Alfredo Guevara-García

Pointing the way to the products? Comparison of the stress tensor and the second-derivative tensor of the electron density

The eigenvectors of the electronic stress tensor can be used to identify where new bond paths form in a chemical reaction. In cases where the eigenvectors of the stress tensor are not available, the gradient-expansion-approximation suggests using the eigenvalues of the second derivative tensor of the electron density instead; this approximation can be made quantitatively accurate by scaling and shifting the second-derivative tensor, but it has a weaker physical basis and less predictive power for chemical reactivity than the stress tensor. These tools provide an extension of the quantum theory of atoms and molecules from the characterization of molecular electronic structure to the prediction of chemical reactivity.

Synthesis and Characterization of a Coupled Binuclear CuI/CuIII Complex

The synthesis through reaction of a C(alpha),C(ortho) dilithiated phosphazene with CuBr and structural characterization of the first example of a binuclear mixed valence [Cu(I)(N(2))/Cu(III)(C(4))] complex showing a metal-metal bond, as well as its applications in cyclopropanation and oxidation reactions, are described.

Are structures with Al–H bonds represented in the photoelectron spectrum of Al3O4H2−?

Photoelectron spectra of Al3O4H2- clusters formed by reactions of Al3O3- with water molecules have been interpreted recently in terms of dissociative absorption products with hydroxide and oxide anions that are coordinated to aluminum cations. Alternative isomers with Al-H bonds have lower energies, but barriers to hydrogen migrations that break O-H bonds and create Al-H bonds are high. Ab initio electron propagator calculations of the vertical electron detachment energies of the anions indicate that the species with hydrides cannot be assigned to the chief features in the photoelectron spectrum. Therefore, the previously studied dissociative absorption products are the structures that are most likely to be probed in the photoelectron spectra.

Electronic Stress as a Guiding Force for Chemical Bonding

In the electron-preceding picture of chemical change, the paramount problem is identifying favorable changes in electronic structure. The electronic stress tensor provides this information; its eigenvectors represent electronic normal modes, pointing the way towards energetically favorable (or unfavorable) chemical rearrangements. The resulting method is well founded in both density functional theory and the quantum theory of atoms in molecules (QTAIM). Stress tensor analysis is a natural way to extend the QTAIM to address chemical reactivity. The definition and basic properties of the electronic stress tensor are reviewed and the inherent ambiguity of the stress tensor is discussed. Extending previous work in which the stress tensor was used to analyze hydrogen-bonding patterns, this work focuses on chemical bonding patterns in organic reactions. Other related material (charge-shift bonding, links to the second-density-derivative tensor) is summarized and reviewed. The stress tensor provides a multifaceted characterization of bonding and can be used to predict and describe bond formation and migration.

Electron binding energies and Dyson orbitals of Al5Om- (m=3,4,5) and Al5O5H2-.

Photoelectron spectra of Al(5)O(m)(-) (m=3-5) and of the anion produced by the dissociative adsorption of a water molecule by Al(5)O(4)(-) are interpreted with density-functional geometry optimizations and electron-propagator calculations of vertical electron detachment energies. For Al(5)O(3)(-), Al(5)O(4)(-), and Al(5)O(5)H(2)(-), the observed signals may be attributed to the most stable isomer of each anion. For Al(5)O(5)(-), the features in the photoelectron spectrum are due to three almost isoenergetic isomers.

Sequential addition of H2O, CH3OH, and NH3 to Al3O3−: A theoretical study

Photoelectron spectra of two species, Al3O3(H2O)2- and Al3O3(CH3OH)2-, that are produced by the addition of two water or methanol molecules to Al3O3- are interpreted with density-functional geometry optimizations and electron propagator calculations of vertical electron detachment energies. In both cases, there is only one isomer that is responsible for the observed spectral features. A high barrier to the addition of a second molecule may impede the formation of Al3O3N2H6- clusters in an analogous experiment with NH3.

Photoelectron and computational studies of the copper-nucleoside anionic complexes, Cu - (cytidine) and Cu - (uridine)

The copper-nucleoside anions, Cu(-)(cytidine) and Cu(-)(uridine), have been generated in the gas phase and studied by both experimental (anion photoelectron spectroscopy) and theoretical (density functional calculations) methods. The photoelectron spectra of both systems are dominated by single, intense, and relatively narrow peaks. These peaks are centered at 2.63 and 2.71 eV for Cu(-)(cytidine) and Cu(-)(uridine), respectively. According to our calculations, Cu(-)(cytidine) and Cu(-)(uridine) species with these peak center [vertical detachment energy (VDE)] values correspond to structures in which copper atomic anions are bound to the sugar portions of their corresponding nucleosides largely through electrostatic interactions; the observed species are anion-molecule complexes. The combination of experiment and theory also reveal the presence of a slightly higher energy, anion-molecule complex isomer in the case of the Cu(-)(cytidine). Furthermore, our calculations found that chemically bond isomers of these species are much more stable than their anion-molecule complex counterparts, but since their calculated VDE values are larger than the photon energy used in these experiments, they were not observed.

Sensing polarization effects through the analysis of the effective C6 dispersion coefficients in NaCl solutions

Density functional theory based ab initio molecular dynamics is used to obtain microscopic details of the interactions in sodium chloride solutions. By following the changes in the atomic C6 coefficients under the Tkatchenko-Schefflers scheme, we were able to identify two different coordination situations for the Cl(-) ion with significant different capabilities to perform dispersion interactions. This capability is enhanced when the ion-ion distance corresponds to the contact ion-pair situation. Also, the oxygen and hydrogen atoms of the water molecules change their aptitudes to interact through van der Waals like terms when they are close to the cation region of the ion-pair. These results have interesting implications on the design of force fields to model electrolyte solutions.

Metal-Molecule Interactions To Produce Hydrogen: What Do They Have in Common?

The main purpose of this work is to study metal-molecule interactions that can lead to the production of molecular hydrogen. Two systems were chosen for this analysis:  yttrium atom and clusters interacting with the simple electron donor ammonia (NH3) and copper atoms and ions with imidazole. For yttrium with ammonia as well as for copper with imidazole there is a charge-transfer process from the metal to the molecule that promotes the dissociation of the hydrogen atoms.