Browsing by Author "Pérez, P"

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    Comparison between experimental and theoretical scales of electrophilicity based on reactivity indexes
    (2002) Pérez, P; Aizman, A; Contreras, R
    A comparative study between a relative experimental scale of electrophilicity and a theoretical absolute scale based on electronic reactivity indexes is presented. The theoretical scale correctly predicts the experimental electrophilicity within the dihalogen and inter-halogen subseries (XY) including F-2, Cl-2, Br-2, BrCl, and CIF and the HX (X = F, Cl, Br) series. It is shown that the best correlation is obtained for the local electrophilic index that encompasses the global electrophilicity power weighted by a local factor described by the electrophilic Fukui function. This result is in agreement with the electrostatic model of Legon (Angew. Chem., Int. Ed. Engl. 1999, 38, 2686), as the electrophilic power of molecules is mainly determined by the local properties of the electrophilic ends of HX and XY species. We also evaluated the electrophilicity of Li-2, LiH, LiF, and LiCl species for which experimental data are not available. Whereas LiH is predicted to have an electrophilic potential comparable to that shown by the dihalogen and inter-halogen series but higher than that of the FIX species, LiF and LiCl are predicted to display an electrophilic pattern even higher that those of the XY and HX molecules. On the other hand, Li-2 displays an electrophilic pattern even lower than that of F-2.
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    Density functional theory study for the cycloaddition of 1,3-butadienes with dimethyl acetylenedicarboxylate. Polar stepwise vs concerted mechanisms
    (2002) Domingo, LR; Arnó, M; Contreras, R; Pérez, P
    The molecular mechanisms for the cycloaddition reactions of four low activated 1,3-butadiene systems (1,3-butadiene, (E)-1,3-pentadiene, (Z)-1,3-pentadiene, and 4-methyl-1,3-pentadiene) with dimethyl acetylenedicarboxylate (DMAD) have been studied using density functional theory method. For these cycloadditions, two competitive mechanisms have been characterized: the First one corresponds to a concerted C-C bond-formation where the asynchronicity depends on the methyl substitution. The second one corresponds to a stepwise process with a larger polar character where first a C-C bond is formed along the nucleophilic attack of 1,3-butadiene system to a conjugate position of the electron-poor substituted acetylene. Although the nonactivated 1,3-butadiene prefers the concerted process, substitution of hydrogen atoms by electron-releasing methyl groups favors the stepwise mechanism along with an increase of the charge-transfer process. A conformational analysis for DMAD reveals that both planar and perpendicular arrangements of the two-carboxylate groups have a decisive role on the dienophile/electrophile nature of this acetylene derivative. Thus, although the planar arrangement is preferred along the concerted process, the perpendicular favors the polar one along an increase of the electrophilicity of DMAD. The global and local electrophilicity power of these 1,3-butadienes and DMAD have been evaluated in order to rationalize these results. The study is completed with an analysis of the electrophilic/nucleophilic site activation, by probing the variations in local properties of DMAD perturbed by a model nucleophile with reference to a model transition structure. Inclusion of solvent effects, by means of a polarizable continuum model, does not modify these gas-phase results.
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    Empirical energy-density relationships for the analysis of substituent effects in chemical reactivity
    (2000) Pérez, P; Simón-Manso, Y; Aizman, A; Fuentealba, P; Contreras, R
    Electronic substituent effects may be rationalized in terms of Hammett-like linear relationships between global energy-dependent quantities and local electronic descriptors of reactivity. These linear relationships are framed on a local hard and soft acids and bases (HSAB) principle in accord with previous results reported by Li and Evens [J. Am. Chem Sec. 1995, 117, 7756]. Chemical substitution is indirectly assessed as local responses at the active center of the substrate, with the Fukui function and local softness as the key quantities within the present approach. This model of chemical substitution has a potential advantage with respect to models based on group properties using the electronegativity equalization principle (EEP), since the transferability of group properties is not required. The formalism is illustrated for the gas-phase basicity of alkylamines, and the gas-phase acidity of alkyl alcohols and alkyl thioalcohols. Our results based on the local HSAB rule agree a ell with those obtained from group properties analysis based on the EEP, suggesting that bath empirical rules consistently complement each other.
