Alexis Maldonado

Venezuelan Institute for Scientific Research, Caracas, Distrito Federal, Venezuela

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Publications (10)22.89 Total impact

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    ABSTRACT: The kinetics and mechanisms of thermal decomposition of phenyl acetate and p-tolyl acetate in the gas-phase were studied by means of electronic structure calculations using Density Functional Theory (DFT) methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), B3PW91/6-31G(d,p), B3PW91/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p)], PBE/6-31G(d,p), PBE/6-31++G(d,p). Two possible mechanisms have been considered: Mechanism A, is a stepwise process involving electrocyclic [1, 5] hydrogen shift to eliminate ketene through concerted six-membered cyclic transition state structure, followed by tautomerization of cyclohexadienone or by 4-methyl cyclohexadienone intermediate to give the corresponding phenol. Mechanism B is a one-step concerted [1,3] hydrogen shift through a four-membered cyclic transition state geometry, to produce ketene and phenol or p-cresol. Theoretical calculations showed reasonable agreement with experimental activation parameters when using the PBE functional, through the stepwise [1, 5] hydrogen shift mechanism. For mechanism B, large deviation for the entropy of activation was observed. No experimental data was available for p-tolyl acetate; however, theoretical calculations showed similar results to phenyl acetate, thus supporting the stepwise mechanism for both phenyl acetate and p-tolyl acetate. Keywords: phenyl acetate, p-tolyl acetate, gas-phase elimination, mechanism, hydrogen shift, DFT calculations.
    Molecular Physics 09/2013; · 1.67 Impact Factor
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    ABSTRACT: The kinetics of the gas-phase thermal decomposition of 2-methyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, and cyclopentanone ethylene ketal were determined in a static system and the reaction vessel deactivated with allyl bromide. The decomposition reactions, in the presence of the free radical suppressor propene, are homogeneous, are unimolecular, and follow first-order law kinetics. The products of these reactions are acetaldehyde and the corresponding ketone. The working temperature range was 459-490 °C, and the pressure range was 46-113 Torr. The rate coefficients are given by the following Arrhenius equations: for 2-methyl-1,3-dioxolane, log k = (13.61 ± 0.12) - (242.1 ± 1.0)(2.303RT)(-1), r = 0.9997; for 2,2-dimethyl-1,3-dioxolane, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998; for cyclopentanone ethylene ketal, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998. Electronic structure calculations using DFT methods B3LYP and MPW1PW91 with 6-31G(d,p), and 6-31++G(d,p) basis sets suggest that the decomposition of these substrates takes place through a stepwise mechanism. The rate-determining step proceeds through a concerted nonsynchronous four-centered cyclic transition state, and the elongation of the C-OCH(3) bond in the direction C(α)(δ+)...OCH(3)(δ-) is predominant. The intermediate products of these decompositions are unstable, at the working temperatures, decomposing rapidly through a concerted cyclic six-centered cyclic transition state type of mechanism.
    The Journal of Physical Chemistry A 08/2012; 116(37):9228-37. · 2.77 Impact Factor
  • The Journal of Physical Chemistry A 07/2012; · 2.77 Impact Factor
  • The Journal of Physical Chemistry A 07/2012; · 2.77 Impact Factor
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    ABSTRACT: The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.
    The Journal of Physical Chemistry A 12/2011; 116(2):846-54. · 2.77 Impact Factor
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    ABSTRACT: The kinetics and mechanisms of the dehydrochlorination of 2-chloro-1- phenylethane, 3-chloro-1-phenylpropane, 4-chloro-1-phenylbutane, 5-chloro-1-phenylpentane, and their corresponding chloroalkanes were examined by means of electronic structure calculation using density functional theory methods B3LYP/6–31G(d,p), B3LYP/6–31++G(d,p), MPW1PW91/6–31G(d,p), MPW1PW91/6–31++G(d,p), PBEPBE/6–31G(d,p), and PBEPBE/6–31++G(d,p). The potential energy surface was investigated for the minimum energy path. Calculated enthalpies and energies of activation are in good agreement with experimental values using the MPW1PW91 and B3LYP methods. The transition state of these reactions is a four-centered cyclic structure. The reported experimental results proposing neighboring group participation by the phenyl group was not supported by theoretical calculations. The rate-determining process in these reactions is the breaking of ClC bond. The reactions are described as concerted moderately polar and nonsynchronous. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 292–302, 2011
