Encontrado(s) 2077391 resultado(s)
Encontrada(s) 207740 página(s)

Theoretical characterization of the lowest-energy absorption band of pyrrole

  • Roos, Björn O.
  • Malmqvist, Per-Ake
  • Molina, Vincent
  • Serrano Andrés, Luis
  • Merchán Bonete, Manuela
The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new assignments are suggested. As illustrated in the paper, the proper treatment of the valence–Rydberg mixing is particularly challenging for ab initio methodologies and can be seen as the main source of deviation among the recent theoretical results as regards the position of the low-lying valence excited states of pyrrole., serrano@uv.es ; merchan@uv.es

Polarizability and optical rotation calculated from the approximate coupled cluster singles and doubles CC2 linear response theory using Cholesky decompositions

  • Pedersen, Bondo Thomas
  • Sánchez de Merás, Alfredo
  • Koch, Henrik
A new implementation of the approximate coupled cluster singles and doubles CC2 linear response model using Cholesky decomposition of the two-electron integrals is presented. Significantly reducing storage demands and computational effort without sacrificing accuracy compared to the conventional model, the algorithm is well suited for large-scale applications. Extensive basis set convergence studies are presented for the static and frequency-dependent electric dipole polarizability of benzene and C60, and for the optical rotation of CNOFH2 and (−)-trans-cyclooctene (TCO). The origin-dependence of the optical rotation is calculated and shown to persist for CC2 even at basis set convergence., Alfredo.Sanchez@uv.es

Una nueva mirada al informe de la Ley Agraria de Jovellanos doscientos años después

  • Llombart Rosa, Vicente
Con motivo del bicentenario (1795-1995) de la publicación del Informe de Ley Agraria de Jovellanos se presenta una visión retrospectiva de esa obra ya clásica del pensamiento español incluyendo un repaso de sus múltiples y desacordes intérpretes modernos y un análisis del interior del texto e incluso del título. Se concluye insistiendo en la plena inserción del Informe en la tradición ilustrada española, en que los usuales calificativos de mercantilista, liberal smithiano o fisiócrata son inadecuados y en que es preciso restituir la obra y su autor a su tiempo., On the occasion of the bicentenary (1795-1995) of the publication oMepart antbe AgraHan Law by Jovellanos, we present a retrospective v.ew of th.s class.c work of Spanish thought, including a review of its widely varying modem ""«P'^*»^'"j^"" aíalysis of the content of the text and its tide. The conclusión ...s.stsAat ^ e ^ ^ j a s m line with the Spanish Enlightenment tradition, that the usual descr.pt.ons "f «e'-""'^^*;' Smithian liberal or physiocrat a,« inappropiate, and that .t .s necessan' to cons.der the work in the period when it was written., vllombar@uv.es

Los negocios de una gran empresa sedera en la Valencia del siglo XVIII: la Compañía de Nuestra Señora de los Desamparados

  • Franch Benavent, Ricardo
La Compañía de Nuestra Señora de los Desamparados fue creada por el artesanado sedero en 1772, pero la Junta General de Comercio cambió su naturaleza al autorizar la participación en su accionariado de todas las clases sociales. Aunque esta circunstancia favoreció su desarrollo, sentó también las bases de su fracaso final al provocar un grave conflicto entre el sector artesanal, que ejercía en exclusiva los cargos directivos, y el resto del accionariado, siendo el motivo fundamental de la discordia las pérdidas sufridas durante la crisis comercial de 1779-82. De todas formas, la conservación de su documentación privada ha permitido analizar su actividad manufacturera y los resultados de sus negocios, entre los que destacó la comercialización de los tejidos de seda en Cádiz., The «Compañía de Nuestra Señora de los Desamparados» was made by the silk crahsmen in 1772, but the General Board of Commerce changed its nature to authoríze. the social classes to take part in share-holders. Although that circumstance favoured its growth, it laid the foundations of its final failure to provoke a serious conflict between the craíitsmen sector, who exercised solé right the managines charges, and the rest ot share-holders, which fundamental discord motive was the loss sufFeted in the com- "lercial crisis of 1779-82. Anyway, the preservation of its prívate documentation has made it possible to analyse iu manufacturíng activity and the result of its business; the commercialization of the silk textile in Cádiz stands ouL, rfranch@uv.es

