Anabinitiostudy of the O(1D)+HCl reaction

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO/ClOX Isomers (X = H, O, and Cl) Using DFT and CCSD(T) Methods

The performance of a group of density functional methods of progressive complexity for the description of the ClO bond in a series of chlorine oxides was investigated. The simplest ClO radical species and the two isomeric structures XClO/ClOX for each X = H, Cl, and O were studied using the PW91, TPSS, B3LYP, PBE0, M06, M06-2X, BMK, and B2PLYP functionals. Geometry optimizations and reaction enthalpies and enthalpies of formation for each species were calculated using Pople basis sets and the (aug)-cc-pVnZ Dunning sets, with n = D, T, Q, 5, and 6. For the calculation of enthalpies of formation, atomization and isodesmic reactions were employed. Both the precision of the methods with respect to the increase of the basis sets, as well as their accuracy, were gauged by comparing the results with the more accurate CCSD(T) calculations, performed using the same basis sets as for the DFT methods. The results obtained employing composite chemical methods (G4, CBS-QB3, and W1BD) were also used for the comparisons, as well as the experimental results when they are available. The results obtained show that error compensation is the key for successful description of molecular properties (geometries and energies) by carefully selecting the method and basis sets. In general, expansion of the one-electron basis set to the limit of completeness does not improve results at the DFT level, but just the opposite. The enthalpies of formation calculated at the CCSD(T)/aug-cc-pV6Z for the species considered are generally in agreement with experimental determinations and the most accurate theoretical values. Different sources of error in the calculations are discussed in detail.

Theoretical study of product polarization of O(1D) + HCl(v = 0; j = 0) → ClO + H and its isotope exchange reaction

O(1D) + HCl(v = 0; j = 0) → ClO + H and its isotope exchange reaction O(1D) + DCl(v = 0; j = 0) → ClO + D are studied in the collision energy range 14.0–20.0 kcal/mol based on the potential energy surface 1[Formula: see text] state. Reaction probabilities, integral cross sections, the two angular distribution functions (concerning the initial/final velocity vector, and the product rotational momentum vector), and the product rotational alignment parameters are calculated as a function of the collision energy for the two reactions. The four generalized polarization dependent differential cross sections are presented to manifest the polarization characters. Also, the effect of the collision energy and the kinetic isotope effect are studied.

Stereodynamic study of the reaction H(²S) + ClO(²Π) → HO(²Π) + Cl(²P) via quasi-classical trajectory calculations

Quasi-classical trajectory calculations were performed to investigate the stereodynamics of the reaction H(²S) + ClO(²Π) → HO(²Π) + Cl(²P) using the ground-state potential energy surface 1¹A'. The alignment and orientation of the product molecules as well as the four polarization-dependent differential cross-sections (PDDCSs) for this reaction across a wide range of collision energies (0.05-1.0 eV) and for the rovibrational state ClO(v = 0 and j = 0) were obtained and are reported here. It was found that the OH product rotational polarization is not very sensitive to the collision energy selected. We discuss this phenomenon in detail. The calculated results indicate that, for this system, the two deep wells in the potential energy surface are very likely to be powerful influences on the degree of product rotational polarization. In addition, the microscopic reaction mechanism that dictates the product angular momentum orientation was investigated. The forward peak in the PDDCS₀₀ at θ = 0° in our study showed a strong dependence on the initial collision energy.

The dynamical study of O(1D) + HCl(v = 0, j = 0) reaction at hyperthermal collision energies

Backgrounds

The quasi-classical trajectory calculations for O(¹D) + HCl → OH + Cl (R1) and O(¹D) + HCl → ClO + H (R2) reactions have been performed at hyperthermal collision energies (60.0, 90.0, and 120.0 kal/mol) on the ¹A' state. Reaction probabilities and integral cross sections are calculated. The product rotational distributions for the two channels, and the product rotational alignment parameters are investigated. Also, the alignment and the orientation of the products have been predicted through the angular distribution functions (concerning the initial/final velocity vector, and the product rotational angular momentum vector). To have a deeper understanding of the natures of the vector correlation between reagent and product relative velocities, a natural generalization of the differential cross section __PDDCS₀₀, is calculated.

Results

The OH + Cl channel is the main product channel and is observed to have essentially isotropic rotational distributions. The ClO + H channel is found to be clearly rotationally polarized.

Conclusions

The dynamical, especially the stereodynamical characters are quite different for the two channels of the title reaction. Most reactions occur directly, except for R2 reaction at the collision energies of 60.0 and 120.0 kcal/mol. The alignment and orientation effects are weak/strong for R1/R2 reaction. The well structure on the potential energy surface and hyperthermal collision energies might result in the dynamical effects.

