Dynamics of the F2 reaction with propene: the effect of methyl substitution.

The reaction of F2 + C3H6 has been investigated with the crossed molecular beam technique. The only observed primary product channel is F + C3H6F while the HF + C3H5F channel cannot be found. The reaction cross section was measured as a function of collision energy and the reaction threshold was determined to be 2.4 +/- 0.3 kcal/mol. Compared to the reaction threshold of the F2 + C2H4 reaction, the methyl substitution effectively reduces the reaction threshold by about 3 kcal/mol. The product time-of-flight spectra and angular distributions were measured and analyzed. The angular distribution displays strongly backward, indicating that the reaction is much faster than rotation. All experimental results support a rebound reaction mechanism, which agrees with the structure of the calculated transition state. The transition state geometry also suggests an early barrier; such dynamics is consistent with the observed small kinetic energy release of the products. Except for the different values of the reaction thresholds, the dynamics of the F2 + C2H4 and F2 + C3H6 reactions are remarkably similar.

Barrierless reactions between two closed-shell molecules. II. Dynamics of F2 + CH3SSCH3 reaction.

A second example of a barrierless reaction between two closed-shell molecules is reported. The reaction F(2)+CH(3)SSCH(3) has been investigated with crossed molecular beam experiments and ab initio calculations. Compared with previous results of the F(2)+CH(3)SCH(3) reaction [J. Chem. Phys. 127, 101101 (2007); J. Chem. Phys. 128, 104317 (2008)], a new product channel leading to CH(3)SF+CH(3)SF is observed to be predominant in the title reaction, whereas the anticipated HF+C(2)H(5)S(2)F channel is not found. In addition, the F+C(2)H(6)S(2)F product channel, the analog to the F+C(2)H(6)SF channel in the F(2)+CH(3)SCH(3) reaction, opens up at collision energies higher than 4.3 kcal/mol. Angular and translational energy distributions of the products are reported and collision energy dependences of the reaction cross section and product branching ratio are shown. The reaction barrier is found to be negligible (<<1 kcal/mol). Multireference ab initio calculations suggest a reaction mechanism involving a short-lived intermediate which can be formed without activation energy.

Dynamics of the F2 reaction with the simplest pi-bonding molecule.

The reaction of F(2)+C(2)H(4) has been investigated with crossed molecular beam experiments and high level ab initio calculations. For a wide range of collision energies up to 11 kcal/mol, only one reaction channel could be observed in the gas phase. The primary products of this channel were identified as F+CH(2)CH(2)F. The experimental reaction threshold of collision energy was determined to be 5.5+/-0.5 kcal/mol. The product angular distribution was found to be strongly backward, indicating that the reaction time scale is substantially shorter than rotation. The calculated transition state structure suggests an early barrier; such dynamics is consistent with the small product kinetic energy release measured in the experiment. All experimental results consistently support a rebound reaction mechanism, which is suggested by the calculation of the intrinsic reaction coordinate. This work provides a clear and unambiguous description of the reaction dynamics, which may help to answer the question why the same reaction produces totally different products in the condensed phase.

Barrierless reactions between two closed-shell molecules. I. Dynamics of F(2)+CH(3)SCH(3) reaction.

A detailed experimental and theoretical investigation of the first-reported barrierless reaction between two closed-shell molecules [J. Chem. Phys. 127, 101101 (2007)] is presented. The translational energy and angular distributions of two product channels, HF+CH(2)SFCH(3) and F+CH(3)SFCH(3), determined at several collision energies, have been analyzed to reveal the dynamics of the studied reaction. Detailed analysis of the experimental and computational results supports the proposed reaction mechanism involving a short-lived F-F-S(CH(3))(2) intermediate, which can be formed without any activation energy. Other possible reaction mechanisms have been discriminated. The decay of the intermediate and competition between the two product channels have been discussed.

Spin-orbit effect in the energy pooling reaction O2(a 1Delta)+O2(a 1Delta)-->O2(b 1Sigma)+O2(X 3Sigma).

