrier still significantly below the entrance channel energy. Higher-energy pathways are available leading to other C(7)H(5) isomers and to a number of C(7)H(4) species + H. Rate constants in the large multiple-well (15) multiple-channel (30) chemically activated system are obtained from a stochastic solution of the one-dimensional master equation, with RRKM theory for microcanonical rate constants. The dominant products of the C(4)H(2) + C(3)H(3) reaction at combustion-relevant temperatures and pressures are i-C(5)H(3) + C(2)H(2) and CH(2)CCHCCCCH + H, along with several quenched C(7)H(5) intermediate species below 1500 K. The major products in the n-C(5)H(3) + C(2)H(2) reaction are i-C(5)H(3) + C(2)H(2) and a number of C(7)H(4) species + H, with C(7)H(5) radical stabilization at lower temperatures. The i-C(5)H(3) + C(2)H(2) reaction predominantly leads to C(7)H(4) + H and to stabilized C(7)H(5) products. The title reactions may play an important role in polycyclic aromatic hydrocarbon (PAH) formation in combustion systems. The C(7)H(5) potential energy surface developed here also provides insight into several other important reacting gas-phase systems relevant to combustion and astrochemistry, including C(2)H + the C(3)H(4) isomers propyne and allene, benzyne + CH, benzene + C((3)P), and C(7)H(5) radical decomposition, for which some preliminary analysis is presented.     

Exploring the dynamics of hydrogen atom release from the radical-radical reaction of O(3P) with C3H5

The gas-phase radical-radical reaction dynamics of O(3P) + C3H5 --> H(2S) + C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-alpha region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H + C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H + C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition.     

Association rate constants for reactions between resonance-stabilized radicals: C3H3 + C3H3, C3H3 + C3H5, and C3H5 + C3H5

Reactions between resonance-stabilized radicals play an important role in combustion chemistry. The theoretical prediction of rate coefficients and product distributions for such reactions is complicated by the fact that the initial complex-formation steps and some dissociation steps are barrierless. In this paper direct variable reaction coordinate transition state theory (VRC-TST) is used to predict accurately the association rate constants for the self and cross reactions of propargyl and allyl radicals. For each reaction, a set of multifaceted dividing surfaces is used to account for the multiple possible addition channels. Because of their resonant nature the geometric relaxation of the radicals is important. Here, the effect of this relaxation is explicitly calculated with the UB3LYP/cc-pvdz method for each mutual orientation encountered in the configurational integrals over the transition state dividing surfaces. The final energies are obtained from CASPT2/cc-pvdz calculations with all pi-orbitals in the active space. Evaluations along the minimum energy path suggest that basis set corrections are negligible. The VRC-TST approach was also used to calculate the association rate constant and the corresponding number of states for the C(6)H(5) + H --> C(6)H(6) exit channel of the C(3)H(3) + C(3)H(3) reaction, which is also barrierless. For this reaction, the interaction energies were evaluated with the CASPT2(2e,2o)/cc-pvdz method and a 1-D correction is included on the basis of CAS+1+2+QC/aug-cc-pvtz calculations for the CH(3) + H reference system. For the C(3)H(3) + C(3)H(3) reaction, the VRC-TST results for the energy and angular momentum resolved numbers of states in the entrance channels and in the C(6)H(5) + H exit channel are incorporated in a master equation simulation to determine the temperature and pressure dependence of the phenomenological rate coefficients. The rate constants for the C(3)H(3) + C(3)H(3) and C(3)H(5) + C(3)H(5) self-reactions compare favorably with the available experimental data. To our knowledge there are no experimental rate data for the C(3)H(3) + C(3)H(5) reaction.     

Radical-molecule reaction C3H+H2O: a mechanistic study

Despite the importance of the C(3)H radical in both combustion and interstellar space, the reactions of C(3)H toward stable molecules have never been studied. In this paper, we report our detailed mechanistic study on the radical-molecule reaction C(3)H+H(2)O at the Becke's three parameter Lee-Yang-Parr-B3LYP6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)6-311G(2d,p) (single-point) levels. It is shown that the C(3)H+H(2)O reaction initially favors formation of the carbene-insertion intermediates HCCCHOH (1a,1b) rather than the direct H- or OH-abstraction process. Subsequently, the isomers (1a,1b) can undergo a direct H- extrusion to form the well-known product propynal HCCCHO (P(5)). Highly competitively, (1a,1b) can take the successive 1,4- and 1,2-H-shift interconversion to isomer H(2)CCCHO(2a,2b) and then to isomer H(2)CCHCO(3a,3b), which can finally take a direct C-C bond cleavage to give product C(2)H(3) and CO (P(1)). The other products are kinetically much less feasible. With the overall entrance barrier 10.6 kcal/mol, the title reaction can be important in postburning processes. Particularly, our calculations suggest that the title reaction may play a role in the formation of the intriguing interstellar molecule, propynal HCCCHO. The calculated results will also be useful for the analogous C(3)H reactions such as with ammonia and alkanes.     

