Quasi-Classical Trajectory Studies of the Insertion Reactions S(1D) + H2, HD, and D2

A new potential energy surface for the H2S system and dynamics study on the S((1)D) + H2(X(1)Σg(+)) reaction

We constructed a new global potential energy surface (PES) for the electronic ground state ((1)A') of H2S based on 21,300 accurate ab initio energy points over a large configuration space. The ab initio energies are obtained from multireference configuration interaction calculations with a Davidson correction using basis sets of quadruple zeta quality. The neural network method is applied to fit the PES, and the root mean square error of fitting is small (1.68 meV). Time-dependent wave packet studies for the S((1)D) + H2(X(1)Σg(+)) → H((2)S) + SH(X(2)Π) reaction on the new PES are conducted to study the reaction dynamics. The calculated integral cross sections decrease with increasing collision energy and remain fairly constant within the high collision energy range. Both forward and backward scatterings can be observed as expected for a barrierless reaction with a deep well on the PES. The calculated integral cross sections and differential cross sections are in good agreement with the experimental results.

INVESTIGATION OF THE EXCHANGE REACTION H + H′S → HS + H′ ON THE 1A′ STATE POTENTIAL ENERGY SURFACE

The quantum time dependent wave packet (TDWP) and quasiclassical trajectory (QCT) calculations were carried out to study the exchange reaction H(2S) + H′S(2Π) → HS(2Π) + H′(2S) on the 1A′ potential energy surface (PES). The integral cross sections of the H + H′S (v = j = 0) → HS + H′ reaction calculated by the two methods were presented. The results reveal that the integral cross sections (ICS) decrease with the collision energy increasing. The result of the QCT calculations is reasonably consistent with the time-dependent wave packet. Moreover, the differential cross sections (DCS) were calculated by the QCT method at the four different collision energies, which display a forward–backward symmetry. A long-lifetime H2S intermediate complex of the exchange reaction was found according to the trajectories. In the stereodynamics investigation, the polar and dihedral angle distribution functions were calculated, which have the distinct oscillations. The oscillations could be attributed to the deep well on the 1A′ PES. However, based on the polar-angle and dihedral angle distribution functions, it could be predicted that the main product rotational angular momentum preferentially point to the positive or negative direction of y-axes.

Rate coefficients from quantum and quasi-classical cumulative reaction probabilities for the S(1D) + H2 reaction

Cumulative reaction probabilities (CRPs) at various total angular momenta have been calculated for the barrierless reaction S((1)D) + H(2) → SH + H at total energies up to 1.2 eV using three different theoretical approaches: time-independent quantum mechanics (QM), quasiclassical trajectories (QCT), and statistical quasiclassical trajectories (SQCT). The calculations have been carried out on the widely used potential energy surface (PES) by Ho et al. [J. Chem. Phys. 116, 4124 (2002)] as well as on the recent PES developed by Song et al. [J. Phys. Chem. A 113, 9213 (2009)]. The results show that the differences between these two PES are relatively minor and mostly related to the different topologies of the well. In addition, the agreement between the three theoretical methodologies is good, even for the highest total angular momenta and energies. In particular, the good accordance between the CRPs obtained with dynamical methods (QM and QCT) and the statistical model (SQCT) indicates that the reaction can be considered statistical in the whole range of energies in contrast with the findings for other prototypical barrierless reactions. In addition, total CRPs and rate coefficients in the range of 20-1000 K have been calculated using the QCT and SQCT methods and have been found somewhat smaller than the experimental total removal rates of S((1)D).

