Spin-orbit effect in the energy pooling reaction O2(a 1Delta)+O2(a 1Delta)-->O2(b 1Sigma)+O2(X 3Sigma)

Five-dimensional nonadiabatic quantum dynamics studies have been carried out on two new potential energy surfaces of S(2)((1)A(')) and T(7)((3)A(")) states for the title oxygen molecules collision with coplanar configurations, along with the spin-orbit coupling between them. The ab initio calculations are based on complete active state second-order perturbation theory with the 6-31+G(d) basis set. The calculated spin-orbit induced transition probability as a function of collision energy is found to be very small for this energy pooling reaction. The rate constant obtained from a uniform J-shifting approach is compared with the existing theoretical and experimental data, and the spin-orbit effect is also discussed in this electronic energy-transfer process.     

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.     

Reaction pathway for the nonadiabatic reaction of Ca(4s3d 1D)+H2-->CaH(X 2Sigma+)+H

The reaction pathway and the nascent CaH product distribution in the reaction Ca(4s3d (1)D)+H(2)-->CaH(X (2)Sigma(+))+H are obtained using a pump-probe technique. The Ca atom is first prepared in the 3 (1)D state by a two-photon absorption, and then in brief time delay the laser-induced fluorescence of the reaction product CaH is monitored. The CaH(v=0,1) distributions appear to be single peaked, as characterized by Boltzmann rotational temperature of 807+/-38 K (v=0) and 684+/-77 K (v=1). The vibrational population ratio of CaH(v=0)/CaH(v=1) is determined to be 3.3+/-0.1, while the v=2 population is not detectable. The fractions of the available energy partitioning into rotation, vibration, and translation are estimated to be 0.36+/-0.05, 0.28+/-0.04, and 0.36+/-0.05, respectively. With the aid of the potential energy surfaces calculations, the current reaction should favor a near C(2v) collision configuration. The temperature dependence measurement yields a positive slope, indicative of the reaction occurrence without any potential barrier. The colliding species are anticipated to follow an attractive 1B(2) (or 2A') surface and then transit nonadiabatically to the reactive ground state surface.     

Reaction dynamics of Si(3PJ)+O2-->SiO(X 1 Sigma+)+O studied by a crossed-beam laser-induced fluorescence technique

Oxidation reaction of the ground state Si atom was studied by using a crossed molecular beam technique at 13.0 kJ/mol of collision energy. The Si atomic beam was generated by laser vaporization and crossed with the oxygen molecular beam at right angle. Products at the crossing region were detected by the laser-induced fluorescence (LIF). The LIF of SiO(A 1 Pi-X 1 Sigma+) was used to determine the vibrational state distribution of the electronic ground state, SiO(X 1 Sigma+). The determined distribution was inverted with the maximum population at v"=4, and in good agreement with the recent quasiclassical trajectory calculation on the singlet potential energy surface. The agreement suggested that an abstraction mechanism is dominant at the collision energy studied here. One of the counterproducts, O(3PJ), was also observed by the vacuum ultraviolet LIF and the distribution of the spin-orbit levels were determined. The formation of O(3PJ) was consistent with the significant population of v"=7 and 8 states of SiO, which could be explained by the presence of the triplet product channel with higher exothermicity.     

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.     

Ab initio analytical potential energy surface and quasiclassical trajectory study of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+))-->OH(+)(X (3)Sigma(-))+H((2)S) reaction and isotopic variants

An analytical potential energy surface (PES) representation of the O(+)((4)S)+H(2)(X (1)Sigma(g) (+)) system was developed by fitting around 600 CCSD(T)/cc-pVQZ ab initio points. Rate constant calculations for this reaction and its isotopic variants (D(2) and HD) were performed using the quasiclassical trajectory (QCT) method, obtaining a good agreement with experimental data. Calculations conducted to determine the cross section of the title reaction, considering collision energies (E(T)) below 0.3 eV, also led to good accord with experiments. This PES appears to be suitable for kinetics and dynamics studies. Moreover, the QCT results show that, although the hypotheses of a widely used capture model are not satisfied, the resulting expression for the cross section can be applied within a suitable E(T) interval, due to errors cancellation. This could be a general situation regarding the application of this simple model to ion-molecule processes.     

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.     

