Transition state in metal atom reactions

Dynamical and Mechanical Insights into the Li(2 S)+ HCl( X 1 Σ + ${X^1 {\rm{\Sigma }}^ + }$ ) Reaction: A Detailed Quantum Wavepacket Study

Quantum wave packet dynamics of the Li(² S)+HCl( X 1 Σ + ${X^1 \Sigma ^ + }$ ) reaction in its electronic ground state is studied. The initial state-selected and energy-resolved dynamical attributes such as reaction probability, integral cross section, and thermal rate constant for the Cl-abstraction and H-abstraction pathways are reported. All partial wave contributions of J up to 120 were found to be necessary for the title reaction up to the collision energy of ∼1.0 eV. The dynamical results reveal that the Cl-abstraction is more favored over the H-abstraction for the different rovibrational (v, j) excitations. Due to the existence of an early barrier in the potential energy surface, the cross sections increase with increasing collision energy. The rate constants also monotonously increase with temperature for both channels. Resonances are identified and characterized in terms of eigenfunctions and lifetimes. Nearly 120 well-resolved eigenstates are reported for the LiHCl complex, and they are categorized as van der Waals (vdW), barrier and product states according to the nodal progressions along (R, r, γ). The vdW resonances reveal a local-mode behavior of quasibound type at low energies and extended progressions at high energies. Further, the single-quantized periodic orbit type is also observed in the barrier region, which decays very fast. Finally, the lifetime analysis reveals that the vdW resonances can survive as long as ∼2.2 ps, which is much longer than the lifetime of the resonances in the barrier region.

Imaging the Complex-Forming Reaction Dynamics in Al + CO2 → AlO + CO

For indirect reactions involving more than one intermediate complex from reactant valley to product valley, the reaction dynamics becomes very complicated for researchers due to competition between pathways. In order to resolve the large discrepancy between theoretical and experimental studies on the linear or bent structures of complexes involved in the title endothermic reaction, we performed a crossed-beam experiment at a large collision energy (E_c) range with mapping of the velocity distributions of Al reactants and AlO products. We found that the reaction proceeds through different complex-forming mechanisms with the increase of E_c: at low E_c near the reaction threshold, only low impact-parameter collisions contribute through a collinear Al-OCO short-lived complex, and at high E_c, the bent-structure complexes, formed by either isomerization or noncollinear collisions, come into play.

The harpooning mechanism as evidenced in the oxidation reaction of the Al atom

The harpooning model is firstly evidenced by the maximum impact parameter derived from AlO(N_max) products with speed v′ ≈ 0 from the Al + O₂ reaction.

The harpooning mechanism has long been proposed for elementary reaction dynamics involving metals. It is characterized by an initial electron transfer (ET) process from the metal to the oxidant molecule. For the titled reaction Al + O₂, the ET distance can be predicted to be 2.6 Å by simply calculating the energy difference between the ionization energy of the Al atom and the electron affinity of the O₂ molecule. Hereby we experimentally derived the maximum impact parameter b_max of 2.5 ± 0.2 Å for the titled reaction, in consistency with the predicted ET distance. This derivation of b_max was achieved by using the crossed molecular beam experiment at a collision energy of 507 cm^–1 (i.e. 1.45 kcal mol^–1) with a high resolution time-sliced ion velocity imaging detection of the state-selective AlO products based on the (1 + 1) resonance-enhanced multiphoton ionization. The small rotational constant of the AlO(X²Σ⁺) radical (B_e = 0.6413 cm^–1) facilitated the formation of the AlO(v = 0) products in high rotational levels up to the energetically limited state, N_max = 52, with an almost zero velocity mapping. Hence, in this extreme angular momentum disposal case, the collisional orbital angular momentum l was nearly completely channeled into the product rotational angular momentum as a consequence of the conservations of energy and angular momentum, offering a reaction system that breaks the restriction of kinematically favored mass combination in order to obtain information on the impact parameters. The present study yields the first direct derivation of b_max from the maximum rotational level of products under the experimental condition with the recoil energy E′_T ≈ 0. This, in turn, provides solid evidence in supporting the harpooning mechanism.

A new ab initio potential energy surface of LiClH (1A') system and quantum dynamics calculation for Li + HCl (v = 0, j = 0-2) → LiCl + H reaction

A new ab initio potential energy surface (PES) for the ground state of Li + HCl reactive system has been constructed by three-dimensional cubic spline interpolation of 36 654 ab initio points computed at the MRCI+Q/aug-cc-pV5Z level of theory. The title reaction is found to be exothermic by 5.63 kcal/mol (9 kcal/mol with zero point energy corrections), which is very close to the experimental data. The barrier height, which is 2.99 kcal/mol (0.93 kcal/mol for the vibrationally adiabatic barrier height), and the depth of van der Waals minimum located near the entrance channel are also in excellent agreement with the experimental findings. This study also identified two more van der Waals minima. The integral cross sections, rate constants, and their dependence on initial rotational states are calculated using an exact quantum wave packet method on the new PES. They are also in excellent agreement with the experimental measurements.