A theory of nonvertical triplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π∗ triplet donors to 1,3,5,7-cyclooctatetraene

  • Frutos, Luis Manuel
  • Castaño, Obis
  • Andrés, José Luis
  • Merchán Bonete, Manuela
  • Acuña Fernández, Alberto Ulises
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 parameter γ = (2g2/κ1)1/2 in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and κ1 is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of π,π∗ triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C�C bond stretchings and C–C bond torsions, which collectively change the ground-state COT bent conformation (D2d) toward a planar triplet state (D8h)., manuela.merchan@uv.es

General occurrence of polar twisted minima in the ionic singlet excited surfaces of polyenes

  • Nebot Gil, Ignacio José
  • Malrieu, Jean Paul
Except for a few cases, the lowest closed shell SCF solutions for the twisted geometries of polyenes involves delocalized nonpolarized MO’s, and represents a meaningless mixture of radicalar gound state and ionic excited states. Higher (quasi) solutions allow a realistic approach of the polarized ionic states. Orthogonality to the radicalar ground state may be imposed without important loss of energy. Further extensive CI does not modify significantly the situation. These statements are illustrated through ab initio calculations of butadiene and hexatriene. This approach allows us to show that slight distortions stablize polar minima even in ’’homosymmetric’’ molecules such as all‐trans hexatriene twisted around the central bond. The existence of polar twisted minima in the ionic singlet excited surfaces of polyenes seems therefore a general phenomenon., Ignacio.Nebot@uv.es

Electronic aspects of the hydride transfer mechanism. Ab initio analytical gradient studies of the cyclopropenyl‐cation/lithium hydride model reactant system

  • Tapia, O.
  • Andres, J.
  • Aulló Reverte, Josep Maria
  • Bränden, C.I.
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 system. The system is thereafter thoroughly studied from the supermolecule approach. Reaction paths for the ground and first triplet electronic states have been calculated. The hypersurface is explored from a geometrical disposition of the reactants that mimics the one found in several dehydrogenases (perpendicular configuration). A hydride ion is found to be the particle transferred on the unconstrained as well as the constrained reaction pathways in the ground electronic state. In the triplet state (perpendicular configuration) the mechanism is stepwise: electron transfer followed by a hydrogen atom transfer. It has been noticed that the perpendicular geometrical disposition of the reactants plays an important role by polarizing the susceptible cyclopropene C–H bond in the sense of increasing the electronic density at the hydrogen nucleus. This provides a clue to rationalize several dehydrogenase’s active site structure and mechanism. The reactant molecular complex found in the inverted potential energy curves, namely the LiH‐‐‐Cp+ association has an electronic distribution which can be described as a hydride ion cementing two electron deficient centers corresponding to the cyclopropenyl and the lithium cations. Direct CI calculations confirm the overall picture obtained above., Josep.M.Aullo@uv.es

A CI study of the CuCO and CuCO+ complexes

  • Merchán Bonete, Manuela
  • Nebot Gil, Ignacio José
  • González Luque, Remedios
  • Ortí Guillén, Enrique
MO CI calculations are carried out using an optimal space of valence virtual MOs obtained by means of a projection technique, as a linear combination of the AOs which are more occupied in the molecular Fock space. Localization of the occupied MOs and nonvalence virtual MOs is also achieved. The overall procedure is proven to be quite advantageous and well suited to obtain potential energy curves which keep the same physical meaning along the range of distances studied. Using a slightly better than double‐zeta quality basis set, a valence CAS‐CI, and selected CI wave function by the CIPSI algorithm have revealed a possible weak van der Waals interaction for the 2Σ+ state of CuCO, which remains when polarization functions are added to the basis set for the carbon and oxygen atoms. Even though the CuCO 2Π and CuCO+ 1Σ+ states are energetically close, the nature of the interactions is quite different, π bonding and mainly electrostatic, respectively. The results give further support to the view of the neutral metal–CO interaction as a balance of σ repulsion and π backbonding. However, it is proposed that the driving force for the positive ion metal–CO interaction becomes essentially electrostatic., Ignacio.Nebot@uv.es ; Manuela.Merchan@uv.es ; Enrique.Orti@uv.es

