Direct gas-liquid interfacial dynamics: the reaction between O(3P) and a liquid hydrocarbon

We report the first measurements of internal energy distributions of the OH produced via a direct mechanism, isolated from other components on the basis of time-of-flight, in the interfacial reaction between gas-phase O((3)P) atoms and the liquid hydrocarbon squalane, C(30)H(62). O((3)P) atoms were generated by laser photolysis of NO(2) above the liquid. Resulting hydroxyl radicals that escape from the surface were detected by laser-induced fluorescence. Time-of-flight profiles demonstrate that the kinetic energy of the fastest OH (nu' = 1) is lower than that of (nu' = 0). Rotational distributions were measured at the rising edge of their appearance for both OH (nu' = 0) and (nu' = 1). They were found to differ substantially more than at the peak of their profiles. They were also less dependent on the bulk liquid temperature. We conclude that the new data confirm strongly that at least two mechanisms contribute to the production of OH. The higher-velocity component has translational and rotational energy distributions, observed cleanly for the first time, consistent with a direct mechanism. The close correspondence of these rotational distributions to those from the corresponding homogeneous gas-phase reaction of O((3)P) with smaller hydrocarbons suggests a very similar, near collinear direct abstraction. This is accompanied by a slower component with kinetic energy and rotational (but not vibrational) distributions reflecting the temperature of the liquid, consistent with a distinct trapping-desorption mechanism.     

Seven-dimensional quantum dynamics study of the O(3P)+CH4 reaction

The initial state selected time-dependent wave packet calculations have been carried out to study the title reaction with seven degrees of freedom included by restricting the nonreacting CH(3) group under C(3V) symmetry and the CH bond length in the group. Total reaction probabilities as well as integral cross sections were calculated for the ground and four vibrationally excited reagent states. Our calculation shows that the reactivity is very small for the reaction for collision energy up to 1.0 eV for all the initial states. Initial vibration excitation of CH(4), in particular, the CH stretch excitation, enhances the reactivity, but only part of the excitation energy deposited can be used to reduce the reaction threshold. The rate constant for the ground initial state agrees rather well with that from a recent quasiclassical trajectory study and is larger than that from the semirigid vibrating rotor target calculations, in particular, in the low temperature region. On the other hand, the thermal rate constant calculated from the integral cross sections for these five vibrational states is about a factor of 20 smaller than that from the multiconfiguration time-dependent Hartree study.     

Quantum-state resolved reaction dynamics at the gas-liquid interface: direct absorption detection of HF(v,J) product from F(2P)+squalane

Exothermic reactive scattering of F atoms at the gas-liquid interface of a liquid hydrocarbon (squalane) surface has been studied under single collision conditions by shot noise limited high-resolution infrared absorption on the nascent HF(v,J) product. The nascent HF(v,J) vibrational distributions are inverted, indicating insufficient time for complete vibrational energy transfer into the surface liquid. The HF(v=2,J) rotational distributions are well fit with a two temperature Boltzmann analysis, with a near room temperature component (T(TD) approximately equal to 290 K) and a second much hotter scattering component (T(HDS) approximately equal to 1040 K). These data provide quantum state level support for microscopic branching in the atom abstraction dynamics corresponding to escape of nascent HF from the liquid surface on time scales both slow and fast with respect to rotational relaxation.     

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.     

Theoretical investigation of hyperthermal reactions at the gas-liquid interface: O (3P) and squalane