Quantum and classical dynamics of H + CaCl(X (2)Σ(+)) → HCl + Ca((1)S) reaction and vibrational energy levels of the HCaCl complex

We carried out accurate quantum wave packet as well as quasi-classical trajectory (QCT) calculations for H + CaCl (νi = 0, ji = 0) reaction occurring on an adiabatic ground state using the recent ab initio potential energy surface to obtain the quantum and QCT reaction probabilities for several partial waves (J = 0, 10, and 20) as well as state resolved QCT integral and differential cross sections. The complete list of vibrational energy levels supported by the intermediate HCaCl complex is also obtained using the Lanczos algorithm. The QCT reaction probabilities show excellent agreement with the quantum ones except for the failure in reproducing the highly oscillatory resonance structure. Despite the fact that the reaction is exothermic and the existence of a barrier that is energetically lower than the bottom of the reactant valley, the reaction probability for J = 0 shows threshold-like behavior and the reactivity all through the energies is very low (<0.1). The dynamical features at two different energy regions (<0.35 eV and >0.35 eV) are found to be different drastically from each other. The analyses of these results suggest that the reaction is governed by one of the two different types of reaction mechanism, one is the direct mechanism at the high energy region and the other is the indirect mechanism at the low energy region by which the reaction proceeds through the long-lived intermediate complex followed by a statistical dissociation into asymptotic channels.

Transition-state spectroscopy of the photoinduced Ca + CH3F reaction. 3. Reaction following the local excitation to Ca(4s3d 1D)

The Ca* + CH3F --> CaF* + CH3 reaction was studied both experimentally and theoretically. The reaction was photoinduced in Ca...CH3F complexes, which were illuminated by a tunable laser in the range 18 000-24 000 cm-1. The absorption band that leads to the reaction extends between 19 000 and 23 000 cm-1. It is formed of three broad overlapping structures corresponding to the excitation of different electronic states of the complex. The two structures of lowest energy were considered in detail. They are associated with two series of respectively 2 and 3 molecular states correlating to Ca(4s3d 1D) + CH3F at infinite separation between Ca and CH3F. The assignment of these structures to specific electronic transitions of the complex stemmed from theoretical calculations where the Ca...CH3F complex is described by a linear Ca-F-C backbone. 2D potential energy surfaces were calculated by associating a pseudopotential description of the [Ca2+] and [F7+] cores, a core polarization operator on calcium, an extensive Gaussian basis, and a treatment of the electronic problem at the CI-MRCI level. All the excited levels correlating to the 4s2 1S, 4s3d 1D, and 4s4p 1P levels of Ca in the Ca + CH3F channel were documented in a calculation that explored the rearrangement channels where either Ca + CH3F or CaF + CH3 are formed. Then, wavepacket calculations on the 2D-PES's allowed one to simulate the absorption spectrum of the complex, in an approximation where the various electronic states of the complex are not coupled together. The assignment above stemmed from this. The second outcome of the calculation was that whatever the excited level of the complex that is considered, the reaction has to proceed through energy barriers. The electronic excitation of the complex on the red side of the absorption band does not seem to deposit enough energy in the system to overcome these barriers (even the lowest one) or to stimulate tunneling reactions. An alternative reaction mechanism involving a transfer to triplet PES's is proposed.

Chemiluminescent reaction of Ba(3P) with N2O at hyperthermal collision energies: rotational alignment of the BaO(A 1Sigma+) product

The chemiluminescent reaction Ba(6s6p (3)P)+N(2)O was studied at an average collision energy of 1.56 eV in a beam-gas arrangement. Ba((3)P) was produced by laser ablation of barium, which resulted in a broad collision energy distribution extending up to approximately 5.7 eV. A series of experiments was made to extract the Ba((3)P) contribution to chemiluminescence from that corresponding to Ba 6s(2) (1)S0 and 6s5d (3)D, which are the other two most populated states in the atomic beam. The fully dispersed polarized chemiluminescence spectra at 400-600 nm from the title reaction were recorded and assigned to a BaO molecule excited in the A (1)Sigma+ level. In addition, the average and wavelength-resolved degrees of polarization associated to the parallel BaO(A (1)Sigma+-->X (1)Sigma+) emission are reported. The analysis of the average polarization degree show that the BaO(A (1)Sigma+) product is significantly aligned, suggesting that the reaction mechanism is predominantly direct. The product rotational alignment was found to depend markedly on the emission wavelength, which revealed a negative correlation with the BaO(A (1)Sigma+) product vibrational state. On the basis of experimental and theoretical investigations on the reactions of N(2)O with both the (1)S0, (3)D, and (1)P1 states of Ba and the lighter group 2 atoms, it is suggested that the Ba((3)P) reaction involves a charge transfer at relatively short reagent separations and that restricted collision geometries at the highest velocity components of the broad distribution are necessary to rationalize the data.

