Search results for "UNESCO::FÍSICA::Química física"
showing 10 items of 81 documents
Molecular spintronics: from magnetic materials to light emitting spintronic devices
2014
En el campo de la electrónica tradicional de semiconductores, la detección de cambios en la corriente eléctrica bajo la influencia de estímulos externos es el principio operacional de los dispositivos fabricados. La manipulación activa del grado de libertad de espín de los portadores de carga, sumado al control sobre la carga eléctrica, es el tema central que concierne a un campo de investigación relativamente joven denominado espintrónica. En el área de la nanotecnología, la espintrónica es una de los campos de investigación más emergentes, suponiendo una rama muy activa dentro del nanomagnetismo. El origen de esta área de conocimiento se puede situar en 1982 con el trabajo de Albert Fert …
Generación de centros estereogénicos cuaternarios heterosustiruidos mediante las reacciones de Henry y aza-Henry
2015
La adición de nitroalcanos a compuestos carbonílicos o sus derivados nitrogenados (reacciones de Henry y aza-Henry, respectivamente) constituye una herramienta fundamental para la formación de enlaces C-C en química orgánica.[1] Los productos resultantes de estas reacciones son "building blocks" muy interesantes en síntesis orgánica ya que permiten el acceso a diferentes motivos estructurales gracias a la versatilidad química del grupo nitro [2] siendo de gran utilidad para la síntesis de fármacos y productos agroquímicos.[3] Además, en el transcurso de estas reacciones pueden formarse uno o más centros estereogénicos por lo que el control de la estereoquímica resulta crucial. Por este moti…
Emisiones difusas: medidas experimentales y modelización aplicadas a instalaciones de tratamiento de aguas residuales
2015
Los olores emitidos por la actividad rutinaria de una planta de tratamiento de aguas residuales (EDAR) representan una importante fuente de molestia medioambiental que, por lo general, implica la liberación atmosférica de sustancias que causan mal olor y problemas de salud asociados. Actualmente, existe una preocupación pública sobre los olores que se generan con trabajos de tratamiento de residuos. Esta preocupación ha aumentado en la última década debido principalmente a la sobrecarga en las zonas de captación y la invasión de núcleos habitados en torno a instalaciones de tratamiento de aguas residuales (reduciendo los márgenes de seguridad impuestos a éstos). Estas potenciales emisiones …
Electronic structure of phthalocyanines : Theoretical investigation of the optical properties of phthalocyanine monomers, dimers, and crystals
1990
We present valence effective Hamiltonian (VEH) calculations on the optical absorptions of a series of phthalocyanine compounds: the metal‐free phthalocyanine molecule, a model system for the lithium phthalocyanine molecule, the metal‐free phthalocyanine dimer, and model systems for the lutetium diphthalocyanine and the lithium phthalocyanine crystal. For these compounds, it is found that the major factor influencing the evolution of the optical transitions is not the electronic structure of the metal but rather the geometric structure: phthalocyanine intraring geometry and, in the dimers and crystals, interring separation and staggering angle. The origin of the so‐called Soret or B absorpti…
Aminoacid zwitterions in solution : Geometric, energetic, and vibrational analysis using density functional theory-continuum model calculations
1998
Glycine and alanine aminoacids chemistry in solution is explored using a hybrid three parameters density functional (B3PW91) together with a continuum model. Geometries, energies, and vibrational spectra of glycine and alanine zwitterions are studied at the B3PW91/6-31+G∗∗ level and the results compared with those obtained at the HF and MP2/6-31+G∗∗ levels. Solvents effects are incorporated by means of an ellipsoidal cavity model with a multipolar expansion (up to sixth order) of the solute’s electrostatic potential. Our results confirm the validity of the B3PW91 functional for studying aminoacid chemistry in solution. Taking into account the more favorable scaling behavior of density funct…
High‐nuclearity mixed‐valence magnetic clusters : A general solution of the double exchange problem
1996
We report here a general solution of the double‐exchange problem in the high‐nuclearity mixed valence systems containing arbitrary number P of the electrons delocalized over the network of N (P<N) localized spins. The developed approach is based on the successive (chainlike) spin‐coupling scheme and takes full advantage from the quantum angular momentum theory. In the framework of this approach the closed‐form analytical expressions are deduced for the matrix elements of the double exchange interaction, two‐electron transfer, and three‐center interaction that can be referred to as the potential exchange transfer. For the arbitrary nuclearity mixed‐valence systems the matrix elements of all …
A quantum mechanics-molecular mechanics study of dissociative electron transfer : The methylchloride radical anion in aqueous solution
2002
The dissociative electron transfer reaction CH3Cl+e−→CH3•+Cl− in aqueous solution is studied by using a QM/MM method. In this work the quantum subsystem (a methylchloride molecule plus an electron) is described using density functional theory while the solvent (300 water molecules) is described using the TIP3P classical potential. By means of molecular dynamics simulations and the thermodynamic integration technique we obtained the potential of mean force (PMF) for the carbon–chlorine bond dissociation of the neutral and radical anion species. Combining these two free energy curves we found a quadratic dependence of the activation free energy on the reaction free energy in agreement with Ma…
A theory ofnonverticaltriplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π* triplet donors to 1,3,5…
2004
Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion paramet…
Full configuration interaction calculation of singlet excited states of Be3
2004
The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of …
Electronic aspects of the hydride transfer mechanism. Ab initio analytical gradient studies of the cyclopropenyl‐cation/lithium hydride model reactan…
1985
The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4‐31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride‐ion mechanism is a likely pathway for this particular sy…