CBS-QB3 + VTST Study of Methyl N-Methylcarbamate + OH Gas-Phase Reaction: Mechanism, Kinetics, and Branching Ratios

A matrix completion algorithm for efficient calculation of quantum and variational effects in chemical reactions

This work examines the viability of matrix completion methods as cost-effective alternatives to full nuclear Hessians for calculating quantum and variational effects in chemical reactions. The harmonic variety-based matrix completion (HVMC) algorithm, developed in a previous study [S. J. Quiton et al., J. Chem. Phys. 153, 054122 (2020)], exploits the low-rank character of the polynomial expansion of potential energy to recover vibrational frequencies (square roots of eigenvalues of nuclear Hessians) constituting the reaction path using a small sample of its entities. These frequencies are essential for calculating rate coefficients using variational transition state theory with multidimensional tunneling (VTST-MT). HVMC performance is examined for four S_N2 reactions and five hydrogen transfer reactions, with each H-transfer reaction consisting of at least one vibrational mode strongly coupled to the reaction coordinate. HVMC is robust and captures zero-point energies, vibrational free energies, zero-curvature tunneling, and adiabatic ground state and free energy barriers as well as their positions on the reaction coordinate. For medium to large reactions involving H-transfer, with the sole exception of the most complex Ir catalysis system, less than 35% of total eigenvalue information is necessary for accurate recovery of key VTST-MT observables.

Theoretical Investigations on the Mechanism and Kinetics of OH Radical Initiated Reactions of Monochloroacetic Acid

The oxidation mechanism of monochloroacetic acid (CH₂ClCOOH) by OH radical has been systematically investigated employing quantum mechanical methods coupled with kinetic calculation using canonical variational transition state theory. Three distinct transition states were identified for the titled reaction, two corresponding to the hydrogen atom abstraction and one corresponding to the chlorine atom abstraction. The rate constants of the titled reactions are computed over the temperature range 278-350 K, and the branching ratios calculated for the hydrogen atom abstraction from the -C(O)OH site and the -CH₂Cl site are 25 and 75%, respectively, at 298 K. The computed branching ratio indicates that the kinetically favorable reaction is the hydrogen atom abstraction from the -CH₂Cl site resulting in the formation of CHClC(O)OH radical, which further undergoes secondary reaction with O₂ and other atmospheric species. The calculated overall rate constant for the hydrogen atom abstraction reactions is in consistent with the reported experimental rate constant. The atmospheric lifetime of CH₂ClCOOH is found to be around 18 days.

An experimental and theoretical study of the kinetics of the reaction between 3-hydroxy-3-methyl-2-butanone and OH radicals

Absolute experimental and theoretical rate constants are determined for the first time for the reaction of 3-hydroxy-3-methyl-2-butanone with OH as a function of temperature. The atmospheric implications are discussed.

Computational study on the kinetics and mechanism of the carbaryl + OH reaction

Carbaryl is released into the atmosphere as a spray drift immediately following the application. In order to evaluate its fate in the atmosphere, a computational study on the kinetics of the OH radical reaction with carbaryl is presented. Different reaction paths are studied at the M05-2X/6-311++G(d,p) level. A complex mechanism involving the formation of a stable reactant complex is proposed and the temperature dependence of the rate coefficients is studied in the 280-650 K temperature range. The principal degradation path is the hydroxyl radical addition to naphthalene, but hydrogen abstractions from the methyl group are identified as a secondary significant path. The rate coefficients, computed using the conventional transition state theory, reproduce quite well the scarce experimental data available.