A crossed molecular beam imaging study of the O(1D2)+HCl-->OH+Cl(2P J=3/2, 1/2) reaction.

A crossed molecular beam study is presented for the O((1)D(2))+HCl-->OH+Cl((2)P(J)) reaction at the collision energy of 6 kcal mol(-1). State-resolved doubly differential cross sections are obtained for the Cl((2)P(J=3/2) ) and Cl*((2)P(J=1/2) ) products by velocity-map ion imaging. Both products are slightly more forward scattered, which suggests a reaction mechanism without a long-lived intermediate in the ground electronic state. A small fraction (23 %) of the energy release into the translational degree of freedom indicates strong internal excitation of the counterpart OH radical. The contribution of the electronic excited states of O--HCl to the overall reaction is also examined from the doubly differential cross sections.     

Soft electron impact ionization in crossed molecular beam reactive scattering: the dynamics of the O((3)P)+C(2)H(2) reaction

Soft ionization by low-energy, tunable electrons is implemented for the first time in crossed molecular beam reactive scattering experiments with mass-spectrometric detection. The power of the method, which permits the suppression of the dissociative ionization of interfering species, is exemplified with the study of the O((3)P)+C(2)H(2) multichannel reaction.     

Theoretical investigation of exchange and recombination reactions in O(3P)+NO(2Pi) collisions

We present a detailed dynamical study of the kinetics of O(3P)+NO(2Pi) collisions including O atom exchange reactions and the recombination of NO2. The classical trajectory calculations are performed on the lowest 2A' and 2A" potential energy surfaces, which were calculated by ab initio methods. The calculated room temperature exchange reaction rate coefficient, kex, is in very good agreement with the measured one. The high-pressure recombination rate coefficient, which is given by the formation rate coefficient and to a good approximation equals 2kex, overestimates the experimental data by merely 20%. The pressure dependence of the recombination rate, kr, is described within the strong-collision model by assigning a stabilization probability to each individual trajectory. The measured falloff curve is well reproduced over five orders of magnitude by a single parameter, i.e., the strong-collision stabilization frequency. The calculations also yield the correct temperature dependence, kr proportional, T-1.5, of the low-pressure recombination rate coefficient. The dependence of the rate coefficients on the oxygen isotopes are investigated by incorporating the difference of the zero-point energies between the reactant and product NO radicals, DeltaZPE, into the potential energy surface. Similar isotope effects as for ozone are predicted for both the exchange reaction and the recombination. Finally, we estimate that the chaperon mechanism is not important for the recombination of NO2, which is in accord with the overall T-1.4 dependence of the measured recombination rate even in the low temperature range.     

The mechanism of proton exchange: guided ion beam studies of the reactions, H(H2O)n+ (n=1-4)+D2O and D(D2O)n+ (n=1-4)+H2O

Reactions of protonated water clusters, H(H(2)O)(n) (+) (n=1-4) with D(2)O and their "mirror" reactions, D(D(2)O)(n) (+) (n=1-4) with H(2)O, are studied using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of collision energy from thermal energy to over 10 eV. At low collision energies, we observe reactions in which H(2)O and D(2)O molecules are interchanged and reactions where H-D exchange has occurred. As the collision energy is increased, the H-D exchange products decrease and the water exchange products become dominant. At high collision energies, processes in which one or more water molecules are lost from the reactant ions become important, with simple collision-induced dissociation processes, i.e., those without H-D exchange, being dominant. Threshold energies of endothermic channels are measured and used to determine binding energies of the proton bound complexes, which are consistent with those determined by thermal equilibrium measurements and previous collision-induced dissociation studies. A kinetic scheme that relies only on the ratio of isomerization and dissociation rate constants successfully accounts for the kinetic energy dependence observed in the branching ratios for H-D and water exchange products in all systems. Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations confirm the feasibility and establish the details of this kinetic model.     

