Browsing by Author "Toro Labbe, Alejandro"

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    Mechanisms of Formation of Hemiacetals: Intrinsic Reactivity Analysis
    (AMER CHEMICAL SOC, 2012) Azofra, Luis Miguel; Alkorta, Ibon; Elguero, Jose; Toro Labbe, Alejandro
    The reaction mechanism of the hemiacetal formation from formaldehyde and methanol has been studied theoretically at the B3LYP/6-311+ +G(d,p) level. In addition to the study of the reaction between the isolated reactants, three different kinds of catalysis have been explored. The first one examines the use of assistants, especially bridging water molecules, in the proton transfer process. The second one attempts to increase the local electrophilicity of the carbon atom in formaldehyde with the presence of a Bronsted acid (H+ or H3O+). The last one considers the combined effect of both catalytic strategies. The reaction force, the electronic chemical potential, and the reaction electronic flux have been characterized for the reaction path in each case. In general, it has been found that structural rearrangements represent an important energetic penalty during the activation process. The barriers for the reactions catalyzed by Bronsted acids show a high percentage of electronic reorganization contribution. The catalytic effects for the reactions assisted by water molecules are due to a reduction of the strain The reaction that includes both acid catalysis and proton assistance transfer shows the lowest in the transition state structures. energy barrier (25.0 kJ mol(-1)).
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    Understanding chemical binding using the Berlin function and the reaction force
    (ELSEVIER, 2012) Chakraborty, Debajit; Cardenas, Carlos; Echegaray, Eleonora; Toro Labbe, Alejandro; Ayers, Paul W.
    We use the derivative of the electron density with respect to the reaction coordinate, interpreted through the Berlin binding function, to identify portions of the reaction path where chemical bonds are breaking and forming. The results agree with the conventional description for S(N)2 reactions, but they are much more general and can be used to elucidate other types of reactions also. Our analysis offers support for, and detailed information about, the use of the reaction force profile to separate the reaction coordinates into intervals, each with characteristic extents of geometry change and electronic rearrangement. (C) 2012 Elsevier B.V. All rights reserved.
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    Understanding the Physics and Chemistry of Reaction Mechanisms from Atomic Contributions: A Reaction Force Perspective
    (AMER CHEMICAL SOC, 2012) Voehringer Martinez, Esteban; Toro Labbe, Alejandro
    Studying chemical reactions involves the knowledge of the reaction mechanism. Despite activation barriers describing the kinetics or reaction energies reflecting thermodynamic aspects, identifying the underlying physics and chemistry along the reaction path contributes essentially to the overall understanding of reaction mechanisms, especially for catalysis. In the past years the reaction force has evolved as a valuable tool to discern between structural changes and electrons' rearrangement in chemical reactions. It provides a framework to analyze chemical reactions and additionally a rational partition of activation and reaction energies. Here, we propose to separate these energies further in atomic contributions, which will shed new insights in the underlying reaction mechanism. As first case studies we analyze two intramolecular proton transfer reactions. Despite the atom based separation of activation barriers and reaction energies, we also assign the participation of each atom in structural changes or electrons' rearrangement along the intrinsic reaction coordinate. These participations allow us to identify the role of each atom in the two reactions and therfore the underlying chemistry. The knowledge of the reaction chemistry immediately leads us to suggest replacements with other atom types that would facilitate certain processes in the reaction. The characterization of the contribution of each atom to the reaction energetics, additionally, identifies the reactive center of a molecular system that unites the main atoms contributing to the potential energy change along the reaction path.