THEORETICAL STUDY OF THE STEREO-DYNAMICS OF THE REACTION O + HCl → ClO + H

Quasi-classical trajectory (QCT) method is used to study the stereo-dynamics of the title reaction on the ground 1 1A′ potential energy surface (PES). Differential cross-sections (DCSs) and alignments of the product rotational angular momentum for the reaction are reported. The influence of collision energy on the product vector properties is also studied in the present work. The distribution of angle between k and j′, P(θr), the distribution of dihedral angle denoting k-k′-j′ correlation, P(ϕr) ⋅ (2π/σ)( d σ00/ d ωt), (2π/σ)( d σ20/ d ωt), (2π/σ)( d σ22+/ d ωt) and (2π/σ)(dσ21-/dωt) have been calculated in the center of mass frame, respectively.

CHEMICAL REACTIONS IN THE O(1D) + HCl SYSTEM I

New global ab initio potential energy surfaces (PES) are presented for the low-lying 11A′, 11A′′ and 21A′ electronic states which are correlated to O (1D) + HCl . These potential energy surfaces are computed by using the multi-reference configuration interaction method with the Davidson correction (MRCI+Q). The reference functions are constructed by the complete active space self-consistent field (CASSCF) calculations using the quadruple zeta + polarization basis set augmented with diffuse functions. The computations are carried out at about 5000 molecular conformations on each three-dimensional potential energy surface. The high accuracy of the computations is confirmed by a comparison with the available most accurate data for the ground state 11A′; thus the present work is the first report of the accurate potential energy surfaces for the two excited states. Three low-lying transition states on the excited surfaces, two (TS2 and TS4) on 11A′′ and one (TS3) on 21A′, are found. Since TS2 and TS3 are as low as 0.07 eV and 0.28 eV, respectively, and correlate to the OH (2Π) + Cl (2P) product, these excited surfaces are expected to play quite important roles in the reaction dynamics. Possible effects of nonadiabatic couplings among the three PESs are also briefy discussed, although the nonadiabatic couplings have not yet been estimated. The quantum reaction dynamics on these three PESs are discussed in the second accompanying paper, Paper II.

CHEMICAL REACTIONS IN THE O(1D) + HCl SYSTEM II

Using the accurate global potential energy surfaces for the 11A′, 11A′′, and 21A′ states reported in the previous sister Paper I, detailed quantum dynamics calculations are performed for these three adiabatic surfaces separately for J = 0 (J: total angular momentum quantum number). Overall reaction probabilities for O + HCl → OH + Cl and H + ClO, the branching ratio between the two reactions, effects of the initial rovibrational excitation, and product rovibrational distributions are evaluated in the total energy region E tot ≤ 0.9 eV. Significant contributions to the overall reaction dynamics are found from the two excited 11A′′ and 21A′ potential energy surfaces, clearly indicating the insufficiency of the dynamics only on the ground 11A′ surface. The detailed dynamics on the excited surfaces are reported in the third paper of this series.

INITIAL ROTATIONAL QUANTUM STATE EXCITATION AND ISOTOPIC EFFECTS FOR THE O(1D)+HCl → OH+Cl (OCl+H) REACTION

We present reaction probabilities, branching ratios and vibrational product quantum state distributions for the reaction O (1D)+ HCl → OH+Cl (OCl+H) , Boltzmann averaged over initial rotational quantum states at a temperature of 300 K and also for the deuterium isotopic variant. The quantum scattering dynamics are performed using the potential energy surfaces for all three contributing electronic states. Comparisons are presented with results computed using only the ground electronic state potential energy surface, with results computed using only the j = 0 initial rotational state and also with results obtained using an equal weighting for the lowest 10 rotational states. Inclusion of the higher initial rotational states significantly changes the form of the reaction probability as a function of collision energy, reducing the threshold for reaction on the 1A" and 2A' excited electronic states. We found that the combined inclusion of higher initial rotational states and all three contributing electronic states is crucial for obtaining a branching ratio that is within the range and trend given by experiment from our J = 0 calculations. Isotopic effects range from tunnelling effects for the hydrogen variant and enhancement of reactivity for the production of OD on the excited electronic states.