A QUASICLASSICAL TRAJECTORY STUDY OF REACTIVE SCATTERING ON AN ANALYTICAL POTENTIAL ENERGY SURFACE FOR GeH2 SYSTEM

A quasiclassical trajectory method was employed to study reaction Ge+H 2 (v=0, j=0) and reverse reaction H+GeH (v=0, j=0) on an analytical potential energy surface obtained from simplified many-body expansion method with fitting to B3P86/CC-pVTZ calculations around a global minimum and a long-range van de Waals well plus spectroscopy data for diatomic molecules GeH and H2 . Reaction probabilities from both reaction and reverse reaction were calculated. Dominant reaction is complex-forming reaction Ge+H2 (v=0, j=0) → GeH2 , and its cross section is 10 times bigger than that of complex-forming reaction from the reverse reaction. There is no threshold effect for complex-forming reaction and the cross sections for both complex-forming reactions decrease with the increase of collision energy. Life time of complex is shown to be decreasing with increase of collision energy. Dominant reverse reaction is reaction H + GeH (v=0,j=0) → Ge+H2 ; the reaction probability decreases with the increase of collision energy and differential cross section shows that this reverse reaction has almost equal angular distribution at low collision energy and mostly forward scattering at high collision energy.

Significant Nonadiabatic Effects in the S(1D) + HD Reaction

A nonadiabatic quantum dynamics calculation involving four coupled potential energy surfaces (two degenerate 3A' ', one 3A', and one 1A') and the spin-orbit coupling matrix for these states is reported for the title reaction. The results show that the important discrepancy between theoretically calculated and experimentally measured intramolecular isotope effects can at least in part be attributed to significant nonadiabatic effects.

Dynamics of insertion reactions of H2 molecules with excited atoms

Recent progress in the study of insertion reactions of hydrogen molecules with excited atoms is reviewed in this article. In particular, the dynamics of the reaction of O(1D), N(2D), C(1D), and S(1D) with H2 and its isotopomers, which have received a great deal of attention over the past decade, are examined in detail. All of these systems have in common the existence of several potential energy surfaces (PES) correlating with the reagents' states, and consequently, they can give rise to reaction following different adiabatic and nonadiabatic pathways. The main contribution, however, arises from their ground singlet PESs which feature the existence of deep wells with small or null barriers for insertion. Accordingly, these reactions proceed mainly via formation of relatively long-lived collision complexes and display an overall nearly statistical behavior. In spite of their similarities, the various reactions have peculiar characteristics caused by important differences of their respective PESs. The contribution of excited PES to the global reactivity, which has also become an important issue and a challenge both for theory and experiment, is also examined. The different theoretical approaches are discussed in the text, along with the experimental results obtained by a variety of techniques. The recent exact quantum treatments of these reactive systems together with the development of a rigorous statistical model have contributed to a very accurate description which in many cases matches very well the detailed measurements. The quasi-classical trajectory (QCT) method has also provided a fairly accurate description of the reaction dynamics for these systems. In particular, the analysis in terms of collision times has yielded interesting clues about the reaction mechanisms.

Quantum statistical and wave packet studies of insertion reactions of S(D1) with H2, HD, and D2

A thorough theoretical investigation of the reactions between S(1D) and various hydrogen isotopomers (H2, D2, and HD) has been carried out using a recent ab initio potential energy surface. State-resolved integral and differential cross sections, thermal rate constants, and their dependence on energy or temperature were obtained from quantum mechanical capture probabilities within a statistical model. For comparison, the J=0 reaction probabilities were also computed using an exact wave packet method. The statistical results are in excellent agreement with available exact differential and integral cross sections. The comparison with experimental results shows that the agreement is reasonably good in general, but some significant differences exist, particularly for the SD/SH branching ratio in the S(1D)+HD reaction.

Determination of the internal state distribution of the SD product from the S(1D)+D2 reaction

The S(1D)+D2-->SD+D reaction has been studied through a photolysis-probe experiment in a cell. S(1D) reagent was prepared by 193 nm photolysis of CS2, and the SD(X 2Pi) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Pi) product was determined. This reaction, previously studied theoretically and in a crossed molecular beam experiment, is known to proceed through formation and decay of a long-lived collision complex involving the deep well in the H2S ground electronic state. The determined SD rotational state distribution in the v=0 vibrational level was found to be approximately statistical, with a small preference for formation of the F1 (Omega=3/2) fine-structure manifold over F2 (Omega=1/2). The branching into the Lambda doublet levels was also investigated, and essentially equal populations of levels of A' and A" symmetry were found. The present results are compared with previous investigations of this reaction and the analogous O(1D)+D2 reaction.