Edgar Marquez | UANL - Academia.edu (original) (raw)

Papers by Edgar Marquez

Research paper thumbnail of Conformation effects on the absorption spectra of macromolecules

Macromolecules, 1993

Applications of scattering theory to the estimation of absorption coefficientaof macromolecules y... more Applications of scattering theory to the estimation of absorption coefficientaof macromolecules yield values that are different from those obtained from the direct application of the Beer-Lambert law. These differences in absorption coefficients can be explained in terms of the size and optical properties of the macromolecules. Thia paper reports on the effects of the molecular conformation on the interpretation of the scattering-corrected absorption spectra of macromolecules. Using Monte Carlo simulations, the conformation of polymer chains modeled ae eelf-avoiding walke on a tetrahedral lattice has been investigated. It is demonstrated that the number of chromophores in the trajectory of the incident light varies ae a function ofthemolecularweight, ThroughanextensionoftheBeer-Lambertlaw,theeffectofelignmentofchromophoric groups is explored and the resulta are compared with experimentally measured spectra. It is concluded that information on the conformation of macromolecules is contained in the Wyie spectra and that this information may be quantitatively extracted.

Research paper thumbnail of Conformation effects on the absorption spectra of macromolecules

Research paper thumbnail of Mechanisms of the homogeneous, unimolecular gas-phase elimination kinetics of triethyl orthoacetate and triethyl orthopropionate

Journal of Physical Organic Chemistry, 2008

Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the tem... more Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of 291–351°C and pressure range of 80–170 Torr. The elimination reactions of these orthoesters in seasoned vessels are homogeneous, unimolecular, and follow a first-order rate law. The reaction products are ethanol, ethylene and the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for triethyl orthoacetate, log k1 (s−1) = (13.76 ± 0.09) − (187.6 ± 1.1) kJ mol−1 (2.303 RT)−1 (r = 0.9993), and for triethyl orthopropionate, log k1 (s−1) = (13.63 ± 0.07) − (193.3 ± 1.8) kJ mol−1 (2.303 RT)−1 (r = 0.9992). A reasonable mechanism of these elimination is to consider that the COCH2CH3 bond, as Cδ+…δ− OCH2CH3 in the TS, is the rate-determining step. The nucleophilicity of the oxygen atom of OCH2CH3 may abstract the hydrogen of the adjacent CH bond for a four-membered cyclic structure to give the corresponding unsaturated ketal. The unstable ketal intermediate decomposes, in a six-membered cyclic transition state, into ethylene and the corresponding ethyl ester. Copyright © 2008 John Wiley & Sons, Ltd.

Research paper thumbnail of Mechanism and structure-reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phase

Journal of Physical Organic Chemistry, 2003

The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a s... more The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435°C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, δ=−0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd.

Research paper thumbnail of Experimental and theoretical study of the mechanism for the kinetic of elimination of methyl carbazate in the gas phase

Journal of Physical Organic Chemistry, 2009

The elimination kinetic of methyl carbazate in the gas phase was determined in a static system ov... more The elimination kinetic of methyl carbazate in the gas phase was determined in a static system over the temperature range of 340–390 °C and pressure range of 47–118 Torr. The reaction is homogeneous, unimolecular, and obeys a first order rate law. The decomposition products are methyl amine, nitrous acid, and CO gas. The variation of the rate coefficients with temperatures is given by the Arrhenius expression: log k1 (s−1) = (11.56 ± 0.34) − (180.7 ± 4.1) kJ mol−1(2.303 RT)−1. The estimated kinetics and thermodynamics parameters are in good agreement to the experimental values using B3LYP/6-31G (d,p), and MP2/6-31G (d,p) levels of theory. These calculations imply a molecular mechanism involving a concerted non-synchronous quasi three-membered ring cyclic transition state to give an unstable intermediate, 1,2-oxaziridin-3-one. Bond order analysis and natural charges implies that polarization of O (alkyl)C (alkyl) bond of the ester is rate determining in this reaction. Copyright © 2008 John Wiley & Sons, Ltd.

Research paper thumbnail of Kinetics and Mechanisms of the Homogeneous, Unimolecular Gas-Phase Elimination of Trimethyl Orthoacetate and Trimethyl Orthobutyrate

Journal of Physical Chemistry A, 2008

The gas-phase elimination kinetics of the title compounds have been examined over the temperature... more The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369°C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k 1 (s -1 ) ) [(13.58 ( 0.10) -(194.7 ( 1.2) (kJ mol -1 )](2.303RT) -1 r ) 0.9998; and for trimethyl orthobutyrate, log k 1 (s -1 ) ) [(13.97 ( 0.37) -(195.3 ( 1.6) (kJ mol -1 )](2.303RT) -1 r ) 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.

