The collision-induced intramolecular energy flow and C–H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C–H stretch and the nearby benzene ring C–H stretch and it is in interaction with Ar. The two excited C–H stretches are coupled to each other through two C–C stretching, two H–C–C bending and one C–C–C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm−1. Above the energy content of 40 000 cm−1, energy loss increases rapidly. Near 65 000 cm−1 energy loss takes a maximum value of about 1000 cm−1. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C–H bonds can dissociate, producing free radicals, C6H5CH2, C6H4CH3, or C6H4CH2. The ring C–H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C–H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl⋯Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.

A quasiclassical trajectory method is used to study the temperature dependence of HBr + OH → Br + H2O using analytic forms of two-, three-, and four-body and long-range interaction potentials. Below 300 K, the reaction is attraction-driven and occurs through formation of a collision complex BrH···OH, which is sufficiently long-lived to enhance H atom tunneling. Strong negative temperature dependence of the complex-mode rate is found between 20 and 300 K, consistent with experimental data reported by various authors. Above 300 K, the reaction occurs primarily through a direct-reaction mechanism. The sum of the complex- and direct-mode rates is shown to describe the reaction over the wide range 20-2000 K. The primary kinetic isotope effect is nearly constant with the normal H reaction faster by a factor of ∼1.7 over the entire temperature range. The product energy distribution in vibration, rotation, and translation at 300 K is found to be 48, 8, and 44%, respectively. The 1:1 resonance leads to efficient flow of energy between the stretching modes. Less than a quarter of the H2O vibrational energy deposits in the bending mode through intramolecular flow from the two stretching modes.

The collision-induced reaction of gas-phase atomic hydrogen with chlorine atoms chemisorbed on a silicon (001)-(2×1) surface is studied by use of the classical trajectory approach. The model is based on reaction zone atoms interacting with a finite number of primary system silicon atoms, which are coupled to the heat bath. The potential energy of the H⋯Cl interaction is the primary driver of the reaction, and in all reactive collisions, there is an efficient flow of energy from this interaction to the Cl–Si bond. All reactive events occur in a single impact collision on a subpicosecond scale, following the Eley–Rideal mechanism. These events occur in a localized region around the adatom site on the surface. The reaction probability is dependent upon the gas temperature and largest near 1000 K, but it is essentially independent of the surface temperature. Over the surface temperature range of 0–700 K and gas temperature range of 300 to 2500 K, the reaction probability lies below 0.1. The reaction energy available for the product state is small, and most of this energy is carried away by the desorbing HCl in its translational and vibrational motions. The Langevin equation is used to consider energy exchange between the reaction zone and the surface.

The reaction of gas-phase atomic chlorine with hydrogen atoms chemisorbed on a silicon surface is studied by use of the classical trajectory approach. In the model the gas atom interacts with the preadsorbed hydrogen atom and adjacent bare surface sites. The reaction zone atoms are configured to interact with a finite number of primary-system silicon atoms, which are coupled to the heat bath. The study shows that the chemisorption of Cl(g) is of major importance. Nearly all of the chemisorption events accompany the desorption of H(ad), i.e., a displacement reaction. Although it is much less important than the displacement reaction, the formation of HCl(g) is the second most significant reaction pathway. At a gas temperature of 1500 K and surface temperature 300 K, the probabilities of these two reactions are 0.829 and 0.082, respectively. The chemisorption of Cl(g) without dissociating H(ad) and collision-induced dissociation of H(ad) are found to be negligible. In the reaction pathway forming HCl, most of the reaction energy is carried by HCl(g). The ensemble-averaged vibrational, rotational, and translational energies are 37.4%, 35.6%, 18.3% of the liberated energy, respectively. Less than 9% of the energy dissipates into the solid phase. Although the majority of HCl produced in the gas phase belongs to a fast component of the time-of-flight distribution for a direct-mode reaction, there is a significant amount of HCl belonging to a slow component, which is characteristic of complex-mode collisions.