Hyperthermal collisions (5 eV) of ground-state atomic oxygen [O ((3)P)] with a liquid-saturated hydrocarbon, squalane (C(30)H(62)), have been studied using QM/MM hybrid "on-the-fly" direct dynamics. The surface structure of the liquid squalane is obtained from a classical molecular dynamics simulation using the OPLS-AA force field. The MSINDO semiempirical Hamiltonian is combined with OPLS-AA for the QM/MM calculations. In order to achieve a more consistent and efficient simulation of the collisions, we implemented a dynamic partitioning of the QM and MM atoms in which atoms are assigned to QM or MM regions based on their proximity to "seed" (open-shell) atoms that determine where bond making/breaking can occur. In addition, the number of seed atoms is allowed to increase or decrease as time evolves so that multiple reactive events can be described. The results show that H abstraction is the most important process for all incident angles, with H elimination, double H abstraction, and C-C bond cleavage also being important. A number of properties of these reactive channels, as well as inelastic nonreactive scattering, are investigated, including angular and translational energy distributions, the effect of incident collision angle, variation with depth of the reactive event within the liquid, with the reaction site on the hydrocarbon, and the effect of dynamics before and after reaction (direct reaction versus trapping reaction-desorption).     

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.     

The effect of spin-orbit coupling on fast neutral chemical reaction O(3P)+CH3-->CH3O

The effect of nonadiabatic transitions through the spin-orbit couplings has been investigated on the fast neutral reaction, O((3)P)+CH(3)-->CH(3)O. Adiabatic potential energies and the spin-orbit coupling terms have been evaluated for the four electronic states of CH(3)O ((2)E, (2)A(2), (4)E, and (4)A(2)) that correlate with the O((3)P)+CH(3) asymptote, as a function of CO distance and OCH angle under the C(3v) symmetry, by ab initio electronic structure calculations using multireference internally contracted single and double excitation configuration interaction method with the 6-311G(2df,2pd) basis sets. Multistate quantum reactive scattering calculations have been carried out with the use of thus obtained potential energies and spin-orbit coupling matrices, based on the generalized R-matrix propagation method. The calculated thermal rate constants show a slight positive dependence on temperature in a range between 50 and 2000 K, supporting the previous experimental results. It is shown that the spin-orbit coupled excited states give rise to reflections over the centrifugal barrier due to the quantum interference. Classical capture calculations yield larger rate constants due to the neglect of quantum reflections. It is concluded that the effect of nonadiabatic transitions is of minor importance on the overall reactivity in this reaction.     

Crossed molecular beam studies on the reaction dynamics of O(1D)+N2O

The reaction of oxygen atom in its first singlet excited state with nitrous oxide was investigated under the crossed molecular beam condition. This reaction has two major product channels, NO+NO and N2+O2. The product translational energy distributions and angular distributions of both channels were determined. Using oxygen-18 isotope labeled O(1D) reactant, the newly formed NO can be distinguished from the remaining NO that was contained in the reactant N2O. Both channels have asymmetric and forward-biased angular distributions, suggesting that there is no long-lived collision complex with lifetime longer than its rotational period. The translational energy release of the N2+O2 channel (fT = 0.57) is much higher than that of the NO+NO channel (fT = 0.31). The product energy partitioning into translational, rotational, and vibrational degrees of freedom is discussed to learn more about the reaction mechanism. The branching ratio between the two product channels was estimated. The 46N2O product of the isotope exchange channel, 18O+44N2O-->16O+46N2O, was below the detection limit and therefore, the upper limit of its yield was estimated to be 0.8%.     

A theoretical study on the reaction mechanisms of O(3P)+1‐butene

The reaction of O(³P) with 1‐butene (CH₃CH₂CH?CH₂) are examined by applying the UMP2 and G3 methods. The minimum energy crossing points (MECPs) between the singlet and triplet potential energy surfaces are located using the Newton‐Lagrange method, and it is shown that the MECPs play a key role in the reaction mechanisms. The complex reaction mechanisms are revealed for both adiabatic and nonadiabatic reaction channels, and the observations in several recent experiments can be rationalized based upon the present calculations. The calculational results indicate that the site selectivity of the addition of O(³P) to either carbon atom of the double bond of 1‐butene is not remarkable. In addition, the formation mechanisms of butenols are discussed. The butenols can be created not only by the keto‐enol tautomerization, but also by the rearrangement and decomposition reaction involving the epoxy compound. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Potential surfaces and dynamics of the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction

We present global potential energy surfaces for the three lowest triplet states in O(3P)+H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P)+H2O(X1A1)-->OH(X2pi)+OH(X2pi) reaction using these surfaces. The surfaces are spline-based fits of approximately 20,000 fixed geometry ab initio calculations at the complete-active-space self-consistent field+second-order perturbation theory (CASSCF+MP2) level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1000-2500 K range using these surfaces. We also compute the total, rovibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approximately 4 km s(-1) (16.9 kcal mol(-1) collision energy) to 11 km s(-1) (122.5 kcal mol(-1) collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approximately 40 kcal mol(-1) collision energy rovibrationally excited OH(X2pi) products are a significant and perhaps dominant contributor to the observed 1-5 micro spectral emission from O(3P)+H2O(X1A1) collisions. Another important result is that OH(X2pi) products are formed in two distinct rovibrational distributions. The "active" OH products are formed with the reagent O atom, and their rovibrational distributions are extremely hot. The remaining "spectator" OH is relatively rovibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approximately 50 kcal mol(-1) collision energy, vibration dominates the OH internal energy, and above approximately 50 kcal mol(-1) rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2pi)+OH(X2pi) collisions.     

A quantum wave-packet study of intersystem crossing effects in the O(3P2,1,0,1D2)+H2 reaction

We present for the first time an exact quantum study of spin-orbit-induced intersystem crossing effects in the title reaction. The time-dependent wave-packet method, combined with an extended split operator scheme, is used to calculate the fine-structure resolved cross section. The calculation involves four electronic potential-energy surfaces of the 1A' state [J. Dobbyn and P. J. Knowles, Faraday Discuss. 110, 247 (1998)], the 3A' and the two degenerate 3A" states [S. Rogers, D. Wang, A. Kuppermann, and S. Wald, J. Phys. Chem. A 104, 2308 (2000)], and the spin-orbit couplings between them [B. Maiti, and G. C. Schatz, J. Chem. Phys. 119, 12360 (2003)]. Our quantum dynamics calculations clearly demonstrate that the spin-orbit coupling between the triplet states of different symmetries has the greatest contribution to the intersystem crossing, whereas the singlet-triplet coupling is not an important effect. A branch ratio of the spin state Pi32 to Pi12 of the product OH was calculated to be approximately 2.75, with collision energy higher than 0.6 eV, when the wave packet was initially on the triplet surfaces. The quantum calculation agrees quantitatively with the previous quasiclassical trajectory surface hopping study.     

Quantum calculations of the O(3P)+H2-->OH+H reaction

Quantum scattering calculations are reported for the O(3P)+H2(v=0,1) reaction using chemically accurate potential energy surfaces of 3A' and 3A" symmetry. We present state-to-state reaction cross sections and rate coefficients as well as thermal rate coefficients for the title reaction using accurate quantum calculations. Our calculations yield reaction cross sections that are in quantitative accord with results of recent crossed molecular beam experiments. Comparisons with results obtained using the J-shifting calculations show that the J-shifting approximation is quite reliable for this system. Thermal rate coefficients from the exact calculations and the J-shifting approximation agree remarkably well with experimental results. Our calculations also reproduce the markedly different OH(v'=0)/OH(v'=1) branching in O(3P)+H2(v=1) reaction, observed in experiments that use different O(3P) atom sources. In particular, we show that the branching ratio is a strong function of the kinetic energy of the O(3P) atom.     

Quantum mechanical investigations of the N(4S)+O2(X 3Sigmag -)-->NO(X 2Pi)+O(3P) reaction

The reaction between energetic nitrogen atoms and oxygen molecules has received important attention in connection with nitric oxide chemistry in the lower thermosphere. We report time-independent quantum mechanical calculations of the N(4S)+O2-->NO+O reaction employing the X 2A' and a 4A' electronic potential energy surfaces of Sayos et al. [J. Chem. Phys. 117, 670 (2002)]. We confirm the production of highly vibrationally excited NO molecules, consistent with previous semiclassical and more recent time-dependent quantum wave packet studies. Calculations are carried out for total angular momentum quantum number J=0 and cross sections and rate coefficients are extracted using the J-shifting approximation. The results are in good agreement with available experimental and theoretical data.     