Research paper thumbnail of DFT Calculations of Triethyl and Trimethyl Orthoacetate Elimination Kinetics in the Gas Phase

Journal of Physical Chemistry A, 2009

The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters ... more The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6-31G(d,p), and MPW1PW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C 3 -O 4 , in the direction C 3 δ+ · · · O 4 δ-

Research paper thumbnail of Combined experimental and theoretical studies of the elimination kinetic of 2-methoxytetrahydropyran in the gas phase

Journal of Physical Organic Chemistry, 2010

The products formed in 2-methoxytetrahydropyran elimination reaction in the gas phase are 3, 4-di... more The products formed in 2-methoxytetrahydropyran elimination reaction in the gas phase are 3, 4-dihydro-2H-pyran and methanol. The kinetic study was carried out in a static system, with the vessels deactivated with allyl bromide, and the presence of the free radical suppressor toluene. Temperature and pressure ranges were 400–450 °C and 25–83 Torr, respectively. The process is homogeneous, unimolecular, and follows a first-order rate law. The observed rate coefficient is represented by the following equation: log k (s−1) = (13.95 ± 0.15) − (223.1 ± 2.1) (kJ mol−1) (2.303RT)−1. The reactant exists mainly in two low energy chair-like conformations, with the 2-methoxy group in axial or equatorial position. However, the transition state (TS) for the elimination of the two conformers is the same. Theoretical calculations of this reaction were carried for two possible mechanisms from these conformations by using DFT functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31G++(d,p). The calculation results demonstrate that 2-methoxytetrahydropyran exists mainly in two conformations, with the 2-methoy group in axial or equatorial position, that are thermal in equilibrium. The average thermodynamic and kinetic parameters, taking into account the populations of the conformers in the equilibrium, are in good agreement with experimental values at B3LYP/6-31++(d,p) level of theory. Copyright © 2010 John Wiley & Sons, Ltd.

Research paper thumbnail of Experimental and Theoretical Studies of the Homogeneous, Unimolecular Gas-Phase Elimination Kinetics of Trimethyl Orthovalerate and Trimethyl Orthochloroacetate

Journal of Physical Chemistry A, 2010

The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters ... more The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6-31G(d,p), and MPW1PW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C 3 -O 4 , in the direction C 3 δ+ · · · O 4 δ-

Research paper thumbnail of Determination of the surface roughness and refractive index of amorphous As 40S 60 films deposited by spin coating

Optical Materials, 2004

An envelope method, based on the optical reflection spectrum taken at normal incidence, has been ... more An envelope method, based on the optical reflection spectrum taken at normal incidence, has been successfully applied to the geometrical–optical characterization of thin dielectric films having significant surface roughness. Such a method allows the determination of the average thickness and the refractive index of the films with accuracies better than 1%, as well as the average amplitude of the surface

Research paper thumbnail of Spectral dependence of the complex refractive index of GeS2 chalcogenide glass thin films

Journal of Materials Science Letters, 1997

Abstracts are not published in this journal

Research paper thumbnail of The shift of the absorption edge of thin chalcogenide films with copper

Journal of Materials Science Letters, 1997

Without Abstract

Research paper thumbnail of Conclusión LCE

El impacto de los plaguicidas sobre los ecosistemas agrícola ha demostrado que estos productos in... more El impacto de los plaguicidas sobre los ecosistemas agrícola ha demostrado que estos productos influyen en la diversidad de especies, en la cadena alimentaria, en el flujo de energía, ciclos de nutrientes, genética de los organismos y en general en la estabilidad del sistema. Por otro lado la falta de conocimiento en cuanto al uso de los plaguicidas y los efectos adversos de estos en la salud humana y del medio ambiente, además de los deseos por mejorar los rendimientos de las cosechas, han llevado a los productores agrícolas en el mundo, a dosificar mal los productos químicos con todos los riesgos que esto significa. Esta situación existente en el mundo se agudiza cuando los productores agrícolas no siguen las recomendaciones técnicas del fabricante, en cuanto al tipo de producto, la dosis a aplicar para cada plaga y cultivo o en el momento de aplicación.

