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An automated method to find reaction mechanisms and solve the kinetics in organometallic catalysis.

ABSTRACT: A novel computational method is proposed in this work for use in discovering reaction mechanisms and solving the kinetics of transition metal-catalyzed reactions. The method does not rely on either chemical intuition or assumed a priori mechanisms, and it works in a fully automated fashion. Its core is a procedure, recently developed by one of the authors, that combines accelerated direct dynamics with an efficient geometry-based post-processing algorithm to find transition states (Martinez-Nunez, E., J. Comput. Chem.2015, 36, 222-234). In the present work, several auxiliary tools have been added to deal with the specific features of transition metal catalytic reactions. As a test case, we chose the cobalt-catalyzed hydroformylation of ethylene because of its well-established mechanism, and the fact that it has already been used in previous automated computational studies. Besides the generally accepted mechanism of Heck and Breslow, several side reactions, such as hydrogenation of the alkene, emerged from our calculations. Additionally, the calculated rate law for the hydroformylation reaction agrees reasonably well with those obtained in previous experimental and theoretical studies.

SUBMITTER: Varela JA

PROVIDER: S-EPMC5577717 | biostudies-literature | 2017 May

REPOSITORIES: biostudies-literature

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An automated method to find reaction mechanisms and solve the kinetics in organometallic catalysis.

Varela J A JA Vázquez S A SA Martínez-Núñez E E

Chemical science 20170307 5

A novel computational method is proposed in this work for use in discovering reaction mechanisms and solving the kinetics of transition metal-catalyzed reactions. The method does not rely on either chemical intuition or assumed <i>a priori</i> mechanisms, and it works in a fully automated fashion. Its core is a procedure, recently developed by one of the authors, that combines accelerated direct dynamics with an efficient geometry-based post-processing algorithm to find transition states (Martin ...[more]

PMID: 28966776

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Project description:Exploration of the chemical reaction space of chemical transformations in multicomponent mixtures is one of the main challenges in contemporary computational chemistry. To remove expert bias from mechanistic studies and to discover new chemistries, an automated graph-theoretical methodology is proposed, which puts forward a network formalism of homogeneous catalysis reactions and utilizes a network analysis tool for mechanistic studies. The method can be used for analyzing trajectories with single and multiple catalytic species and can provide unique conformers of catalysts including multinuclear catalyst clusters along with other catalytic mixture components. The presented three-step approach has the integrated ability to handle multicomponent catalytic systems of arbitrary complexity (mixtures of reactants, catalyst precursors, ligands, additives, and solvents). It is not limited to predefined chemical rules, does not require prealignment of reaction mixture components consistent with a reaction coordinate, and is not agnostic to the chemical nature of transformations. Conformer exploration, reactive event identification, and reaction network analysis are the main steps taken for identifying the pathways in catalytic systems given the starting precatalytic reaction mixture as the input. Such a methodology allows us to efficiently explore catalytic systems in realistic conditions for either previously observed or completely unknown reactive events in the context of a network representing different intermediates. Our workflow for the catalytic reaction space exploration exclusively focuses on the identification of thermodynamically feasible conversion channels, representative of the (secondary) catalyst deactivation or inhibition paths, which are usually most difficult to anticipate based solely on expert chemical knowledge. Thus, the expert bias is sought to be removed at all steps, and the chemical intuition is limited to the choice of the thermodynamic constraint imposed by the applicable experimental conditions in terms of threshold energy values for allowed transformations. The capabilities of the proposed methodology have been tested by exploring the reactivity of Mn complexes relevant for catalytic hydrogenation chemistry to verify previously postulated activation mechanisms and unravel unexpected reaction channels relevant to rare deactivation events.

Project description:Isocyanic acid, HNCO, mainly emitted by combustion processes, is doubted to be detrimental to human health if its concentration surpasses ∼1 ppbv. Very little information has been found regarding the HNCO loss in the gas phase. This study aims to close this knowledge gap by performing a theoretical kinetic study on the reaction of HNCO with the propargyl radical. The potential energy surface of the HNCO + C3H3 reaction was characterized utilizing high-level CCSD(T)/CBS(TQ5)//B3LYP/6-311++G(3df,2p) quantum-chemical approaches, followed by TST and RRKM/ME kinetic computations. The obtained results reveal that the reaction can proceed via H-abstraction, leading to the C3H4 + NCO bimolecular products with energy barriers of 23-25 kcal/mol, and/or addition, resulting in C4H4NO intermediates with 23-26 kcal/mol barrier heights. The C4H4NO adducts when formed can decompose to products and/or return to HNCO + C3H3 in which the reverse decompositions are found to be dominant with a branching ratio that accounts for nearly 100% at 300 K and 760 Torr. The calculated P-independent rate coefficients indicate that at low temperatures, the H-abstraction channels are insignificant. However, at high temperatures (T > 1500 K), the H-abstraction path leading to H3CCCH + NCO prevails with a branching ratio of ∼50-53% in the descending 1800-1500 K temperature range at 760 Torr, while the H-abstraction leading to H2CCCH2 + NCO is favorable at T > 1800 K, with the yield reaching above 50% at 760 Torr. In contrast to the H-abstraction rate constants, the calculated values for the additions and the C4H4NO decompositions show a positive pressure dependence. Both the total rate constants for the reactions HNCO + C3H3 → products and C4H4NO → products, which are, respectively, k _total_bimo(T) = 3.53 × 10-23 T 3.27 exp[(-21.35 ± 0.06 kcal/mol)/RT] cm3 molecule-1 s-1 and k _total_uni(T) = 1.13 × 1025 T -4.02 exp[(-11.77 ± 0.16 kcal/mol)/RT] s-1, increase with the increasing temperature in the 300-2000 K range at 760 Torr. The rate constant of HNCO + C3H3 → products is about 8 orders of magnitude smaller than the value of HCHO + C3H3 → products, showing that HCHO is more reactive toward the C3H3 free radicals than HNCO. The computed heats of formation for several species agree well with the available literature data with the deviation less than 1.0 kcal/mol, indicating that the methods used in this study are extremely reliable. With the given results, it is vigorously suggested that the predicted rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling HNCO-related systems under atmospheric and combustion conditions.

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An automated method to find reaction mechanisms and solve the kinetics in organometallic catalysis.

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An automated method to find reaction mechanisms and solve the kinetics in organometallic catalysis.

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