Computational Kinetics by Variational Transition-State Theory with Semiclassical Multidimensional Tunneling: Direct Dynamics Rate Constants for the Abstraction of H from CH₃OH by Triplet Oxygen Atoms

Abstract

Rate constants and the product branching ratio for hydrogen abstraction from CHOH by O(P) were computed with multistructural variational transition-state theory including microcanonically optimized multidimensional tunneling. Benchmark calculations of the forward and reverse classical barrier heights and the reaction energetics have been carried out by using coupled cluster theory and multireference calculations to select the most reliable density functional method for direct dynamics computations of the rate constants. The dynamics calculations included the anharmonicity of the zero-point energies and partition functions, with specific-reaction-parameter scaling factors for reactants and transition states, and multistructural torsional anharmonicity was included for the torsion around the C-O bond in methanol and in the transition states. The resulting rate constants are presented over a wider range than they are available from experiment, but in the temperature range where experiments are available, they agree well with experimental values, which is encouraging for their reliability over the wider temperature range and for future computations of oxygen atom reaction rates. In contrast to a previous computational prediction, the branching ratio predicted by the present work shows that the formation of CHOH + OH is the dominant channel over the whole range of temperature from 250 to 2000 K.

Effects of multidimensional tunneling in the kinetics of hydrogen abstraction reactions of O (3P) with CH3OCHO

“…Examination of the normal‐mode generalized frequencies along the MEP demonstrated that the slight bump of of R1 is caused by a large decrease in two bending modes of the OCOH group around s = 0.02 a.u. The frequency decreasing in reactive modes sometimes occur because hydrogen abstraction reaction is related to weakening of force constant along the reaction path …”

“…This large difference between the TST/Eckart rate constants and those ones computed by Dooley et al could be related to the uncertainties of the thermal kinetics measurements, or the lack of large tunneling corrections in the kinetics model employed since a hydrogen atom is transferred between two heavy atoms, and also due to some known deficiencies on the reactive potential surface built with the B3LYP method …”

“…Since the calculations were based only the lowest surface, it was assumed the A′ and A″ surfaces make almost equal contributions to the rate constants, and the electronic partition coefficients of the triplet transition state was multiplied by 2 using an ad hoc criterion. In another recent study for the O ( 3 P) + CH 3 OH hydrogen abstraction reactions performed at the C 1 point group geometry, MS‐CASPT2 calculations were employed to compute vertical excitation energies of reactions R1 and R2 with two states electronic states with the same A symmetry. The computed vertical excitations energies were employed to estimate the electronic partition functions of the triplet transition states.…”

Computational Chemistry: The Fate of Current Methods and Future Challenges

“…As tunneling reactivity currently emerges as the third paradigm next to kinetic and thermodynamic control of chemical reactions, it can no longer be ignored. Pertinent recent examples include the conformational preferences of carboxylic acids including amino acids, the reactivity of carbenes, photoreactions, and radical abstractions, to name only a few.…”

Computerchemie: das Schicksal aktueller Methoden und zukünftige Herausforderungen

“…Da sich die “Tunnel”‐Reaktivität derzeit als das dritte Paradigma neben der kinetischen und thermodynamischen Kontrolle chemischer Reaktionen entwickelt, kann sie nicht länger ignoriert werden. Als aktuelle Beispiele seien hier die Konformationspräferenzen von Carbonsäuren einschließlich Aminosäuren, die Reaktivität von Carbenen, Photoreaktionen und Radikalabstraktionen genannt.…”