Adiabatic capture theory applied to N+NH-->N2+H at low temperature

The adiabatic capture centrifugal sudden approximation (ACCSA) has been applied to the ground state reaction N+NH-->N2+H over the temperature range 2-300 K using an existent potential energy surface. The resultant thermal rate constants are in agreement with available rate constants from quasi-classical trajectory calculations but are significantly larger than the available experimentally derived rate. The calculated rate constants monotonically increase with increasing temperature but could only be approximately described with a simple Arrhenius-like form. Subtle quantum effects are evident in the initial rotational state resolved cross sections and rate constants.     

A coupled states calculation of accurate quantum rate constants for H + H2

In this article we present the results of converged quantum reactivescattering calculations of thermal rate constants for H + H₂ using the Liu-Siegbahn-Truhlar-Horowitz (LSTH) potential energy surface. These calculations are based on the coupled states (CS) approximation wherein rotational states having different body fixed angular momentum projection quantum numbers are decoupled. By comparision with accurate coupled channel results on the Porter-Karplus No. 2 (PK2) potential surface, we estimate that the maximumerror in thermal rate constants arising from both this approximation and from other numerical approximations in the calculation is less than 25%. We also show that the sum over projection quantum numbers Ω associated with the CS calculation may be approximated quite accurately in terms of the Ω = 0 rate constants by assuming that the |Ω| > 0 rate constants differ from Ω = 0 by a shift in activation energy, which reflects the vibrationally adiabatic bending energy associated with each Ω. Comparison of the LSTH rate constants with experiment indicates average errors of 16% and 24% relative to the two modern measurementsof the rate constants for H + H₂. These errors are reduced to 18% and 9% if the CS rate constants are multiplied by exp(0.0065 eV/kT). The expected error (based on recent quantum structure calculations) associated with the 0.425 eV barrier of theLSTH potential surface is 0.0065 eV. Overall, the agreement of either the LSTH or modified LSTH rate constants with experiment is within the 32% maximum disagreement between the two experimental measurements at all butthe lowest temperature that has been studied. Comparison of our CS rate constants with the results from simpler theories is considered using both the LSTH and PK2 potential surfaces. The best overall agreement is with transition state theories that use accurate dynamical methods to calculate tunnelling factors. These include reduced dimensionality quantum dynamics methods and variational transition state theory using either the Marcus-Coltrin or least action ground-state tunnelling paths. Comparison with the results of quasiclassical trajectory calculations indicates substantial errors at low temperatures.     

The reaction H + D2 → HD + D: Distorted wave calculations at Etrans (υ = 0,j 0) = 0.55 and 1.3 eV

Static—static distorted wave and vibrationally adiabatic distorted wave calculations have been performed for the product rotational distributions of the H + D₂ → HD + D reaction using an accurate ab initio potential energy surface. Comparison is made with coupled states and quasiclassical trajectory calculations as well as with experimental measurements.     

Quantum capture, adiabatic channel, and classical trajectory study of the high pressure rate constant of the reaction H+O2-->HO2 between 0 and 5000 K

Limiting high pressure rate constants for the recombination reaction H+O(2)-->HO(2) are modeled between 0 and 5000 K on an ab initio potential. Quantum capture theory is employed for the temperature range from 0 to about 1 K, while classical trajectory calculations are suitable for covering temperatures above about 200 K. The intermediate temperature range is analyzed by adiabatic channel capture theory. The system is characterized by transition-state switching from outer transition states in the long-range-C(6)R(6) potential to inner transition states in the range of a "shoulder" of the potential. The limiting high pressure rate constants from the trajectory calculations are sufficient for comparison with the experimental data which are available over the range from 300 to 900 K. Specific rate constants k(E,J) for HO(2) dissociation are also given and analyzed with respect to internal consistency with capture cross sections.     

