Carlos Sieiro | Universidad Autónoma de Madrid (original) (raw)
Papers by Carlos Sieiro
Spectrochimica Acta Part A: Molecular Spectroscopy, 1982
The charge-transfer (CT) complexes between five chloronaphthazarin derivatives and DMSO (dimethyl... more The charge-transfer (CT) complexes between five chloronaphthazarin derivatives and DMSO (dimethylsulphexide) are studied by electron spin resonance and vis.-u.v, spectroscopy. The compounds studied are 2-chloro-(l), 2,6-dichloro-(2), 2,7-dichloro-(3), 2,3,6-trichloro-(4) and tetrachloronaphthazarin ($). The radical anions are obtained by ionization of the corresponding CT complexes. The same donor molecule (DMSO) is used as ionizing solvent. The absorption CT bands are obtained using CHCI3 as solvent. Values are given for the hyperfine splittings and g factors of the radical anions, together with the Am~, and • of the electronic transitions of 1-$ and the corresponding An of the CT bands. The importance of the previous cyclic voltammetry when the radical anions or cations of quinones With electron withdrawing groups are obtained by electrolysis is emphasized.
Chemischer Informationsdienst, 1979
Spectrochimica Acta Part A: Molecular Spectroscopy, 1985
Abstract We report the chemical and in situ electrochemical generation of the 2-amino-1,4-naphtho... more Abstract We report the chemical and in situ electrochemical generation of the 2-amino-1,4-naphthosemiquinone radical anion. This radical with a complex hyperfine structure presents in an alkaline medium effects of g tensor anisotropy and dipolar interactions. Two previous papers devoted to this radical presented different hyperfine coupling constants and also different assignments of them. In this paper, we analysed the spectra obtained in both cases and made an alternative assignment of the hccs.
Journal of Molecular Structure: THEOCHEM, 1992
A potential energy function for the BeHF system has been obtained which fits published ab initio ... more A potential energy function for the BeHF system has been obtained which fits published ab initio data at a quantitative level, i.e. with an r.m.s. error about 1 kcal mol-', without any quantity of an ad hoc character, and preserves the accuracy of the ab initio points. The features of this potential have been compared with previously reported potential functions for this system. The true transition state for the Be + HF + BeF + H reaction is 32.2 kcal mall' above the reactants at R
Journal of Molecular Structure: THEOCHEM, 1998
Reaction of methoxybenzene derivatives with phenyliodine(III) bis(trifluoro-acetate), PIFA, yield... more Reaction of methoxybenzene derivatives with phenyliodine(III) bis(trifluoro-acetate), PIFA, yielded the corresponding radical cations, which were observed and characterized by ESR and visible spectroscopies. The application of this methodology to this kind of compound has allowed well-resolved ESR spectra to be obtained of the one-electron oxidized compounds, thus determining the hyperfine coupling constants of the cation radical when the solvent was 1,1,1,3,3,3-hexafluoropropanol (HFP). The requirements of the substitution pattern of the aromatic ring have been studied. In contrast, it was not possible to obtain the ESR spectra of N,N-dialkylaniline radical cations in the same reaction conditions. The ESR spectra obtained corresponded to non-identified secondary reaction products. The spectral resolution obtained by application of this method has been compared with that obtained when the radical cations were generated by in situ electrochemical oxidation. Semiempirical calculations have allowed the coupling constants to be assigned to the magnetic centers in the radical cation. The spin densities obtained from these calculations have been correlated with bibliographic data on the reactivity of radical cations under nucleophilic attack.
Journal of the Chemical Society, Perkin Transactions 2, 1982
Twelve naphthazarin derivatives were reduced electrochemically giving rise to the corresponding s... more Twelve naphthazarin derivatives were reduced electrochemically giving rise to the corresponding semiquinone anion radicals. Their e.s.r. spectra were recorded in situ and their hyperfine coupling constants and g factor were evaluated. For this purpose, SCF INDO MO calculations were performed in order to obtain an approximate value of the different coupling constants. The structure of these radicals was also deduced from these calculations, showing a strong linear O–H–O hydrogen bond, free rotation of the methyl group, and the absence of hyperfine interactions in the methoxy-group.
Spectrochimica Acta Part A: Molecular Spectroscopy, 1988
Abstract The radical anions of several methyl methoxy benzoates are obtained by an in situ electr... more Abstract The radical anions of several methyl methoxy benzoates are obtained by an in situ electrochemical method. ESR spectra of these radicals and u.v. spectra of their parent compounds are analyzed in order to study the change of conformation in the ortho substituted esters, when these are reduced. The effects of the substituents on the hyperfine coupling constants (HCCs) are analyzed by means of an additivity model.
Spectrochimica Acta Part A: Molecular Spectroscopy, 1987
Abstract The hyperfine coupling constants and g factor of the mono-, di- and 2,4,6-trimethylbenzo... more Abstract The hyperfine coupling constants and g factor of the mono-, di- and 2,4,6-trimethylbenzoate radical anions are analyzed in terms of a planar conformation (in contrast to ortho-substituted benzoic acids). Theoretical studies support this conclusion.