Electronic structure of metal‐free phthalocyanine : A valence effective Hamiltonian theoretical study

  • Ortí Guillén, Enrique
  • Brédas, J. L.
We present a valence effective Hamiltonian (VEH) nonempirical investigation of the electronic properties of metal‐free phthalocyanine. The valence one‐electron energy levels are related to those of the phthalocyanine components: benzene, pyrrole, and isoindole. From the electronic structure standpoint, phthalocyanine has to be viewed as formed by joining four benzene moieties to the central carbon–nitrogen ring rather than by combining four isoindole units through nitrogen bridges. Comparison of the VEH density‐of‐valence‐states curves with the experimental ultraviolet photoelectron spectroscopy (UPS) data is quantitatively excellent and allows for a complete interpretation of the experimental spectra., Enrique.Orti@uv.es

Bueno, hasta luego: el uso de bueno en conversaciones

  • Gregori-Signes, Carmen
En las páginas que siguen presentamos un posible análisis de bueno en su función de marcador del discurso. Este análisis es parte (y a la vez resultado) de un trabajo contrastivo en torno a well/bueno y de otros marcadores del discurso en inglés y en castellano, dado su frecuente uso en la conversación cotidiana. La razón por la que se comparan estos dos marcadores es porque se pensó que podían coincidir en su función en ambos idiomas, y se procedió al análisis para comprobar esta hipótesis. El corpus utilizado no es muy amplio: incluye quince conversaciones relativamente breves1 en las que se han clasificado cincuenta ejemplos del uso de bueno como marcador del discurso; si bien el corpus es limitado, creemos que es suficiente para una primera aproximación al tema. Un análisis posterior, con una ampliación sustancial también del corpus objeto de estudio, permitirá clasificar con más acierto el uso y la función de los marcadores discursivos de forma sistemática y científica. El corpus en que nos hemos basado son conversaciones cotidianas, y que podrían ser clasificadas como “phatic communion” en el sentido que Malinowski (1923) le dió inicialmente: a type of speech in which ties of union are created by a mere exchange of words.... They fulfil a social function and that is their principal aim, but they are neither the result of intellectual reflection, nor do they necessarily arouse reflection in the listener. Once again we may say that language does not function here as means of transmission of thought. Las conversaciones fueron grabadas sin conocimiento ni previo aviso a los participantes, para no restar naturalidad, aunque posteriormente se les solicitó permiso para la utilización de esos datos. Queremos subrayar que este tipo de conversaciones, aparentemente banales, son sumamente interesantes para nuestro propósito y creemos que los marcadores del discurso desempeñan aquí una función fundamental y decisiva. En este tipo de conversaciones, más que los temas tratados, lo importante consistiría en evitar posibles conflictos en las relaciones entre los participantes, ya sea a fin de evitar un clima “tenso” o ese “silencio” que tanto temen la mayoría de los hablantes occidentales (no es ése el caso con los finlandeses)., This paper presents a possible approach to the analysis and use of the Spanish discourse marker bueno in everyday conversation. The analysis is part (and at the same time a result) of a project in which the functions of bueno are compared to those of well in English conversations. In this project English and Spanish discourse markers are being compared in order to find similarities and differences between their function and use in conversation. In this paper we do not try to reach definitive conclusions since the corpus needs to be extended and it is necessary to elaborate a framework in order to classify every occurrence of the different markers in a systematic way., gregoric@uv.es

Buscador avanzado