Quasiclassical trajectory study of the C(1D) + H2 reaction and isotopomeric variants: kinetic isotope effect and CD/CH branching ratio

The recently proposed ab initio single-sheeted double many-body expansion potential energy for the methylene molecule has been used to perform quasiclassical trajectory (QCT) calculations for the title reaction. Thermal and initial state-specific (v = 0, j = 0) rate constants for the C((1)D) + H(2)/HD/D(2) reactions have been obtained over a wide range of temperatures. Cross sections for the reaction C((1)D) + H(2) and its deuterated isotopes have also been calculated, as well as the CD/CH branching ratios for the C((1)D) + HD reaction. It is found that the CD + H product channel in the C((1)D) + HD reaction is preferred relative to the CH + D channel. The estimated rate constants are predicted to be in the order k(H2) > k(HD) > k(D2) and the calculated cross sections and rate constants compared with available theoretical and experimental data.     

Quasi-classical trajectory study of the adiabatic reactions occurring on the two lowest-lying electronic states of the LiH2+ system

Quasi-classical trajectory calculations have been performed on the adiabatically allowed reactions taking place on the two lowest-lying electronic states of the LiH2+ system, using the ab initio potential energy surfaces of Martinazzo et al. (J. Chem. Phys., 2003, 119, 11 241). These reactions comprise: (i) the exoergic H2 and H2+ formation occurring through LiH+ + H and LiH + H+ collisions in the ground and in the first electronically excited state, respectively; (ii) the endoergic (ground state) LiH+ dissociation induced by collisions with H atoms; and (iii) the endoergic (excited state) Li + H2+ --> LiH + H+ reaction. The topic is of relevance for a better understanding of the lithium chemistry in the early universe. Thermal rate constants for the above reactions have been computed in the temperature range 10-5000 K and found in reasonably good agreement with estimates based on the capture model.     

Combining transition state calculations with quasiclassical trajectory calculations: II. Collinear collisions involving vibrationally excited reagents

Conventional quasiclassical trajectory simulations of collinear reactive collisions, A + BC (v = 0, 1 and 2) → AB + C, have been compared with trajectories integrated forward and backward in time from points along a dividing line S^* in the strong interaction zone. Calculations have been performed on three different potential energy surfaces, ranging from a strongly attractive surface with a 1 kcal mol⁻¹ barrier to a strongly repulsive surface with a 10 kcal mol⁻¹ barrier, and for all combinations of light (1 amu) and heavy (35 amu) atoms. Two methods of selecting S^* have been examined. The first, based on defining vibrationally adiabatic states orthogonal to the minimum energy path by an approximate analysis, works well for many combinations of potential energy surface and atomic masses. However, a better method is to use pods (periodic orbiting dividing surfaces) for which the action over one cycle of the pods motion is equal to (v + 1/2)h. In only a few cases, where the pods cross the minimum energy path after substantial curvature in the latter, is the agreement between the two sets of calculations less than very good. The results confirm that reagent vibrational motion is in many cases strongly adiabatic up to S^* (i.e. the transition state), and suggest that similar combined calculations on three-dimensional systems should provide a substantial saving in computer effort compared with conventional quasiclassical trajectory methods.     

Quantum statistical study of O + O2 isotopic exchange reactions: cross sections and rate constants

Using a wave packet based statistical model, we compute cross sections and thermal rate constants for various isotopic variants of the O + O2 exchange reaction on a recently modified ab initio potential energy surface. The calculation predicts a highly excited rotational distribution and relatively cold vibrational distribution for the diatomic product. A small but important threshold effect was identified for the (16)O + 18O2 reaction, which is suggested to contribute to the experimentally observed negative temperature dependence of the rate ratio, k(18O + 16O2)/k(16O + 18O2). Despite reasonable agreement with quasiclassical trajectory results, however, the calculated thermal rate constants are smaller than experimental measurements by a factor from 2 to 5. The experimentally observed negative temperature dependence of the rate constants is not reproduced. Possible reasons for the theory-experiment discrepancies are discussed.     