Research paper thumbnail of Conformation effects on the absorption spectra of macromolecules

Macromolecules, 1993

Applications of scattering theory to the estimation of absorption coefficientaof macromolecules y... more Applications of scattering theory to the estimation of absorption coefficientaof macromolecules yield values that are different from those obtained from the direct application of the Beer-Lambert law. These differences in absorption coefficients can be explained in terms of the size and optical properties of the macromolecules. Thia paper reports on the effects of the molecular conformation on the interpretation of the scattering-corrected absorption spectra of macromolecules. Using Monte Carlo simulations, the conformation of polymer chains modeled ae eelf-avoiding walke on a tetrahedral lattice has been investigated. It is demonstrated that the number of chromophores in the trajectory of the incident light varies ae a function ofthemolecularweight, ThroughanextensionoftheBeer-Lambertlaw,theeffectofelignmentofchromophoric groups is explored and the resulta are compared with experimentally measured spectra. It is concluded that information on the conformation of macromolecules is contained in the Wyie spectra and that this information may be quantitatively extracted.

Research paper thumbnail of Conformation effects on the absorption spectra of macromolecules

Research paper thumbnail of Mechanisms of the homogeneous, unimolecular gas-phase elimination kinetics of triethyl orthoacetate and triethyl orthopropionate

Journal of Physical Organic Chemistry, 2008

Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the tem... more Triethyl orthoacetate and triethyl orthopropionate were pyrolyzed in a static system over the temperature range of 291–351°C and pressure range of 80–170 Torr. The elimination reactions of these orthoesters in seasoned vessels are homogeneous, unimolecular, and follow a first-order rate law. The reaction products are ethanol, ethylene and the corresponding ethyl ester. The Arrhenius expressions of these eliminations were found as follow: for triethyl orthoacetate, log k1 (s−1) = (13.76 ± 0.09) − (187.6 ± 1.1) kJ mol−1 (2.303 RT)−1 (r = 0.9993), and for triethyl orthopropionate, log k1 (s−1) = (13.63 ± 0.07) − (193.3 ± 1.8) kJ mol−1 (2.303 RT)−1 (r = 0.9992). A reasonable mechanism of these elimination is to consider that the COCH2CH3 bond, as Cδ+…δ− OCH2CH3 in the TS, is the rate-determining step. The nucleophilicity of the oxygen atom of OCH2CH3 may abstract the hydrogen of the adjacent CH bond for a four-membered cyclic structure to give the corresponding unsaturated ketal. The unstable ketal intermediate decomposes, in a six-membered cyclic transition state, into ethylene and the corresponding ethyl ester. Copyright © 2008 John Wiley & Sons, Ltd.

Research paper thumbnail of Mechanism and structure-reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phase

Journal of Physical Organic Chemistry, 2003

The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a s... more The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435°C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, δ=−0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd.

Research paper thumbnail of Experimental and theoretical study of the mechanism for the kinetic of elimination of methyl carbazate in the gas phase

Journal of Physical Organic Chemistry, 2009

The elimination kinetic of methyl carbazate in the gas phase was determined in a static system ov... more The elimination kinetic of methyl carbazate in the gas phase was determined in a static system over the temperature range of 340–390 °C and pressure range of 47–118 Torr. The reaction is homogeneous, unimolecular, and obeys a first order rate law. The decomposition products are methyl amine, nitrous acid, and CO gas. The variation of the rate coefficients with temperatures is given by the Arrhenius expression: log k1 (s−1) = (11.56 ± 0.34) − (180.7 ± 4.1) kJ mol−1(2.303 RT)−1. The estimated kinetics and thermodynamics parameters are in good agreement to the experimental values using B3LYP/6-31G (d,p), and MP2/6-31G (d,p) levels of theory. These calculations imply a molecular mechanism involving a concerted non-synchronous quasi three-membered ring cyclic transition state to give an unstable intermediate, 1,2-oxaziridin-3-one. Bond order analysis and natural charges implies that polarization of O (alkyl)C (alkyl) bond of the ester is rate determining in this reaction. Copyright © 2008 John Wiley & Sons, Ltd.

Research paper thumbnail of Kinetics and Mechanisms of the Homogeneous, Unimolecular Gas-Phase Elimination of Trimethyl Orthoacetate and Trimethyl Orthobutyrate

Journal of Physical Chemistry A, 2008

The gas-phase elimination kinetics of the title compounds have been examined over the temperature... more The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369°C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k 1 (s -1 ) ) [(13.58 ( 0.10) -(194.7 ( 1.2) (kJ mol -1 )](2.303RT) -1 r ) 0.9998; and for trimethyl orthobutyrate, log k 1 (s -1 ) ) [(13.97 ( 0.37) -(195.3 ( 1.6) (kJ mol -1 )](2.303RT) -1 r ) 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.