Coupled diabatic potential energy surfaces for studying the nonadiabatic dynamics at conical intersections in angular resolved photodetachment simulations of OHF--->OHF+e-

An energy-based method is proposed for the diabatization of the OH(2Pi)+F(2P)-->O(3P)+HF(1Sigma+) reaction. It is demonstrated that the diabatic representation obtained is regularized, i.e., the residual derivative couplings do not present singularities at the conical intersections appearing along the reaction path. This method only requires the knowledge of the 1,2 3A" and 1 3A' eigenvalues and does not require any adjustable parameter. Thus, many convergence problems arising in other derivative-based diabatization methods are avoided, and the description of the configuration space along the reaction path is enormously simplified. Three-dimensional coupled diabatic energy surfaces are obtained by an interpolation procedure using approximately 4000 accurate ab initio points. The angular resolved photodetachment cross sections are obtained in the diabatic and adiabatic representations using a wave packet method. An excellent agreement is obtained with recent experimental data [D. M. Neumark, Phys. Chem. Chem. Phys. 7, 433 (2005)] for high electron kinetic energies where only the triplet electronic states contribute.

Coordinate transformation methods to calculate state-to-state reaction probabilities with wave packet treatments

A procedure for the transformation from reactant to product Jacobi coordinates is proposed, which is designed for the extraction of state-to-state reaction probabilities using a time-dependent method in a body-fixed frame. The method consists of several steps which involve a negligible extra computational time as compared with the propagation. Several intermediate coordinates are used, in which the efficiency depends on the masses of the atoms involved in the reaction. A detailed study of the relative efficiency of using reactant and product Jacobi coordinates is presented for several systems, and simple arguments are found depending on the masses of the atoms involved in the reaction. It is found that the proposed method is, in general, more efficient than the use of product Jacobi coordinates, specially for nonzero total angular momentum. State-to-state reaction probabilities are obtained for Li+FH-->LiF+H and F+HO-->FH+O collisions for several total angular momenta.

Collisional and photoinitiated reaction dynamics in the ground electronic state of Ca-HCl

Ca+HCl(upsilon,j) reactive collisions were studied for different rovibrational states of the HCl reactant using wave-packet calculations in reactant Jacobi coordinates. A recently proposed potential-energy surface was used with a barrier of approximately 0.4 eV followed by a deep well. The possibility of an insertion mechanism due to this last well has been analyzed and it was found that once the wave packet passes over the barrier most of it goes directly to CaCl+H products, which shows that the reaction dynamics is essentially direct. It was also found that there is no significant change in the reaction efficiency as a function of the initial HCl rovibrational state, because CaHCl at the barrier has an only little elongated HCl bond. Near the threshold for reaction with HCl(upsilon=0), however, the reaction shows significant steric effects for j > 0. In a complementary study, the infrared excitation from the Ca-HCl van der Waals well was simulated. The spectrum thus obtained shows several series of resonances which correspond to quasibound states correlating to excited HCl(upsilon) vibrations. The Ca-HCl binding energies of these quasibound states increase dramatically with upsilon, from 75 to 650 cm(-1), because the wave function spreads increasingly over larger HCl bond lengths. Thus it explores the region of the barrier saddle point and the deep insertion well. Although also the charge-transfer contribution increases with upsilon, the reaction probability for resonances of the upsilon=2 manifold, which are well above the reaction threshold, is still negligible. This explains the relatively long lifetimes of these upsilon=2 resonances. The reaction probability becomes significant at upsilon=3. Our simulations have shown that an experimental study of this type will allow a gradual spectroscopic probing of the barrier for the reaction.

Ab initiopotential-energy surface for the reaction Ca+HCl→CaCl+H

The potential-energy surface of the ground electronic state of CaHCl has been obtained from 6400 ab initio points calculated at the multireference configuration-interaction level and represented by a global analytical fit. The Ca+HCl-->CaCl+H reaction is endothermic by 5100 cm(-1) with a barrier of 4470 cm(-1) at bent geometry, taking the zero energy in the Ca+HCl asymptote. On both sides of this barrier are potential wells at linear geometries, a shallow one due to van der Waals interactions in the entrance channel, and a deep one attributed to the H(-)Ca(++)Cl(-) ionic configuration. The accuracy of the van der Waals well depth, approximately 200 cm(-1), was checked by means of additional calculations at the coupled-cluster singles and doubles with perturbative triples level and it was concluded that previous empirical estimates are unrealistic. Also, the electric dipole function was calculated, analytically fitted in the regions of the two wells, and used to analyze the charge shifts along the reaction path. In the insertion well, 16,800 cm(-1) deep, the electric dipole function confirmed the ionic structure of the HCaCl complex and served to estimate effective atomic charges. Finally, bound rovibrational levels were computed both in the van der Waals well and in the insertion well, and the infrared-absorption spectrum of the insertion complex was simulated in order to facilitate its detection.

Dissociative multiphoton ionization of NO[sub 2] studied by time-resolved imaging

We have studied dissociative multiphoton ionization of NO2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO+ with approximately 0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO+ decay. We attribute the signal to two competitive mechanisms. The first involving three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the 2B2 state of NO2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO+ ion signal exhibits biexponential decay, 0.72 exp(-t/90+/-10)+0.28 exp(-t/4000+/-400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO+ signal suggests a direct dissociation via the bending mode of the pumped state. .