EFFECTS OF ROTATIONAL AND VIBRATIONAL EXCITATION ON THE STEREODYNAMICS OF THE O (D-1) + HCl → OH + Cl REACTION

The stereodynamics of the title reaction on the ground 1 1A′ potential energy surface (PES) has been studied using quasi-classical trajectory (QCT) method. Collision energy of 6.4 kcal/mol is considered, and vector properties including angular momentum alignment distributions and polarization-dependent differential cross-sections (PDDCS) of the product OH are presented. Furthermore, the influence of reagent rotational excitation and vibrational excitation on the product vector properties has also been studied in the present work. The results indicate that the distribution of the P(θr) and P(ϕr) are sensitively affected by the rotational and vibrational excitation. The rotational excitation decreases the degree of alignment and orientation, while vibrational excitation increases the degree of alignment and orientation. The PDDCS (2π/σ)(dσ20/dωt) and (2π/σ)(dσ22+/dωt) are sensitively influenced by rotational and vibrational excitations, while the PDDCS ((2π/σ)(dσ00/dωt)) and (2π/σ)(dσ21-/dωt) are not. The preference of forward scattering has been found from the results of PDDCS ((2π/σ)(dσ00/dωt)), which is in good agreement with the experimental results.

ACCURATE AND HIGHLY EFFICIENT CALCULATION OF THE O(1D)HCl VIBRATIONAL BOUND STATES, USING A COMBINATION OF METHODS

Hypochlorous acid, HOCl, is an important intermediate in the O (1D) HCl reactive system. Due in part to a large number of vibrational bound states (over 800), extremely large direct product basis sets (around 300,000) are required to compute the energy levels just below the dissociation threshold. This situation, combined with a very high density of states, results in difficult convergence for iterative methods — e.g. Lanczos requires 50,000 iterations, and filter diagonalization uses 60,000 iterations. In contrast, using new methodologies, we are able to compute the highest-lying bound states with only 271 iterations, although the CPU cost per iteration is substantially greater. Lower lying states are also computed, for a fraction of the CPU cost of the highest energy calculation.

The dynamics of the O(1D) + HCl --> OH + Cl reaction at a 0.26 eV collision energy: a comparison between theory and experiment

The dynamics of the O((1)D) + HCl(v = 0, j = 0) --> Cl + OH reaction at a 0.26 eV collision energy has been investigated by means of a quasiclassical trajectory (QCT) and statistical quantum and quasiclassical methods. State-resolved cross sections and Cl atom velocity distributions have been calculated on two different potential energy surfaces (PESs): the H2 surface (Martinez et al. Phys. Chem. Chem. Phys. 2000, 2, 589) and the latest surface by Peterson, Bowman, and co-workers (PSB2) (J. Chem. Phys. 2000, 113, 6186). The comparison with recent experimental results reveals that the PSB2 PES manages to describe correctly differential cross sections and the velocity distributions of the departing Cl atom. The calculations on the H2 PES seem to overestimate the OH scattering in the forward direction and the fraction of Cl at high recoil velocities. Although the comparison of the corresponding angular distributions is not bad, significant deviations with a statistical description are found, thus ruling out a complex-forming mechanism as the dominant reaction pathway. However, for the ClO + H product channel, the QCT and statistical predictions are found to be in good agreement.

A Crossed Molecular Beam Imaging Study of the O(1D2)+HCl→OH+Cl(2PJ=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.

Accurate and highly efficient calculation of the highly excited pure OH stretching resonances of O(1D)HCl, using a combination of methods

Accurate calculation of the energies and widths of the resonances of HOCl--an important intermediate in the O(1D)HCl reactive system--poses a challenging benchmark for computational methods. The need for very large direct product basis sets, combined with an extremely high density of states, results in difficult convergence for iterative methods. A recent calculation of the highly excited OH stretch mode resonances using the filter diagonalization method, for example, required 462,000 basis functions, and 180,000 iterations. In contrast, using a combination of new methods, we are able to compute the same resonance states to higher accuracy with a basis less than half the size, using only a few hundred iterations-although the CPU cost per iteration is substantially greater. Similar performance enhancements are observed for calculations of the high-lying bound states, as reported in a previous paper [J. Theo. Comput. Chem. 2, 583 (2003)].

EXPERIMENTS ON THE DYNAMICS OF MOLECULAR PROCESSES: A Chronicle of Fifty Years

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▪ Abstract  This paper reviews the way in which, in the Italy of the years immediately after World War II, interest in the dynamics of molecular processes was awakened. The narrative begins with the work of a small number of chemists and physicists who, in the initial stage, interacted closely. In the course of the years, their interests diverged and younger people joined the newly formed groups. Even now, after half a century, a common approach can still to be seen regarding how to attack problems and perform experiments. Experimental work is discussed, bringing out the common viewpoint of fields as diverse as mass spectrometry, isotope effects, chemical kinetics, molecular beams, molecule-molecule interactions, molecule-ion interactions, molecule-surface interactions, and plasma chemistry.