Browsing by Author "Schaefer, Henry F., III"

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    Characterizing the Mechanism of the Double Proton Transfer in the Formamide Dimer
    (2011) Hargis, Jacqueline C.; Vohringer-Martinez, Esteban; Lee Woodcock, H.; Toro Labbé, Alejandro; Schaefer, Henry F., III
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    Contrasting the Mechanism of H2 Activation by Monomeric and Potassium-Stabilized Dimeric AlI Complexes: Do Potassium Atoms Exert any Cooperative Effect?
    (2021) Villegas-Escobar, Nery; Toro-Labbe, Alejandro; Schaefer, Henry F., III
    Aluminyl anions are low-valent, anionic, and carbenoid aluminum species commonly found stabilized with potassium cations from the reaction of Al-halogen precursors and alkali compounds. These systems are very reactive toward the activation of sigma-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cationMIDLINE HORIZONTAL ELLIPSIS pi interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing KMIDLINE HORIZONTAL ELLIPSISH bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H-2 molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H-2 utilizing a NON-xanthene-Al dimer, [K{Al(NON)}](2) (D) and monomeric, [Al(NON)](-) (M) complexes are studied using density functional theory and high-level coupled-cluster theory to reveal the potential role of K+ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H-2. The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol(-1)). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H-2 distorts the most, usually over 0.77 Delta Edist not equal . Overall, it is found here that electrostatic and induction energies between the complexes and H-2 are the main stabilizing components up to the respective transition states. The results suggest that the K+ atoms act as stabilizers of the dimeric structure, and their cooperative role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H-2. Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H-2 is wholly disengaged.
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    Decomposition of the electronic activity in competing [5,6] and [6,6] cycloaddition reactions between C60 and cyclopentadiene
    (2019) Villegas-Escobar, Nery; Poater, Albert; Sola, Miquel; Schaefer, Henry F., III; Toro-Labbe, Alejandro
    Fullerenes, in particular C-60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. However, chemical transformations have to be done in order to transform C-60 in added-value compounds with increased applicability. The most common procedure corresponds to the classical Diels-Alder cycloaddition reaction. In this research, a comprehensive study of the electronic activity that takes place in the cycloaddition between C-60 and cyclopentadiene toward the [5,6] and [6,6] reaction pathways is presented. These are competitive reaction mechanisms dominated by sigma and fluctuating activity. To better understand the electronic activity at each stage of the mechanism, the reaction force (RF) and the symmetry-adapted reaction electronic flux (SA-REF, J(i)()) have been used to elucidate whether or sigma bonding changes drive the reaction. Since the studied cycloaddition reaction proceeds through a C-s symmetry reaction path, two SA-REF emerge: J(A)() and J(A)(). In particular, J(A)() mainly accounts for bond transformations associated with bonds, while J(A)() is sensitive toward sigma bonding changes. It was found that the [6,6] path is highly favored over the [5,6] with respect to activation energies. This difference is primarily due to the less intensive electronic reordering of the sigma electrons in the [6,6] path, as a result of the pyramidalization of carbon atoms in C-60 (sp(2) sp(3) transition). Interestingly, no substantial differences in the electronic activity from the reactant complex to the transition state structure were found when comparing the [5,6] and [6,6] paths. Partition of the kinetic energy into its symmetry contributions indicates that when a bond is being weakened/broken (formed/strengthened) non-spontaneous (spontaneous) changes in the electronic activity occur, thus prompting an increase (decrease) of the kinetic energy. Therefore, contraction (expansion) of the electronic density in the vicinity of the bonding change is expected to take place.