Electronic transitions in collinear H+ + D2 (ν = 0) → HD+ (ν = 0,1) + D via classical trajectories in complex time and complex phase space

A semiclassical treatment of electronic transitions in the collinear rearrangement H⁺ + D₂ (ν = 0) → HD⁺ (ν = 0,1) + D is presented. The treatment represents an extension of Stueckelberg's method for a single nuclear degree of freedom to collisions involving several nuclear degrees of freedom. The classical limit of scattering amplitudes (S-matrix elements) is calculated for the transition between the two adiabatic potential energy surfaces corresponding to the two lowest singlet states of HD⁺₂. S-matrix elements are constructed from trajectories propagating in complex time and complex phase space, which make localized transitions between the two surfaces by crossing their complex line of intersection. The action along each trajectory acquires an imaginary part, which contributes exponential damping to the corresponding amplitude for electronic transition.     

Ab initio prediction of the structure and vibration-rotation spectroscopic properties of Li2OH and Li2OH+

The equilibrium structures and potential energy surfaces of the Li2OH radical and the Li2OH+ cation in their ground electronic states have been determined from accurate ab initio calculations. The vibration-rotation energy levels and spectroscopic constants of three isotopic species (Li2OH, Li2OD, 6Li2OH) were calculated by a perturbational approach. The predicted spectroscopic constants may serve as a useful guide for detecting these species by vibration-rotation spectroscopy and for assigning their spectra.     

Time-dependent quantum study of the kinetics of the H(2S) + FO(2II) OH(2II) + F(2P) reaction

Quantum mechanical wave packet calculations are carried out for the H((2)S) + FO((2)II) --> OH((2)II) + F((2)P) reaction on the adiabatic potential energy surface of the ground (3)A' triplet state. The state-to-state and state-to-all reaction probabilities for total angular momentum J = 0 have been calculated. The probabilities for J > 0 have been estimated from the J = 0 results by using J-shifting approximation based on a capture model. Then, the integral cross sections and initial state-selected rate constants have been calculated. The calculations show that the initial state-selected reaction probabilities are dominated by many sharp peaks. The reaction cross section does not manifest any sharp oscillations and the initial state-selected rate constants are sensitive to the temperature.     

A Study of the Reaction Dynamics for the H+BrCH_3 System Using Quasi-classical Trajectory Theory

In the present paper, scattering probabilities and rate constants of different channels for the H + BrCH_3 reaction system have been calculated by means of quasiclassical trajectory (QCT) method. Several important kinetic effects such as vibrational enhancement, channel competition, vibrational adiabaticity, mass combination, coupling of angular momenta and the relation between the kinetic effects and the feature of the potential energy surface have been discussed. Based on these analyses, a direct-type rebonded mechanism for this reaction has been inferred and used to explain the nonsymmetric angular distribution of the products crossed-molecular beam experiment. The agreement of calculation with experimental results is satisfactory.     

Collision energy dependence of the O(1D) + HCl --> OH + Cl(2P) reaction studied by crossed beam scattering and quasiclassical trajectory calculations on ab initio potential energy surfaces

The dynamics of the O(1D) + HCl --> OH + Cl(2P) reaction are investigated by a crossed molecular beam ion-imaging method and quasiclassical trajectory calculations on the three ab initio potential energy surfaces, the ground 1(1)A' and two excited (1(1)A' and 2(1)A') states. The scattering experiment was carried out at collision energies of 4.2, 4.5, and 6.4 kcal/mol. The observed doubly differential cross sections (DCSs) for the Cl(2P) product exhibit almost no collision energy dependence over this inspected energy range. The nearly forward-backward symmetric DCS indicates that the reaction proceeds predominantly on the ground-state potential energy surface at these energies. Variation of the forward-backward asymmetry with collision energy is interpreted using an osculating complex model. Although the potential energy surfaces obtained by CASSCF-MRCI ab initio calculations exhibit relatively low potential barriers of 1.6 and 6.5 kcal/mol for 1(1)A' and 2(1)A', respectively, the dynamics calculations indicate that contributions of these excited states are small at the collision energies lower than 15.0 kcal/mol. Theoretical DCSs calculated for the ground-state reaction pathway agree well with the observed ones. These experimental and theoretical results suggest that the titled reaction at collision energies less than 6.5 kcal/mol is predominantly via the ground electronic state.     