Evidence for the production of N4 via the N2 A3Sigma(u)+ + N2 A3Sigma(u)+ energy pooling reaction

We report mass spectrometric evidence supporting our proposed mechanistic pathway for the production of N4 through the energy pooling reaction N2 A3Sigma(u)+ + N2 A3Sigma(u)+. N2 A3Sigma(u)+ is generated from the quenching of resonantly excited xenon in a mixture of xenon, 15N2, and 14N2 that is illuminated with xenon resonant lamps (147 nm). Mass spectra are periodically taken of the mixture. Over time, we observe significant isotopic scrambling of the 15N2 and 14N2, generating 15N14N in concentrations approaching 10% (approximately 2 Torr) of the initial 15N2 concentration. Though we do not observe the direct formation of N4, the isotopic ratios indicate that an excited complex (15N2(14)N2) exists long enough so that scrambling of the nitrogen atoms can occur, offering a possible route to the formation of tetrahedral nitrogen (1Td N4).     

Reaction dynamics of Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3: reaction pathway and energy disposal for the CaH product

The reaction pathway for Ca(4s3d 1D2)+CH4-->CaH(X 2Sigma+)+CH3 has been investigated by using a pump-probe technique in combination with potential-energy surface (PES) calculations. The nascent product distributions of CaH have been characterized with Boltzmann rotational temperatures of 1013+/-102 and 834+/-70 K for the v=0 and 1 levels, respectively, and a Boltzmann vibrational temperature of 1313+/-173 K. The rotational and vibrational energy partitions in CaH have been estimated to be 461+/-45 and 252+/-15 cm(-1), respectively. According to the PES calculations, the pathway favors an insertion mechanism. Ca(3 1D2) approaches CH4 along an attractive potential surface in a C2v (or Cs) symmetry and then the collision complex undergoes nonadiabatic transition to the reactive ground-state surface. An Arrhenius plot shows a potential-energy requirement of 2695+/-149 cm(-1), which accounts for the endothermicity of 2930 cm(-1) for the reaction scheme. The Ca-C bond distance in the transition state structure is short enough to allow for tight orbital overlap between CaH and CH3. The strong coupling between the moieties renders the energy transfer sufficient from CaH into the CH3 radical. As compared to the Ca(4 1P1) reaction, the dissociation lifetime of the intermediate complex with less excess energy is prolonged so as to cause much less vibrational energy disposal into CaH.     

Differential cross sections and product energy distributions for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a quasiclassical trajectory method

Quasiclassical trajectory calculations have been carried out for the C((3)P)+OH(X (2)Pi)-->CO(X (1)Sigma(+))+H((2)S) reaction using a recent ab initio potential energy surface for the ground electronic state X (2)A(') of COH. Differential cross sections (DCSs), and product vibrational, rotational and translational distributions have been determined for a wide range of collision energies (0.001-1 eV). The role of excitations (rotation or vibration) of the OH reactant on these quantities has been investigated. Product vibrational, rotational, and translational distributions are found to be almost independent on the rovibrational state of OH, whereas DCSs show a weak dependence on the initial rotational state of OH. We also analyze the results using a study based on the lifetime of the intermediate complex and on the kinematic constraint associated with the mass combination.     

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."     

Cross sections and rate constants for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a quasiclassical trajectory method

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state, X 2A', of HCO/COH. Total and state-specific integral cross sections have been determined for a wide range of collision energies (0.001-1 eV). Then, thermal and state-specific rate constants have been calculated in the 1-500 K temperature range. The thermal rate constant varies from 1.78x10(-10) cm3 s-1 at 1 K down to 5.96x10(-11) cm3 s-1 at 500 K with a maximum value of 3.39x10(-10) cm3 s-1 obtained at 7 K. Cross sections and rate constants are found to be almost independent of the rovibrational state of OH.     

A quasiclassical trajectory study for the N(4S)+O2(X 3Sigmag-)-->NO(X 2Pi)+O(3P) reaction on the new 2A' and 4A' potential-energy surfaces

A quasiclassical trajectory study with the sixth-order explicit symplectic algorithm of the N(4S)+O2(X 3Sigmag-)-->NO(X 2Pi)+O(3P) atmospheric reaction has been performed by employing the new 2A' and 4A' potential-energy surfaces reported by Sayos et al. [J. Chem. Phys. 117, 670 (2002)]. For the translational temperature considered up to 10,000 K, the larger relative translational energy and the higher rovibrational levels of O2 molecule with respect to the previous works have been taken into account, and a clearer database about the character of the total reaction cross section has been presented in this work. The dependence of microscopic rate constants on the vibrational level of O2 molecule at T=3000, 5000, and 10,000 K has been exhibited, and we can see that the values of log10 k(T,v,J) vary almost linearly with the vibrational level of O2 molecule. The thermal rate constants at the translational temperature between 300 and 10,000 K have been evaluated and compared with the experimental and previous theoretical results. It is found that the thermal rate constants determined in this work have a better agreement with the experimental data and can provide a more valid theoretical reference at the translational temperature considered for the title reaction.     

