Dynamics of HO2+O3 reaction using a test DMBE potential energy surface: does it occur via oxygen or hydrogen atom abstraction?

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Difficulties and Virtues in Assessing the Potential Energy Surfaces of Carbon Clusters via DMBE Theory: Stationary Points of Cκ (κ = 2-10) at the Focal Point

The previously reported potential energy surfaces (PESs) of ground-state singlet C₃ and triplet C₄ are here utilized as input for the construction of approximate cluster expansions for larger C_κ (κ = 5-10) species. Relying primarily on the double many-body expansion (DMBE) approach, global potentials are obtained by summing the total interaction energies of all atomic subclusters up to four-body terms. The notable capability of the final forms in predicting good estimates of the linear global minima and their thermochemical/structural properties may provide important insights into the structure-determining nature of the (2 + 3 + 4) terms. The main difficulties and virtues in assessing C_κ's via DMBE theory are analyzed and new prospects given for the construction of global reliable PESs for the target species.

Quantum modeling of the reaction between ozone and hydrogen cyanide

This work consists of an investigation, using current methods of quantum chemistry and, at first, on the basis of the available experimental results, about the new mechanisms of the reaction between ozone and hydrogen cyanide (HCN) in gaseous phases. Three possible reaction pathways which we have determined as the most probable and, all three, leading exactly to the same products, are proposed here. For each of these pathways, several steps for which we performed a kinetic study were identified in the singlet potential energy surface. To confirm the proposed mechanisms, we have achieved a study including the intrinsic reaction coordinate (IRC), the topological analysis of atoms in molecule and the harmonic vibrational frequencies calculations. The obtained results reveal that the final products have considerable thermodynamic stability and this reaction is exothermic in standard conditions.

Computational study of the kinetics and mechanisms for the HCO + O3 reaction

The mechanisms of radical-molecule reactions between HCO (formyl radical) and O3 (ozone) have been investigated by using BH&HLYP and QCISD methods with the 6-311++G(3df,2p) basis set. The energetics have been refined with CCSD(T) and QCISD(T) theoretical approaches with the same basis set based on the geometries calculated at the QCISD method. The intermediates of hydrogen-bonded complexes and the critical transition states are also examined with the multireference methods. Two possible reaction pathways containing hydrogen-abstraction and association-elimination processes for the interaction of HCO with O3 are proposed. Both reaction mechanisms can occur via the prereactive hydrogen-bonded complex, O3-HCO, with 2.45 kcal/mol stability at the CCSD(T) approach with respect to the reactants; even so, the hydrogen-abstraction mechanism exhibits a lower energy barrier. The rate constants for both processes are also predicted. The total rate constant at 298 K is calculated to be in close agreement with the experimental value of 8.3 × 10(-13) cm(3) molecule(-1) s(-1).

Ab initio-based double many-body expansion potential energy surface for the first excited triplet state of the ammonia molecule

A global single-sheeted double many-body expansion potential energy surface is reported for the first excited triplet state of NH(3). It employs an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments. Four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. A new switching function formalism has been reported to approximate the true multisheeted nature of NH(3)((3)A(2) ('')) potential energy surface, thus allowing the correct behavior at the NH(2)((2)A(")) + H((2)S) and NH(2)((4)A(")) + H((2)S) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under the permutation of identical atoms, and predicts the correct behavior at all dissociation channels while providing a realistic representation at all interatomic separations. The major attributes of the NH(3) double many-body expansion potential energy surface have also been characterized, and found to be in good agreement, both with the calculated ones from the raw ab initio energies and the theoretical results available in the literature.

The Gas-Phase Hydrogen-Bonded Complex between Ozone and Hydroperoxyl Radical. A Theoretical Study

We report a theoretical study on the gas-phase hydrogen-bonded complexes formed between ozone and hydroperoxyl radical, which are of interest in atmospheric chemistry. We have employed CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches employing 6-311+G(2df,2p) and aug-cc-pVTZ basis sets, and we have found three complexes whose stabilities are computed to be 2.02, 1.19, and 1.34 kcal/mol, respectively, at 0 K. In addition, we have also found three transition states connecting these complexes that lie below the energy of the separate reactants. To help for possible experimental identification of these hydrogen-bonded complexes, we report also the computed harmonic vibrational frequencies along with the frequency shifts of the complexes, relative to the monomers, and the computed rotational constants.