Rotational state-resolved reaction cross section in the reactions of state-selected CH with NO and with O2

A pure and highly intense state-selected pulsed supersonic CH(X (2)Pi) radical beam source was developed by use of the C((1)D)+H(2) reaction with the combination of the state selection and purification by an electrostatic hexapole field. Under the beam-cell condition, the elementary reactions of CH+NO and CH+O(2) were studied by using this state-selected CH beam. NH(A (3)Pi) [and NCO(A (2)Sigma(+))] formations and OH(A (2)Sigma(+)) formation were directly identified in the elementary reaction of CH+NO and CH+O(2), respectively. For the CH+NO reaction, the relative branching ratio sigma(NCO*)sigma(NH) of NCO(A (2)Sigma(+)) formation to NH(A (3)Pi) formation was determined to be 0.35+/-0.15. The state-selected reaction cross sections were determined for each rotational state of CH. In the CH+NO reaction, a remarkable rotational state dependence of the reactive cross section was revealed, while the CH+O(2) reaction showed little rotational state dependence.     

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.     

Calculations on the competition between association and reaction for C3H(+) + H2

A potential energy surface has been calculated for the competing associative and reactive ion-molecule processes involving the reactants C3H(+) + H2. Our ab initio results show that the linear ion C3H+ and H2 can directly access the deep potential well of the propargyl ion H2CCCH+, which is calculated to lie 390 kJ mol-1 below the zero-point energy of the reactants. Isomerization between the propargyl ion and the lower energy, cyclic C3H3+ ion, calculated to lie 501 kJ mol-1 below the zero-point energy of reactants, can subsequently occur via two pathways. One of these pathways involves a transition state lying 22 kJ mol-1 below the energy of the reactants while the other, which occurs at much lower energies, involves two transition states and an intermediate. The dissociation of c-C3H3+ into c-C3H2(+) + H is calculated to occur directly, without any intermediate potential energy maximum, but the energy of the products lies 7.3 kJ mol-1 above the energy of the reactants. Using the minimum energy potential pathway and properties of the stationary point structures determined via ab initio methods, we have calculated both the association rate coefficient to produce C3H3+ as a function of density and the branching ratio between the propargyl and cyclic structures of the ion. Our results are in good agreement with some experimental results and in conflict with others. Specifically, we agree with the 1:1 branching ratio measured for the propargyl and cyclic isomers of C3H3+ at 80 and 300 K and we agree with the rate coefficient for radiative association measured at 80 K. We cannot reproduce reported measurements that the reactive channel (C3H2(+) + H) is the dominant channel at 80 K and at low gas densities, or that the association channel at high densities saturates at an effective rate coefficient well below the Langevin value -2x10(-11) cm3 s-1 at 300K and 1x10(-10) cm3 s-1 at 80K.     

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.     

Vibrational and rotational distributions of the CH(A2Delta) product of the C(2)H + O(3P) reaction studied by fourier transform visible (FTVIS) emission spectroscopy

The C(2)H + O((3)P) --> CH(A) + CO reaction is investigated using Fourier transform visible emission spectroscopy. The O((3)P) and C(2)H radicals are produced by simultaneous 193 nm photolysis of SO(2) and C(2)H(2) precursors, respectively. The nascent vibrational and rotational distributions of the CH(A) product are obtained under time-resolved, but quasi-steady-state, conditions facilitated by the short lifetime of the CH(A) emission. The vibrational temperature of the CH(A) product is found to be appreciably hotter (2800 +/- 100 K) than the rotational distributions in the v' = 0 (1400 +/- 100 K) and v' = 1 (1250 +/- 250 K) levels. The results suggest that the reaction may proceed through an electronically excited HCCO() intermediate; moreover, the vibrational excitation compared to rotational excitation is higher than expected based on a statistical distribution of energy and may be the result of geometrical changes in the transition state. The CH(A) emission is also observed in a C(2)H(2)/O/H reaction mixture using a microwave discharge apparatus to form O atoms, with subsequent H atom production. The nascent rotational and vibrational distributions of the CH(A) determined by the microwave discharge apparatus are very similar to the CH(A) distributions obtained in the photodissociation experiment. The results support the idea that the C(2)H + O((3)P) reaction may play a role in low-pressure C(2)H(2)/O/H flames, as previously concluded.     