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    Epidemiological investigations of the introduction of porcine reproductive and respiratory syndrome virus in Chile, 2013-2015
    (2017) Neira, V; Brito, B; Mena, J; Culhane, M; Apel, M I; Max, V; Pérez, P; Moreno, V; Mathieu, C; Medina Silva, Rafael Andrés; Johow, M; Badía, C; Torremorell, M; Ortega, R
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    On the condensed Fukui function
    (2000) Fuentealba, P; Pérez, P; Contreras, R
    A critical comparison among recently proposed methods for evaluating the condensed Fukui function neglecting relaxation effects is presented. The sign of the condensed Fukui function is discussed and arguments for a positive definite condensed Fukui function are given. Our numerical calculations in two series of molecules show that: (i) the condensed Fukui function can give, in general, valuable information about the site selectivity in chemical reactions and systematization in a family of molecules. In particular, it has been shown that the selectivity towards protonation in anilines and derivatives molecules can be correctly assessed by the electrophilic Fukui function described in this paper. Within this approach non-negative values for the condensed Fukui function are obtained for the relevant protonation sites in these polyfunctional systems; and (ii) the solvent effects on the condensed Fukui function are negligible, confirming a recently presented theoretical prediction. (C) 2000 American Institute of Physics. [S0021-9606(00)30331-2].
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    Quantitative characterization of the global electrophilicity power of common diene/dienophile pairs in Diels-Alder reactions
    (2002) Domingo, LR; Aurell, MJ; Pérez, P; Contreras, R
    The global electrophilicity power, omega, of a series of dienes and dienophiles commonly used in Diels-Alder reactions may be conveniently classified within a unique relative scale. Useful information about the polarity of transition state structures expected for a given reaction may be obtained from the difference in the global electrophilicity power, Deltaomega. of the diene/dienophile interacting pair. Thus the polarity of the process can be related with non-polar (Deltaomega small, pericyclic processes) and polar (Deltaomega big, ionic processes) mechanisms. (C) 2002 Elsevier Science Ltd. All rights reserved.
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    Spin-philicity and spin-donicity as auxiliary concepts to quantify spin-catalysis phenomena
    (2002) Pérez, P; Andrés, J; Safont, VS; Tapia, O; Contreras, R
    For molecular systems Susceptible to undergo a change of their spin state as a result of a chemical reaction with a given reactant, the spin-polarized density functional theory is used to define the concepts of "spin-philicity" (omega(S)(+)) and "spin-donicity" (omega(S)(-)) as global reactivity indexes. They are defined as the maximum energy change when a molecular system acquires or donates a spin number DeltaN(S) to increase (omega(S)(+)) or decrease (omega(S)(-)) its spin multiplicity. The spin transformation of chemically reactive species induced by the interaction of these molecules with external spin carriers-a phenomenon known as spin catalysis-is discussed on the basis of an absolute scale for omega(S)(+) and omega(S)(-). As an illustration of the method, a selection of paramagnetic and diamagnetic molecules, commonly used as spin catalyst. is classified within this scale and the hierarchy obtained is compared with the available experimental information.
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    Theoretical study of the effect of the substituents on the gas phase acidity of alcohols and silanols
    (2001) Pérez, P
    The opposite polarizability and inductive effects of the substituent groups on the gas phase acidity of alcohols and silanols are analyzed on the basis of a simple model that relates the proton affinity to the dipole polarizability. Inductive (electrostatic) effects are represented in terms of the variation in electronic chemical potential mediated by local softness at the basic center of the conjugated bases. Electronic (polarization) effects may be probed by the variations in local softness at the basic site. In alkyl alcohols, polarizability effects outweigh inductive effects, while in the corresponding silanol series, the opposite trend is observed. These results are in agreement with the experimental model of substituent effects proposed by Damrauer et al. (J. Am. Chem. Sec. 1991, 113, 4431). alkyl groups decrease silanol acidity, in contrast to alkyl group effects in alcohols.