Research paper thumbnail of DFT Calculations of Triethyl and Trimethyl Orthoacetate Elimination Kinetics in the Gas Phase

Journal of Physical Chemistry A, 2009

The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters ... more The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6-31G(d,p), and MPW1PW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C 3 -O 4 , in the direction C 3 δ+ · · · O 4 δ-

Research paper thumbnail of Combined experimental and theoretical studies of the elimination kinetic of 2-methoxytetrahydropyran in the gas phase

Journal of Physical Organic Chemistry, 2010

The products formed in 2-methoxytetrahydropyran elimination reaction in the gas phase are 3, 4-di... more The products formed in 2-methoxytetrahydropyran elimination reaction in the gas phase are 3, 4-dihydro-2H-pyran and methanol. The kinetic study was carried out in a static system, with the vessels deactivated with allyl bromide, and the presence of the free radical suppressor toluene. Temperature and pressure ranges were 400–450 °C and 25–83 Torr, respectively. The process is homogeneous, unimolecular, and follows a first-order rate law. The observed rate coefficient is represented by the following equation: log k (s−1) = (13.95 ± 0.15) − (223.1 ± 2.1) (kJ mol−1) (2.303RT)−1. The reactant exists mainly in two low energy chair-like conformations, with the 2-methoxy group in axial or equatorial position. However, the transition state (TS) for the elimination of the two conformers is the same. Theoretical calculations of this reaction were carried for two possible mechanisms from these conformations by using DFT functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31G++(d,p). The calculation results demonstrate that 2-methoxytetrahydropyran exists mainly in two conformations, with the 2-methoy group in axial or equatorial position, that are thermal in equilibrium. The average thermodynamic and kinetic parameters, taking into account the populations of the conformers in the equilibrium, are in good agreement with experimental values at B3LYP/6-31++(d,p) level of theory. Copyright © 2010 John Wiley & Sons, Ltd.

Research paper thumbnail of Experimental and Theoretical Studies of the Homogeneous, Unimolecular Gas-Phase Elimination Kinetics of Trimethyl Orthovalerate and Trimethyl Orthochloroacetate

Journal of Physical Chemistry A, 2010

The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters ... more The reaction paths for the gas-phase molecular elimination of triethyl and trimethyl orthoesters were examined at B3LYP/6-31G(d,p), B3LYP/6-31G++(d,p), B3PW91/6-31G(d,p), B3PW91++G(d,p), MPW1PW91/6-31G(d,p), and MPW1PW91/6-31++G(d,p) levels of theory. The thermal decomposition of ethyl and methyl orthoesters involves similar transition state configurations in a four-membered ring arrangement. Products formed are ethanol and the corresponding unsaturated ketal for ethyl orthoesters, while in methyl orthoesters are methanol and the corresponding unsaturated ketal. Calculated thermodynamic and kinetic parameters from B3LYP calculations were found to be in good agreement with the experimental values. The calculated data imply the polarization of the C 3 -O 4 , in the direction C 3 δ+ · · · O 4 δ-

Research paper thumbnail of Determination of the surface roughness and refractive index of amorphous As 40S 60 films deposited by spin coating

Optical Materials, 2004

An envelope method, based on the optical reflection spectrum taken at normal incidence, has been ... more An envelope method, based on the optical reflection spectrum taken at normal incidence, has been successfully applied to the geometrical–optical characterization of thin dielectric films having significant surface roughness. Such a method allows the determination of the average thickness and the refractive index of the films with accuracies better than 1%, as well as the average amplitude of the surface

Research paper thumbnail of Spectral dependence of the complex refractive index of GeS2 chalcogenide glass thin films

Journal of Materials Science Letters, 1997

Abstracts are not published in this journal

Research paper thumbnail of The shift of the absorption edge of thin chalcogenide films with copper

Journal of Materials Science Letters, 1997

Without Abstract

Research paper thumbnail of Conclusión LCE

El impacto de los plaguicidas sobre los ecosistemas agrícola ha demostrado que estos productos in... more El impacto de los plaguicidas sobre los ecosistemas agrícola ha demostrado que estos productos influyen en la diversidad de especies, en la cadena alimentaria, en el flujo de energía, ciclos de nutrientes, genética de los organismos y en general en la estabilidad del sistema. Por otro lado la falta de conocimiento en cuanto al uso de los plaguicidas y los efectos adversos de estos en la salud humana y del medio ambiente, además de los deseos por mejorar los rendimientos de las cosechas, han llevado a los productores agrícolas en el mundo, a dosificar mal los productos químicos con todos los riesgos que esto significa. Esta situación existente en el mundo se agudiza cuando los productores agrícolas no siguen las recomendaciones técnicas del fabricante, en cuanto al tipo de producto, la dosis a aplicar para cada plaga y cultivo o en el momento de aplicación.