Ab initio adiabatic and quasidiabatic potential energy surfaces of lowest four electronic states of the H+ + O2 system

Ab initio global adiabatic and quasidiabatic potential energy surfaces of lowest four electronic (1-4 (3)A(")) states of the H(+)+O(2) system have been computed in the Jacobi coordinates (R,r,γ) using Dunning's cc-pVTZ basis set at the internally contracted multireference (single and double) configuration interaction level of accuracy, which are relevant to the dynamics studies of inelastic vibrational and charge transfer processes observed in the scattering experiments. The computed equilibrium geometry parameters of the bound [HO(2)](+) ion in the ground electronic state and other parameters for the transition state for the isomerization process, HOO(+)?OOH(+) are in good quantitative agreement with those available from the high level ab initio calculations, thus lending credence to the accuracy of the potential energy surfaces. The nonadiabatic couplings between the electronic states have been analyzed in both the adiabatic and quasidiabatic frameworks by computing the nonadiabatic coupling matrix elements and the coupling potentials, respectively. It is inferred that the dynamics of energy transfer processes in the scattering experiments carried out in the range of 9.5-23 eV would involve all the four electronic states.     

Dynamics of the 18O + 16O2(υ=0) exchange reaction on a new potential energy surface for ground-state ozone

A quasiclassical trajectory study of the thermoneutral isotopic oxygen exchange reaction was made using a recently reported potential energy surface for the ground state of ozone. The calculated macroscopic rate constants show an improvement over the previous quasiclassical results when compared with the experimental values.     

A dynamical study of the Si(+) + H(2)O reaction

A dynamical study of the Si(+) + H(2)O reaction has been carried out by means of a quasiclassical trajectory method that decomposes the reaction into a capture step, for which an accurate analytical potential is employed, and an unimolecular step, in which the evolution of the collision complex is studied through a direct dynamics BHandHLYP/6-31G(d,p) method. The capture rate coefficient has been computed for thermal conditions corresponding to temperatures ranging from 50 to 1000 K. It is concluded that the main reason why the reaction rate is about 10 times smaller than the capture rate (at T = 298 K) is the topology of the potential energy surface of the ground state. It is also concluded that the ratio between the rates of product and reactant generation from the collision complex decreases quite steeply with increasing temperature, and therefore, the reaction rate decreases even more sharply. Exciting the stretching normal modes of water substantially increases that ratio, and moderate rotational excitation does not appear to have a relevant effect. The collision complex is always initially SiOH(2)(+), but in some trajectories, it becomes HSiOH(+), which generates the products, although the former species is the main intermediate.     

辛准经典轨迹法研究Cl+H2反应

用辛准经典轨迹法模拟了Cl+H2反应在mBW2势能面上的动力学行为。研究了各种初始条件下的反应碰撞截面,产物的能量分配,角度分布和态分布。另外,我们还比较了反应物的三种能量形式(平动能,转动能和振动能)对反庆的有效性。     

Total reactive cross section for K + Br2 in the energy range of 0–4 eV

3-D classical trajectory calculations were performed using diabatic as well as adiabatic potential energy surfaces. Non-adiabatic transitions were allowed and localized at the avoided crossing of the two adiabatic surfaces. The transition probability was calculated according to the Landau—Zener formalism. The total cross sections for the reaction K + Br₂ → KBr + Br were calculated and compared with experimental data. The total cross sections, calculated with the aid of adiabatic potential energy surfaces, were, contrary to those with diabatic surfaces, in very good agreement with the measured total reactive cross sections over the whole energy range of 0–4 eV.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.