Browsing by Author "Aizman, A"

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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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    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.