Prediction of product branching ratios in the C(3P)+C2H2-->l-C3H+H/c-C3H+H/C3+H2 reaction using ab initio coupled clusters calculations extrapolated to the complete basis set combined with Rice-Ramsperger-Kassel-Marcus and radiationless transition theories

Ab initio CCSD(T) calculations of intermediates and transition states on the singlet and triplet C3H2 potential energy surfaces extrapolated to the complete basis set limit are combined with statistical computations of energy-dependent rate constants of the C(3P)+C2H2 reaction under crossed molecular beam conditions. Rice-Ramsperger-Kassel-Marcus theory is applied for isomerization and dissociation steps within the same multiplicity and radiationless transition and nonadiabatic transition state theories are used for singlet-triplet intersystem crossing rates. The calculated rate constants are utilized to predict product branching ratios. The results demonstrate that, in qualitative agreement with available experimental data, c-C3H+H and C3+H2 are the most probable products at low collision energies, whereas l-C3H+H becomes dominant at higher Ec above approximately 25 kJ/mol.     

Reaction mechanism of HCN+ + C2H4: a theoretical study

The complex doublet potential energy surface for the ion-molecule reaction of HCN(+) with C(2)H(4) is investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. The initial association between HCN(+) and C(2)H(4) forms three energy-rich addition intermediates, 1 (HCNCH(2)CH(2)(+)), 2 (HC-cNCH(2)CH(2)(+)), and 3 (N-cCHCH(2)CH(2)(+)), which are predicted to undergo subsequent isomerization and decomposition steps. A total of nine kinds of dissociation products, including P(1) (HCN + C(2)H(4)(+)), P(2) (HCNCHCH(2)(+) + H), P(3) (NCCH(2) + CH(3)(+)), P(4) (CN + C(2)H(5)(+)), P(5) (NCCHCH(2)(+) + H(2)), P(6) (HNCCHCH(2)(+) + H), P(7) (c-CHCCH(2)N(+) + H(2)), P(8) (c-NHCCH(2)C(+) + H(2)), and P(9) (HNCCCH(+) + H(2) + H), are obtained. Among the nine products, P(1) is the most abundant product. P(2) is the second feasible product but is much less competitive than P(1). P(3), P(4), P(5), and P(6) may have the lowest yields observed. Other products, P(7), P(8), and P(9), may become feasible at high temperature. Because the intermediates and transition states involved in the most favorable pathway all lie below the reactant, the HCN(+) + C(2)H(4) reaction is expected to be rapid, which is confirmed by experiment. The present calculation results may provide a useful guide for understanding the mechanism of HCN(+) toward other unsaturated hydrocarbons.     

Theoretical study on the mechanism and kinetics for the self-reaction of C2H5O2 radicals

Oxygen-to-oxygen coupling, direct H-abstraction and oxygen-to-(α)carbon nucleophilic substitution processes have been investigated for both the singlet and triplet self-reaction of C(2)H(5)O(2) radicals at the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) level to evaluate the reaction mechanisms, possible products and rate constants. The calculated results show that the title reaction mainly occurs through the singlet oxygen-to-oxygen coupling mechanism with the formation of entrance tetroxide intermediates, and the most dominant product is C(2)H(5)O + HO(2) + CH(3)CHO (P5) generated in channel R5. Beginning from the radical products of P5 (C(2)H(5)O, HO(2)) and reactant (C(2)H(5)O(2)), five secondary reactions HO(2) + HO(2) (a), HO(2) + C(2)H(5)O (b), C(2)H(5)O + C(2)H(5)O (c), HO(2) + C(2)H(5)O(2) (d), and C(2)H(5)O + C(2)H(5)O(2) (e) mainly proceed on the triplet potential energy surface. Among these reactions, (a), (b), and (d) are kinetically favorable because of lower barrier heights. The calculated rate constants of channel R5 between 200 and 295 K are almost independent of the temperature, which is in agreement with the experimental report. With regard to the final products distribution, CH(3)CHO, C(2)H(5)OH, C(2)H(5)OOH, H(2)O(2), and (3)O(2) are predicted to be major, whereas C(2)H(5)OOC(2)H(5) should be in minor amount.