Browsing by Author "Santos, José G."

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    Linear relationship between emission quantum yield and Stokes shift in 3-styryl aza-coumarin based dyes in the presence of cyclodextrins
    (2023) Alcázar, Jackson; García-Río, Luis; Robles, Agustín I.; Dinamarca-Villarroel, Luis; Fierro, Angélica; Santos, José G.; Aliaga, Margarita E.
    The effect of the cyclodextrins inclusion on the Stokes shifts and emission quantum yield of three 3-styryl aza-coumarin dyes (SACs) was experimentally and theoretically studied. Preliminary results show a relationship between the emission quantum yield and the calculated binding constants. Supramolecular inclusion was supported by changes in the fluorescence spectra, high-resolution mass spectrometry and molecular dynamics studies. 2,6-di-O-methyl-b-cyclodextrin (DM-b-CD) presented higher binding constants than b-cyclodextrin (b-CD), along with up to a 6-fold increase in emission quantum yield for the SACs. Additionally, a linear negative correlation was obtained between the Stokes shift and the emission quantum yield. This linear and empirical relationship was explained by the action of a unique intramolecular rotation and charge transfer phenomenon in the dyes, which was modulated by cyclodextrins, and supported by calculations based on density functional theory.
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    Reactions of Aryl Acetates with Secondary Alicyclic Amines in Ethanol/Water Mixtures: Effect of the Solvent Composition on the Kinetics and Mechanism
    (2011) Castro, Enrique A.; Millan, Daniela; Aguayo, Raúl; Campodónico, Paola; Santos, José G.
    We report a kinetic study on the reactions of secondary alicyclic amines toward 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl acetates (1, 2, and 3) in ethanol/water mixtures of different compositions. It is found that (i) the intermediate in the reaction of 1 is stabilized in a mixture of 90 vol% ethanol; (ii) for the reaction of 2, the mechanism is stepwise in water but concerted in the mixtures; (iii) For the reaction of 3, the mechanism is concerted along the whole range of composition; (iv) the effect of -NO(2) outweighs the solvent effect; (v) preferential solvation in the core of reaction can be ruled out. (C) 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 687-693, 2011
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    Reactivity of benzohydrazide derivatives towards acetylation reaction. Experimental and theoretical studies
    (2010) Campodónico, Paola; Aliaga, Margarita E.; Santos, José G.; Castro, Enrique A.; Contreras, Renato
    We herein report an experimental and theoretical study on the acetylation reaction of benzohydrazide derivatives towards p-nitrophenyl acetate (NPA). The kinetic data are consistent with a stepwise mechanism with the nucleophilic attack as the rate determining step. From the theoretical analysis it is found that benzohydrazide derivatives establish intramolecular proton rearrangement. The enol form appears as the active species for nucleophilic attack. A reaction mechanism incorporating keto-enol pre-equilibria is proposed. The study is completed with a local reactivity analysis describing the most reactive centers for nucleophilic attack together with a site activation analysis describing inductive substituent effect
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    The reactions of O-(4-nitrophenyl) S-aryl dithiocarbonates with anilines: Effects on the relative nucleofugality
    (2019) Santos, José G.; Gazitúa, Marcela
    Kinetic and high-performance liquid chromatography studies were investigated for the reactions of S-phenyl, S-(4-chlorophenyl), and S-(4-nitrophenyl) O-(4-nitrophenyl) dithiocarbonates with anilines. These were performed in the presence of 0.1M borate buffer in 44 wt% aqueous ethanol. For reactions of the 3 substrates, the mechanism is stepwise with 2 tetrahedral intermediates, one zwitterionic (T±), and the other anionic (T−), where the intermediate T− is formed by proton transfer from T± to the borate buffer. The nonleaving group is not passive, playing an important role in the relative nucleofugality of the groups, which depend largely on its electron withdrawing capability. The nucleofugacity of 4-nitrophenolate ion and of 4-nitrophenylthiolate from the same tetrahedral intermediate is similar, despite the differences in their basicities (3 pKa units). In the reactions of S-(4-nitrophenyl) O-(4-nitrophenyl) dithiocarbonate, the change of nucleophile from pyridines (only 4-nitrophenolate ion is nucleofuge) to anilines (2 nucleofuges) shows that the nature of the amine impacts on the relative nucleofugality of groups.

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