Quantum real wave-packet dynamics of the N(4S)+NO(X2Pi)-->N2(X1Sigma(g)+)+O(3P) reaction on the ground and first excited triplet potential energy surfaces: rate constants, cross sections, and product distributions

The reaction N+NO-->N(2)+O was studied by means of the time-dependent real wave-packet (WP) method and the J-shifting approximation. We consider the ground 1 (3)A(") and first excited 1 (3)A(') triplet states, which correlate with both reactants and products, using analytical potential energy surfaces (PESs) recently developed in our group. This work extends our previous quantum dynamics study, and probabilities, cross sections, and rate constants were calculated and interpreted on the basis of the different shapes of the PESs (barrierless 1 (3)A(") and with barrier 1 (3)A(') surfaces, respectively). The WP rate constant (k(1)) shows a weak dependence on T(200-2500 K), as the dominant contribution to reactivity is provided by the barrierless ground PES. There is a good agreement of WP k(1) with the measurements and variational transition state theory (VTST) data, and also between the WP and VTST k(1)(1 (3)A(")) results. Nevertheless, there is a large discrepancy between the WP and VTST k(1)(1 (3)A(')) results. Product state distributions were also calculated for the much more reactive 1 (3)A(") PES. There is an excellent agreement with the experimental average fraction of vibrational energy in N(2)(25+/-3%), the only measured dynamics property of this reaction.     

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.     

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.     

Full dimensional time-dependent quantum dynamics study of the H + NH3 --> H2 + NH2 reaction

A rigorous full dimensional time-dependent wave packet method has been developed for the reactive scattering between an atom and a tetra-atomic molecule. The method has been applied to the hydrogen abstraction reaction H+NH(3)-->H(2)+NH(2). Initial state-selected total reaction probabilities are investigated for the reactions from the ground vibrational state and from four excited vibrational states of ammonia. The total reaction probabilities from two lowest "tunneling doublets" due to the inversion barrier for the umbrella bending motion of NH(3) and from two pairs of doubly degenerate vibrational states of NH(3) are also inspected. Integral cross sections and rate constants are calculated for the reaction from the ground state with the centrifugal-sudden approximation. The calculated results are compared with those from the previous seven dimensional calculations [M. Yang and J. C. Corchado, J. Chem. Phys. 126, 214312 (2007)]. This work shows that the full dimensional rate constants are a factor of 3 larger than the corresponding seven dimensional calculated values at T=200 K and are overall smaller than those obtained from the variational transition state theory in the whole temperature region. The work also reveals that nonreactive NH bonds of NH(3) cannot be treated as spectators due to the fact that three NH bonds are coupled with each other during the reaction process.     

Inelastic scattering matrix elements for the nonadiabatic collision B(2P1/2)+H2(1Sigmag+,j)<-->B(2P3/2)+H2(1Sigmag+,j')

Inelastic scattering matrix elements for the nonadiabatic collision B(2P1/2)+H2(1Sigmag+,j)<-->B(2P3/2)+H2(1Sigmag+,j') are calculated using the time dependent channel packet method (CPM). The calculation employs 1 2A', 2 2A', and 1 2A" adiabatic electronic potential energy surfaces determined by numerical computation at the multireference configuration-interaction level [M. H. Alexander, J. Chem. Phys. 99, 6041 (1993)]. The 1 2A' and 2 2A', adiabatic electronic potential energy surfaces are transformed to yield diabatic electronic potential energy surfaces that, when combined with the total B+H2 rotational kinetic energy, yield a set of effective potential energy surfaces [M. H. Alexander et al., J. Chem. Phys. 103, 7956 (1995)]. Within the framework of the CPM, the number of effective potential energy surfaces used for the scattering matrix calculation is then determined by the size of the angular momentum basis used as a representation. Twenty basis vectors are employed for these calculations, and the corresponding effective potential energy surfaces are identified in the asymptotic limit by the H2 rotor quantum numbers j=0, 2, 4, 6 and B electronic states 2Pja, ja=1/2, 3/2. Scattering matrix elements are obtained from the Fourier transform of the correlation function between channel packets evolving in time on these effective potential energy surfaces. For these calculations the H2 bond length is constrained to a constant value of req=1.402 a.u. and state to state scattering matrix elements corresponding to a total angular momentum of J=1/2 are discussed for j=0<-->j'=0,2,4 and 2P1/2<-->2P1/2, 2P3/2 over a range of total energy between 0.0 and 0.01 a.u.