Reaction-Path Dynamics Calculations Using Integrated Methods. The CF3CH3 + OH Hydrogen Abstraction Reaction

Toward Efficient Direct Dynamics Studies of Chemical Reactions: A Novel Matrix Completion Algorithm

This paper describes the development and testing of a polynomial variety-based matrix completion (PVMC) algorithm. Our goal is to reduce computational effort associated with reaction rate coefficient calculations using variational transition state theory with multidimensional tunneling (VTST-MT). The algorithm recovers eigenvalues of quantum mechanical Hessians constituting the minimum energy path (MEP) of a reaction using only a small sample of the information, by leveraging underlying properties of these eigenvalues. In addition to the low-rank property that constitutes the basis for most matrix completion (MC) algorithms, this work introduces a polynomial constraint in the objective function. This enables us to sample matrix columns unlike most conventional MC methods that can only sample elements, which makes PVMC readily compatible with quantum chemistry calculations as sampling a single column requires one Hessian calculation. For various types of reactions─S_N2, hydrogen atom transfer, metal-ligand cooperative catalysis, and enzyme chemistry─we demonstrate that PVMC on average requires only six to seven Hessian calculations to accurately predict both quantum and variational effects.

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.

Assessment of theoretical methods for the study of hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation (IUPAC Technical Report)

Global climate change is a major concern as it leads to an increase in the average temperature of the earth’s atmosphere. The existence and persistence of some gaseous species in the atmosphere contribute to global warming. Experimental techniques are used to study the kinetics and degradation of global warming gases. However, quantum mechanical methods are also useful for the kinetic and radiative forcing study of global warming species and can precede experimental investigations. Research has also been targeted to develop more adapted procedures using ab initio and density functional theory (DFT) methods. This report provides a global perspective, in simplified manner, of the theoretical studies of the degradation of gas species in the atmosphere with an emphasis on the hydrogen abstraction kinetics of global warming gas species during their degradation and byproduct formation. En route, the results obtained from these studies are analysed and compared with experimental data where available. Our analyses indicate that the theoretical predictions are in agreement with experimental findings but the predicted parameters are dependent on the method being used. Theoretical methods are used to predict the thermodynamic parameters of reactions, and, with relevance to this report, the global warming potential (GWP) index can also be calculated. This report can be useful for future investigations involving global warming gaseous species while providing suggestions on how computations can fill in data gaps when experimental data are unavailable.

Rate coefficients for the reaction of OH with CF3CH2CH3 (HFC-263fb) between 200 and 400 K: ab initio, DFT, and transition state theory calculations

Rate coefficients for the reaction of the hydroxyl radical with CF(3)CH(2)CH(3) (HFC-263fb) were computed using ab initio methods, viz. MP2, G3MP2, and G3B3 theories between 200 and 400 K. Structures of the reactants in the ground state (GS) and transition state (TS) were optimized at MP2(FULL)/6-31G*, MP2(FULL)/6-311+ +G**, and B3LYP/6-31G* level of theories. Seven TSs were identified for the title reaction in the above theories. However, five out of seven TSs were found to be symmetrically distinct. The kinetic parameters due to these five different TSs are presented in this manuscript. Intrinsic reaction coordinate (IRC) calculations were performed to confirm the existence of transition states. The contributions of all the individual hydrogens in the substrate for the reaction are estimated and compared with the results obtained using Structure Additivity Relationships. The rate coefficients for the title reaction were computed to be k = (7.96 +/- 0.93) x 10(-13) exp [-(2245 +/- 30)/T] cm(3) molecule(-1) s(-1) at MP2, k = (9.50 +/- 0.93) x 10(-13) exp [-(1162 +/- 30)/T] cm(3) molecule(-1) s(-1) at G3MP2, and k = (7.01 +/- 0.88) x 10(-13) exp [-(753 +/- 35)/T] cm(3) molecule(-1) s(-1)at G3B3 theories. The theoretically computed rate coefficients are found to be in excellent agreement with the experimentally determined ones. The OH-driven atmospheric lifetimes of this compound are computed to be 132, 2.2, and 0.7 years at, MP2, G3MP2, and G3B3 level of theories, respectively.

Theoretical Investigation of the Hydrogen Abstraction Reaction of the OH Radical with CH3CHF2 (HFC152-a): A Dual Level Direct Density Functional Theory Dynamics Study

The hydrogen abstraction reaction of the OH radical with CH(3)CHF(2) (HFC152-a) has been studied theoretically over a wide temperature range, 200-3000 K. Two different reactive sites of the molecule, CH(3) and CHF(2) groups have been investigated precisely, and results confirm that CHF(2) position of the molecule is a highly reactive site. In this study, three recently developed hybrid density functional theories, namely, MPWB1K, MPW1B95, and MPW1K, are used. The MPWB1K/6-31+G(d,p) method gives the best result for kinetic calculations, including barrier heights, reaction path information and geometry of transition state structures and other stationary points. To refine the barrier height of each channel, a single point energy calculation was performed in MPWB1K/MG3S method. The obtained rate constants by dual level direct dynamics with the interpolated single point energy method (VTST-ISPE) using DFT quantum computational methods, are consistent with available experimental data. The canonical variational transition state theory (CVT) with the zero-curvature and also the small-curvature tunneling correction methods is used to calculate the rate constants. Over the temperature range 200-3000 K, the variation effect, tunneling contribution, branching ratio of each channel are calculated. The rate constants and their temperature dependency in the form of a fitted three-parameter Arrhenius expression are k(1)(T) = 2.00 x 10(-19)(T)(2.24) exp(-1273/T), k(2)(T) = 1.95 x 10(-19)(T)(2.46) exp(-2374/T), and k(T) = 3.13 x 10(-19)(T)(2.47) exp(- 1694/T) cm(3) molecule(-1) s(-1). For the H abstraction from the CHF(2) group, a nonclassical reflection effect is detected as a dominant quantum effect.

Glycolaldehyde + OH Gas Phase Reaction: A Quantum Chemistry + CVT/SCT Approach

We present a theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glycolaldehyde. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all stationary points, as well as for additional points along the minimum energy path (MEP). Energies are obtained by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p) to produce the potential energy surface. The rate coefficients are calculated for the temperature range 200-500 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel, for all the modeled paths. The overall agreement between the calculated and experimental kinetic data that are available at 298 K is very good. This agreement supports the reliability of the parameters obtained for the temperature dependence of the glycolaldehyde + OH reaction. The expressions that best describe the studied reaction are k(overall) = 7.76 x 10(-13) e(1328/)(RT) cm(3).molecule(-1).s(-1) and k(overall) = 1.09 x 10(-21)T(3.03) e(3187/)(RT) cm(3) molecule(-1) s(-1), for the Arrhenius and Kooij approaches, respectively. The predicted activation energy is (-1.36 +/- 0.03) kcal/mol, at about 298 K. The agreement between the calculated and experimental branching ratios is better than 10%. The intramolecular hydrogen bond in OO-s-cis glycolaldehyde is found to be responsible for the discrepancies between SAR and experimental rate coefficients.