    International Journal of Chemical Kinetics 06/2011; 43(6). · 1.19 Impact Factor
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    ABSTRACT: The design and synthesis of novel sterol hydrazone analogues (9, 10, 11 and 12) are described, followed by their evaluation as inhibitors of fungal growth, using Paracoccidioides brasiliensis as the biological tester. Compounds 9, 10, 11 and 12 generated a dose-dependent effect in fungal growth, particularly 9, 11 and 12, which were active at nanomolar concentrations (100 nM). When P. brasiliensis in its pathogenic yeast-like phase was treated individually with each of the aforementioned compounds at concentrations that reduced growth rate around 50%, the analysis of sterol composition in the resulting surviving cells demonstrated a 50% reduction of the final sterols brasicasterol and ergosterol, and concomitant increase in the levels of lanosterol. These results indicate that these compounds inhibit the enzyme Δ(24)-sterol methyl transferase (SMT), in a manner dependent on the stereochemical location of the hydrazone group. Compound 12, instead, induced a good antiproliferative activity not associated with blockage of any step in the pathway to sterol biosynthesis, suggesting a different mode of action. The X-ray crystal structure of H1 was determined to obtain information regarding the rings and side chain conformation of the sterol hydrazones. Comparison of the inhibitory effects of sterol hydrazones (9-12) and azasterols (AZA1-AZA3) on SMT with the molecular electrostatic potential, negative isopotential energy surfaces (-10 kcal/mol) and local ionization potential calculated via DFT methods, showed that changes in the electronic moiety introduced by the N and O atoms were not as important as the additional flexibility of the side chain introduced by an extra methylene group.
    Steroids 05/2011; 76(10-11):1069-81. · 2.80 Impact Factor
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    ABSTRACT: The rates of gas-phase elimination of several β-substituted diethyl acetals have been determined in a static system and seasoned with allyl bromide. The reactions, inhibited with toluene, are homogeneous, unimolecular, and follow first-order law kinetics. These elimination processes involve two parallel reactions. The first parallel reaction yields ethanol and the corresponding ethyl vinyl ether. The latter product is an unstable intermediate and further decomposes to ethylene and the corresponding substituted aldehyde. The second parallel reaction gives ethane and the corresponding ethyl ester. The kinetics has been measured over the temperature range of 370–441 °C and pressure range of 23–160 torr. The rate coefficients are given by the following Arrhenius equations: The differences in the rates of ethanol formation may be attributed to electronic transmission of the β-substituent. The comparative kinetic and thermodynamic parameters of the parallel reactions suggest two different concerted polar four-membered cyclic transition state types of mechanisms. Copyright © 2010 John Wiley & Sons, Ltd.
    Journal of Physical Organic Chemistry 01/2010; · 1.58 Impact Factor
  • Maldonado A, Mora J.R, Córdova T, Chuchani G
    Journal of Molecular Structure THEOCHEM 01/2010; · 1.37 Impact Factor
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    ABSTRACT: Leishmaniasis is a parasitic zoonosis caused by protozoans of the genus Leishmania transmitted by insects known as phlebotomines, which are found in wild or urban environments. The disease occurs in tropical and sub-tropical areas, mainly in Asia, Europe, Africa and the Americas. At present, there is no effective treatment for this disease. In the search for new rational chemotherapeutic alternatives, two novel trans [Pt(Hpy1)(2)(Cl)(2)] (1) and trans [Pt(Hpy2)(2) (Cl)(2)] (2) complexes were synthesized by the reaction of K(2)PtCl(4) with sterol hydrazone ligands 20-hydrazone-pyridin-2-yl-5alpha-pregnan-3beta-ol (Hpy1) and 22-hydrazone-pyridin-2-yl-chol-5-ene-3beta-ol (Hpy2). These organic compounds are specific inhibitors of sterol methyl transferase (SMT). The new platinum complexes were characterized by a combination of ESI-MS (electrospray ionization-mass spectroscopy), UV-vis, infrared and NMR spectroscopies; elemental analysis and molar conductivity. Promastigotes of Leishmania (L.) mexicana were treated for 48 h with 10 microM of the sterol hydrazones Hpy1 or Hpy2 alone or coordinated to Pt. Hpy1 produced higher leishmanistatic activity than Hpy2 (39% growth inhibition vs. 16%), which significatively increased (71%, p<0.001) when the complex trans-[Pt(Hpy1)(2)(Cl)(2)] was used. This complex represents a new chemotherapeutic alternative to be evaluated in depth in experimental models of leishmaniasis.
    Journal of Inorganic Biochemistry 03/2008; 102(3):547-54. · 3.20 Impact Factor