The transition-state region of the O((3)P)+O(2)((3)Sigma(g) (-)) potential energy surface

New electronic structure calculations for the transition-state region of the lowest ozone potential energy surface are reported. A two-dimensional potential energy surface in the asymptotic channel is calculated with the O(2) bond distance being fixed. The calculations are performed at the multireference average quadratic coupled cluster level of theory using full-valence complete active space self-consistent field wave functions and the augmented correlation consistent polarized V6Z atomic basis set. The general shape of the potential energy surface as predicted in earlier studies, that is, a narrow transition state below the O+O(2) asymptote, is confirmed by the present calculations. The transition state is 181 cm(-1) below the asymptote and 72 cm(-1) above the van der Waals-like minimum. The changes in the O+O(2)-->O(3) (*) capture cross section and rate constant when the new potential energy surface is employed are investigated by means of classical trajectory calculations.     

Dynamics of the gas-liquid interfacial reaction of O(1D) with a liquid hydrocarbon

The dynamics of the gas-liquid interfacial reaction of the first electronically excited state of the oxygen atom, O((1)D), with the surface of a liquid hydrocarbon, squalane (C(30)H(62); 2,6,10,15,19,23-hexamethyltetracosane) has been studied experimentally. Translationally hot O((1)D) atoms were generated by 193 nm photolysis of a low pressure (nominally 1 mTorr) of N(2)O a short distance (mean = 6 mm) above a continually refreshed liquid squalane surface. Nascent OH (X(2)Π, v' = 0) reaction products were detected by laser-induced fluorescence (LIF) on the OH A(2)Σ(+)-X(2)Π (1,0) band at the same distance above the surface. The speed distribution of the recoiling OH was characterized by measuring the appearance profiles as a function of photolysis-probe delay for selected rotational levels, N'. The rotational (and, partially, fine-structure) state distributions were also measured by recording LIF excitation spectra at selected photolysis-probe delays. The OH v' = 0 rotational distribution is bimodal and can be empirically decomposed into near thermal (~300 K) and much hotter (~6000 K) Boltzmann-temperature components. There is a strong positive correlation between rotational excitation and translation energy. However, the colder rotational component still represents a significant fraction (~30%) of the fastest products, which have substantially superthermal speeds. We estimate an approximate upper limit of 3% for the quantum yield of OH per O((1)D) atom that collides with the surface. By comparison with established mechanisms for the corresponding reactions in the gas phase, we conclude that the rotationally and translationally hot products are formed via a nonstatistical insertion mechanism. The rotationally cold but translationally hot component is most likely produced by direct abstraction. Secondary collisions at the liquid surface of products of either of the previous two mechanisms are most likely responsible for the rotationally and translationally cold products. We do not think it likely, a priori, that they could be produced in the observed significant yield via a statistical insertion mechanism for a molecule the size of squalane embedded in a surrounding liquid surface.     