Global potential energy surfaces for O((3)P) + H2O((1)A1) collisions

Global analytic potential energy surfaces for O((3)P) + H(2)O((1)A(1)) collisions, including the OH + OH hydrogen abstraction and H + OOH hydrogen elimination channels, are presented. Ab initio electronic structure calculations were performed at the CASSCF + MP2 level with an O(4s3p2d1f)/H(3s2p) one electron basis set. Approximately 10(5) geometries were used to fit the three lowest triplet adiabatic states corresponding to the triply degenerate O((3)P) + H(2)O((1)A(1)) reactants. Transition state theory rate constant and total cross section calculations using classical trajectories to collision energies up to 120?kcal mol(-1) (～11?km s(-1) collision velocity) were performed and show good agreement with experimental data. Flux-velocity contour maps are presented at selected energies for H(2)O collisional excitation, OH + OH, and H + OOH channels to further investigate the dynamics, especially the competition and distinct dynamics of the two reactive channels. There are large differences in the contributions of each of the triplet surfaces to the reactive channels, especially at higher energies. The present surfaces should support quantitative modeling of O((3)P) + H(2)O((1)A(1)) collision processes up to ～150?kcal mol(-1).     

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.     

CH2((X)3B1)自由基与N2O反应的研究

The reaction dynamics of methylene radicalCH2((x)3B1)with N2O was investigated by Time Resolved Fourier Transform Infrared Spectroscopy(TR-FTIRS). Pure CH2((x)3B1) radical was produced via laser photolysis of ketene at 351 nm.. Nascent vibrationally excited products CO , NO and HCN were observed.. Some reaction pathways which may lead to these products were proposed and a possible reaction mechanism was outlined..     

N(4S)+ NO(X2Π)→N2(X3Σg-)+O(3P) 反应的立体动力学研究

采用准经典轨线方法研究了在不同碰撞能下，碰撞反应N(4S)+NO(X2Π)→ N2(X3Σg- )+O(3P)在两个最低势能面3A 和 3A'上产物与反应物之间的矢量相关. 结果表明，对于不同的碰撞能，在两个势能面上反应产物的转动取向展示了不同的特征和趋势. 随着碰撞能的增加，发生在3A 势能面上的反应主要受外平面机理支配，而发生在 3A' 势能面上的反应倾向于受内平面机理支配. 这些差异来自于两个势能面的不同构型.     

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.     

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.     

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.     

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.     

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 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.     

O(1D)与CF3Br的反应研究

研究了CF₃Br-O₃体系在253.7nm紫外光照射下所引发的O(¹D)与CF₃Br的反应.O⁽¹D)与CF₃Br反应的最终产物为CF₂O、F₂和Br₂,反应速率常数k为1.32×10^-10cm³·mol^-1·s^-1.讨论了O(3P)与CF₃Br反应的可能性、O(¹D)与CF₃Br反应的机理及外加气体对反应的影响等.     

Isotope branching and tunneling in O(3P)+HD-->OH+D; OD+H reactions

The O((3)P)+HD and O((3)P)+D(2) reactions are studied using quantum scattering calculations and chemically accurate potential energy surfaces developed for the O((3)P)+H(2) system by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)]. Cross sections and rate coefficients for OH and OD products are calculated using accurate quantum methods as well as the J-shifting approximation. The J-shifting approach is found to work remarkably well for both O+HD and O+D(2) collisions. The reactions are dominated by tunneling at low temperatures and for the O+HD reaction the hydrogen atom transfer leading to the OH product dominates at low temperatures. Our result for the OH/OD branching ratio is in close agreement with previous calculations over a wide range of temperatures. The computed OH/OD branching ratios are also in close agreement with experimental results of Robie et al. [Chem. Phys. Lett. 134, 579 (1987)] at temperatures above 400 K but the theoretical results do not reproduce the rapid rise in the experimental values of the branching ratio for temperatures lower than 350 K. We believe that new measurements could resolve the long-standing discrepancy between experiment and theory for this benchmark reaction.     