Five-dimensional nonadiabatic quantum dynamics studies have been carried out on two new potential energy surfaces of S(2)((1)A(')) and T(7)((3)A(")) states for the title oxygen molecules collision with coplanar configurations, along with the spin-orbit coupling between them. The ab initio calculations are based on complete active state second-order perturbation theory with the 6-31+G(d) basis set. The calculated spin-orbit induced transition probability as a function of collision energy is found to be very small for this energy pooling reaction. The rate constant obtained from a uniform J-shifting approach is compared with the existing theoretical and experimental data, and the spin-orbit effect is also discussed in this electronic energy-transfer process.

Nonadiabatic reactant-product decoupling calculation for the F(2P(1/2)) + H2 reaction.

In this paper we present a theoretical study using time-dependent nonadiabatic reactant-product decoupling method for the state-to-state reactive scattering calculation of F((2)P(1/2))+H(2) (nu=j=0) reaction on the Alexander-Stark-Werner potential energy surface. In this nonadiabatic state-to-state calculation, the full wave function is partitioned into reactant component and a sum of all product components. The reactant and product components of the wave function are solved independently. For the excited state reaction, the state-to-state reaction probabilities for J=0.5 are calculated. Comparing the state-to-state reaction probabilities, it is found that the vibrational population of the HF product is dominated by vibrational levels nu=2 and 3. The rotation specific reaction probabilities of HF product in j=1 and 2 are larger than those in other rotational levels. As the rotation quantum number j increases, the positions of the peak in the rotational reaction probability of HF product in nu=3 shift to higher collision energy.

Nonadiabatic energy transfer studies of O((1)D)+N(2)(X (1)Sigma(g) (+))-->O((3)P)+N(2)(X (1)Sigma(g) (+)) by time-dependent wave packet.

Three-dimensional time-dependent quantum calculations have been performed on two/three coupled potential surfaces, including the singlet surface 1 (1)A(') and two triplet surfaces 1 (3)A(') and 1 (3)A("), for the electronic quenching process of O((1)D)+N(2)(X (1)Sigma(g) (+))-->O((3)P)+N(2)(X (1)Sigma(g) (+)). An extended split-operator scheme was employed to study this nonadiabatic process. Two types of singlet surface 1 (1)A('), namely, double many body expansion (DMBE2) were used in the calculations, along with spin-orbit couplings of Nakamura-Kato and with a constant value of 80 cm(-1). All the calculated probabilities are resonance dominated, with a general decreasing trend within the investigated collision energy range. The probability involving three potential energy surfaces is approximately two times as high as that on two potential energy surfaces. At low collision energies, the calculations on the ZPM2 surface produced much larger probability than that on the DMBE2 surface, but the difference was diminishing as the collision energy became high. The behavior of the probability on DMBE2/ZPM2 surfaces at low energies indicates that the ZPM2 surface dominates over the DMBE2 surface in the description of the process. However, the DMBE2 surface has been modified by removing the unreasonable barrier. The estimated quenching cross sections both on the ZPM2 surface and on the modified DMBE2 surface in the three-coupled-surface calculations agree with the experimental measurement. Also, a rather insensitive characteristic of the probability relative to the analytical function form of spin-orbit coupling is revealed.

The investigation of spin-orbit effect for the F((2)P)+HD reaction.

In this paper, we employ the time-dependent quantum wave packet method to study the reaction of F((2)P(3/2), (2)P(1/2)) with HD on the Alexander-Stark-Werner potential energy surface. The reaction probabilities and total integral cross sections of the spin-orbit ground and excited states for the two possible products of the system are calculated. Because the reaction channel of the excited spin-orbit state is closed at the resonance energy, the resonance feature does not appear in the reaction probabilities and cross section for the F((2)P(1/2))+HD(v=j=0)-->HF+D reaction, in contrast with that found for the ground spin--orbit state. We also compare the average cross sections of the two possible products with the experimental measurement. The resonance peak in the present average cross section for the HF+D product is slightly larger than the experimental result, but much smaller than that of the single-state calculations on the potential energy surface of Stark and Werner. It seems that the spin--orbit coupling would play a relatively important role in this reaction. Moreover, the isotope effects of the ground and excited spin--orbit states and the reactivity of the two product channels from the excited spin--orbit state are presented.