Adiabatic and nonadiabatic dynamics in the CH3(CD3)+HCl reaction

The scattering dynamics leading to the formation of Cl (2P(3/2)) and Cl* (2P(1/2)) products of the CH(3)+HCl reaction (at a mean collision energy =22.3 kcal mol(-1)) and the Cl (2P(3/2)) products of the CD(3)+HCl reaction (at =19.4 kcal mol(-1)) have been investigated by using photodissociation of CH(3)I and CD(3)I as sources of translationally hot methyl radicals and velocity map imaging of the Cl atom products. Image analysis with a Legendre moment fitting procedure demonstrates that, in all three reactions, the Cl/Cl* products are mostly forward scattered with respect to the HCl in the center-of-mass (c.m.) frame but with a backward scattered component. The distributions of the fraction of the available energy released as translation peak at f(t)=0.31-0.33 for all the reactions, with average values that lie in the range =0.42-0.47. The detailed analysis indicates the importance of collision energy in facilitating the nonadiabatic transitions that lead to Cl* production. The similarities between the c.m.-frame scattering and kinetic energy release distributions for Cl and Cl* channels suggest that the nonadiabatic transitions to a low-lying excited potential energy surface (PES) correlating to Cl* products occur after passage through the transition state region on the ground-state PES. Branching fractions for Cl* are determined to be 0.14+/-0.02 for the CH(3)+HCl reaction and 0.20+/-0.03 for the CD(3)+HCl reaction. The difference cannot be accounted for by changes in collision energy, mass effects, or vibrational excitation of the photolytically generated methyl radical reagents and instead suggests that the low-frequency bending modes of the CD(3)H or CH(4) coproduct are important mediators of the nonadiabatic couplings occurring in this reaction system.     

O(3P)+O2H→OH+O2反应机理的密度泛函理论研究

用密度泛函理论方法研究了O(3P)与O2H反应生成羟基和氧分子的反应机理. 在PW91/6-31+G水平上用梯度解析技术全自由度优化上述反应物、产物和反应路径上的中间体及过渡态几何构型, 并通过频率振动分析加以确认, 计算IRC反应路径及中间体异构化过程, 确定了此反应的可能反应通道. 结果表明: 该反应是多通道多步骤的强放热反应. 首先形成顺式或反式O3H富能中间体, 此过程无能垒; 然后跨过一个能垒分解成产物OH和O2. 通道IM1→TS1比IM2→TS2克服的能垒要大, 反应放热372.822 kJ*mol-1. IM1TS3IM2 可相互转化.     

Rate constant for OH(2 Pi)+O(3P)-->H(2S)+O2(3 Sigma g-) reaction on an improved ab initio potential energy surface and implications for the interstellar oxygen problem

The authors report a global potential energy surface for the ground electronic state of HO(2)(X (2)A(")), which improves upon the XXZLG potential [Xu and et al., J. Chem. Phys. 122, 244305 (2005)] with additional high-level ab initio points for the long-range interaction potential in the O+OH channel. Exact J=0 quantum mechanical reaction probabilities were calculated on the new potential and the rate constant for the title reaction was obtained using a J-shifting method. The calculated rate constant is in good agreement with available experimental values and our results predict a significantly lower rate at temperature range below 30 K, offering a possible explanation for the "interstellar oxygen problem."     

Experimental and theoretical investigations of rate coefficients of the reaction S(3P)+O2 in the temperature range 298-878 K

Rate coefficients of the reaction S+O(2) with Ar under 50 Torr in the temperature range 298-878 K were determined with the laser photolysis technique. S atoms were generated by photolysis of OCS with a KrF excimer laser at 248 nm; their concentration was monitored via resonance fluorescence excited by atomic emission of S produced from microwave-discharged SO(2). Our measurements show that k(298 K)=(1.92+/-0.29)x10(-12) cm(3) molecule(-1) s(-1), in satisfactory agreement with previous reports. New data determined for 505-878 K show non-Arrhenius behavior; combining our results with data reported at high temperatures, we derive an expression k(T)=(9.02+/-0.27)x10(-19)T(2.11+/-0.15) exp[(730+/-120)/T] cm(3) molecule(-1) s(-1) for 298< or =T< or =3460 K. Theoretical calculations at the G2M (RCC2) level, using geometries optimized with the B3LYP/6-311+G(3df) method, yield energies of transition states and products relative to those of the reactants. Rate coefficients predicted with multichannel RRKM calculations agree satisfactorily with experimental observations; the reaction channel via SOO(1A') dominates at T<500 K, whereas channels involving formation of SOO(3A") followed by isomerization to SO(2) before dissociation, and formation of SOO(1A") followed by direct dissociation, become important at high temperatures, accounting for the observed rapid increase in rate coefficient.     

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.