A. I. Maergoiz

Statistical rate theory for the HO + O H + O2 reaction system: SACM/CT calculations between 0 and 5000 K.

The potential energy surface of the HO+O⇔HO2⇔H+O2 reaction system is characterized by ab initio calculations. The complex-forming bimolecular reaction is then treated by statistical rate theory, using statistical adiabatic channel and classical trajectory calculations for the HO+O⇔HO2 and HO2⇔H+O2 association/dissociation processes. Specific rate constants k(E,J) of both reactions as well as thermal rate constants are calculated over wide ranges of conditions. Open shell quantum effects are important up to room temperature. The good agreement with experimental results suggests that the ab initio potential is of sufficient accuracy. There is no evidence for non-statistical effects or for a significant contribution from electronically excited states. The comparison with rate data for the H+O2→HO+O reaction, because of the remaining uncertainty in the heat of formation of HO, is somewhat inconclusive. Apart from this problem, the calculated rate constants appear reliable between 0 and 5000 K.

Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. IV. Valence interactions between atoms and linear rotors

The combination of two linear rotors forming linear or nonlinear adducts is treated using standardized valence potentials. Classical trajectory (CT) and statistical adiabatic channel (SACM) calculations are used for the calculation of thermal capture rate constants. At very low temperatures, only SACM applies. At intermediate temperatures SACM and CT approach each other; however, Landau–Zener-type multiple crossings of adiabatic channel potentials introduce local nonadiabaticity which has to be accounted for. The high-temperature transition from globally adiabatic to nonadiabatic (sudden) dynamics is studied by CT. Thermal rigidity factors, accounting for the influence of the anisotropy of the potential on the capture rate constant, are expressed in simple analytical form which facilitates practical applications. The present work complements similar studies on the addition of atoms to linear molecules in standardized valence potentials (part IV of this series).

Low-temperature behavior of capture rate constants for inverse power potentials

The energy dependence of the capture cross section and the temperature dependence of the capture rate constants for inverse power attractive potentials V∝−R−n is considered in the regime where the quantum character of the relative motion of colliding partners is important. For practically interesting cases n=4 and n=6, a simple formula for the cross section is suggested which interpolates between the classical and the quantum Bethe limits. We have shown that the classical approximation for the capture cross section performs well far below the simple estimations of the onset the quantum regime. This seemingly “classical” feature of the cross section and the rate constant is due to the large quantum effects of the waves in transmission through and reflection above the centrifugal potential barriers.

Adiabatic channel potential curves for two linear dipole rotors. I: Classification of states and numerical calculations for identical rotors

Adiabatic channel potential curves for a system of two linear dipole rotors are discussed. A general classification of states is given and a numerical procedure for calculating eigenvalues as a function of interrotor distance is formulated, both in a limited and extended basis set. A system of identical (but distinguishable) rotors is treated explicitly. Unexpectedly, the adiabatic potential curves show narrow avoided crossings which suggests the possibility of constructing diabatic channel potential curves. The validity of the adiabatic assumption for the relative motion of the dipoles is discussed.

Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. VI. Properties of transitional modes and specific rate constants k(E,J)

Transitional modes in simple unimolecular bond fission and in the reverse recombination reactions are characterized quantitatively by statistical adiabatic channel (SACM) and classical trajectory (CT) calculations. Energy E- and angular momentum J-specific numbers of open channels (or activated complex states) W(E,J) and capture probabilities w(E,J) are determined for a series of potentials such as ion—dipole, dipole–dipole, and various model valence potentials. SACM and CT treatments are shown to coincide under classical conditions. Adiabatic as well as nonadiabatic dynamics are considered. The dominant importance of angular momentum couplings is elaborated. A sequence of successive approximations, from phase space theory neglecting centrifugal barriers E0(J), via phase space theory accounting for centrifugal barriers E0(J), toward the final result, expressing the effects of the anisotropy of the potentials by specific rigidity factors frigid(E,J), is described. This approach emphasizes the importance to...

Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: Experiments and kinetic modeling

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

Statistical adiabatic channel calculation of accurate low‐temperature rate constants for the recombination of OH radicals in their ground rovibronic state

Accurate low‐energy capture cross sections and low‐temperature capture rate constants for two OH radicals in their ground rovibronic states X 2Π3/2(v=0, j=3/2) were calculated within the statistical adiabatic channel approach. The rate constants calculated in first order provide a good approximation to the true rate constant below 4 K. The rate constants calculated in second order provide a correction of about 25% to the first order rate constant at 20 K and indicate an only weak temperature dependence at T≳20 K. At higher temperatures deviation of the potential from long‐range electrostatic interaction have to be accounted for.

Electronic nonadiabatic effects in low temperature radical-radical reactions. I. C(3P) + OH(2Π)

The formation of collision complexes, as a first step towards reaction, in collisions between two open-electronic shell radicals is treated within an adiabatic channel approach. Adiabatic channel potentials are constructed on the basis of asymptotic electrostatic, induction, dispersion, and exchange interactions, accounting for spin-orbit coupling within the multitude of electronic states arising from the separated reactants. Suitable coupling schemes (such as rotational + electronic) are designed to secure maximum adiabaticity of the channels. The reaction between C((3)P) and OH((2)Π) is treated as a representative example. The results show that the low temperature association rate coefficients in general cannot be represented by results obtained with a single (generally the lowest) potential energy surface of the adduct, asymptotically reaching the lowest fine-structure states of the reactants, and a factor accounting for the thermal population of the latter states. Instead, the influence of non-Born-Oppenheimer couplings within the multitude of electronic states arising during the encounter markedly increases the capture rates. This effect extends up to temperatures of several hundred K.