A combined crossed beam and theoretical investigation of O(3P) + C3H3 --> C3H2 + OH

The radical-radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated in a crossed beam configuration. The radical reactants O(3P) and C3H3 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor propargyl bromide, respectively. A new exothermic channel of O(3P) + C3H3 --> C3H2 + OH was identified and the nascent distributions of the product OH in the ground vibrational state (X 2Pi:nu" = 0) showed bimodal rotational excitations composed of the low- and high-N" components without spin-orbit propensities. The averaged ratios of Pi(A')/Pi(A") were determined to be 0.60 +/- 0.28. With the aid of ab initio theory it is predicted that on the lowest doublet potential energy surface, the reaction proceeds via the addition complexes formed through the barrierless addition of O(3P) to C3H3. The common direct abstraction pathway through a collinear geometry does not occur due to the high entrance barrier in our low collision energy regime. In addition, the major reaction channel is calculated to be the formation of propynal (CHCCHO) + H, and the counterpart C3H2 of the probed OH product in the title reaction is cyclopropenylidene (1c-C3H2) after considering the factors of barrier height, reaction enthalpy and structural features of the intermediates formed along the reaction coordinate. On the basis of the statistical prior and rotational surprisal analyses, the ratio of population partitioning for the low- and high-N" is found to be about 1:2, and the reaction is described in terms of two competing addition-complex mechanisms: a major short-lived dynamic complex and a minor long-lived statistical complex. The observed unusual reaction mechanism stands in sharp contrast with the reaction of O(3P) with allyl radical (C3H5), a second significant conjugated hydrocarbon radical, which shows totally dynamic processes [J. Chem. Phys. 117, 2017 (2002)], and should be understood based upon the characteristic electronic structures and reactivity of the intermediates on the potential energy surface.     

Exploring the dynamics of reaction N((2)D)+C2H4 with crossed molecular-beam experiments and quantum-chemical calculations

We conducted the title reaction using a crossed molecular-beam apparatus, quantum-chemical calculations, and RRKM calculations. Synchrotron radiation from an undulator served to ionize selectively reaction products by advantage of negligibly small dissociative ionization. We observed two products with gross formula C(2)H(3)N and C(2)H(2)N associated with loss of one and two hydrogen atoms, respectively. Measurements of kinetic-energy distributions, angular distributions, low-resolution photoionization spectra, and branching ratios of the two products were carried out. Furthermore, we evaluated total branching ratios of various exit channels using RRKM calculations based on the potential-energy surface of reaction N((2)D)+C(2)H(4) established with the method CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p)+ZPE[B3LYP/6-311G(d,p)]. The combination of experimental and computational results allows us to reveal the reaction dynamics. The N((2)D) atom adds to the C=C π-bond of ethene (C(2)H(4)) to form a cyclic complex c-CH(2)(N)CH(2) that directly ejects a hydrogen atom or rearranges to other intermediates followed by elimination of a hydrogen atom to produce C(2)H(3)N; c-CH(2)(N)CH+H is the dominant product channel. Subsequently, most C(2)H(3)N radicals, notably c-CH(2)(N)CH, further decompose to CH(2)CN+H. This work provides results and explanations different from the previous work of Balucani et al. [J. Phys. Chem. A, 2000, 104, 5655], indicating that selective photoionization with synchrotron radiation as an ionization source is a good choice in chemical dynamics research.     

B(2)A(')-X(2)A(') detection of vibrationally excited HCO produced by the O((3)P)+C(2)H(4) reaction

The distribution of rotational and vibrational energy in HCO produced by the O((3)P)+C(2)H(4) reaction has been measured using laser-induced fluorescence detection via the B(2)A(')-X(2)A(') transition. Over a detection wavelength range of 248-290 nm, our experiments have shown that HCO is formed in both the ground state and in at least six vibrationally excited states with up to two quanta of energy in the C-O stretch and the bending mode. Dispersed fluorescence experiments were conducted to positively assign all of the HCO vibrational bands. The experiments confirmed that many bands, including the B(000)-X(000) band, are affected by overlap with other HCO bands. Spectral modeling was used to separate the contributions of overlapping HCO B-X bands and to determine a nascent HCO rotational temperature of approximately 600 K, corresponding to approximately 6% of the total energy from the O((3)P)+C(2)H(4) reaction. HCO vibrational distributions were determined for two different average collision energies and were fit with vibrational temperatures of 1850+/-80 K and 2000+/-100 K, corresponding to approximately 15% of the total energy. The observed Boltzmann distribution of vibrational energy in HCO indicates that HCO and CH(3) are formed by the dissociation of an energized intermediate complex.