Journal of the Chemical Society, Perkin Transactions 2, 1990
A mechanism for glucose degradation in alkaline media has been proposed based upon ESR spectrosco... more A mechanism for glucose degradation in alkaline media has been proposed based upon ESR spectroscopy studies. This degradative process has been carried out with aqueous Bu4NOH in dimethyl sulphoxide (DMSO) at room temperature. Several radicals with semidione-like structures are generated during this process, the presence of an α-hydroxycarbonyl group in the molecule being a crucial requisite for the formation of such radicals. Cleavage of the C(3)–C(4) bond, due to a base catalysed retroaldolization is the first step of the degradation. The important role of DMSO in this process as a methylating agent has also been demonstrated. The corresponding hyperfine splittings (hfs) and g-factors of the radicals detected are given.
Ultrasonics Sonochemistry, 2001
The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform ... more The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform in the presence of oxygen, at 20 and 475 kHz ultrasound frequencies has been investigated by the ESR-spin trapping technique. 5,5-Dimethyl-1-pyrroline-Noxide (DMPO), 3,3,5,5-tetramethylpyrroline-N-oxide (TMPO) and N-tert-butyl-a-phenyl nitrone (PBN) were able to trap superoxide radical anion, generated as the result of the sonication of the organic media. The addition of superoxide dismutase (SOD) resulted in a dramatic decrease of the ESR signal intensity of the superoxide radical adduct. In addition, the thermolysis of the liquids under ultrasound was shown by ESR detection of the spin adducts of the radicals formed by homolytic fragmentation. Occasionally, the nature of the detected spin adduct was dependent on the sonication time or on the frequency of the ultrasonic radiation. Experiments carried out in the presence of 2-methyl-2-nitrosopropane (MNP) resulted in the detection of radicals originating from thermal decomposition of the spin trap, showing its lability under ultrasonic radiation.
The Journal of Physical Chemistry B, 2008
The temperature dependence of the cw-EPR spectra corresponding to the propagating radical respons... more The temperature dependence of the cw-EPR spectra corresponding to the propagating radical responsible for the polymerization of methacrylic monomers has been simulated by an integrated computational approach that determines the structural and magnetic parameters (via quantum mechanical calculations) and diffusive properties (via hydrodynamic based methods), which represent the overall input of the stochastic Liouville equation that yields the spectrum. The system has been modeled as a rotator with only one relaxation process, the rotation around the C alpha-C beta bond. The simulations clearly indicate that the change of the spectral shape with the temperature is essentially related to the internal flexibility of the radical end.
The Journal of Physical Chemistry A, 2005
, an adequate computational protocol for the calculation of isotropic hyperfine coupling constant... more , an adequate computational protocol for the calculation of isotropic hyperfine coupling constants (hfcc's) was proposed. The main conclusion concerns the reliability of the scheme B3LYP/TZVP//B3LYP/ 6-31G* in the predictions of hfcc's with low computational cost. In the present study, we gain insight into the behavior of the above functional/basis set scheme on nuclei of the third row, for which few systematic studies have been carried out up to the present date. The systems studied are neutral, cationic, anionic, localized, and conjugated radicals, containing 29 Si, 31 P, and 33 S nuclei. After carrying out a regression analysis, we conclude that density functional theory (DFT) predictions on the hfcc's of the third-row nuclei are reliable for B3LYP/TZVP by using an optimized geometry with B3LYP/6-31G* combination. By comparison with other much more computationally demanding schemes, namely, B3LYP/cc-pVTZ and B3LYP/cc-pVQZ, we conclude that the B3LYP functional in conjunction with the TZVP basis set is the most useful computational protocol for the assignment of experimental hfcc's, not only for nuclei of first and second rows, but also for those of the third row.
Polymer, 2001
The kinetics of the radical polymerization of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate... more The kinetics of the radical polymerization of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate was studied in benzene and dioxane/hexane solutions at different conditions of temperature, monomer and initiator concentrations by using ultraviolet and electron paramagnetic resonance spectroscopic techniques. The values of the ratios between the propagation rate constant and the square root of the termination rate constant (kp/kt1/2) were determined by UV spectroscopy by
Phosphorus, Sulfur, and Silicon and the Related Elements, 2005
Abstract
Macromolecules, 2007
The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studi... more The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studied. The experimental two-step method consists of a short initial photoinduced polymerization period and subsequent register of the monomer concentration evolution in the dark. After switching off the light, the radical concentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerization reaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units). By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values of the termination rate coefficients, 〈k t 〉, scarcely exceeding 10 000 L mol-1 s-1 , and radical concentrations at the beginning of the postpolymerization about 10-8 mol L-1 are determined. The reliability of these fitting results is checked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecular weight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomer and to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of the instantaneous chain length distribution. The extremely low 〈k t 〉 values obtained by the analysis suggest that the power-law exponents which describe the chain-length dependency of k t should be higher than the unity for long radicals even in dilute solution (i.e., at low monomer conversions).
Journal of Molecular Structure, 1982
... 10 YG Smeyers and MN Bellido, Int. J. Quantum Chem., 19 (1981) 553. 11 NL AUinger, MA Miller,... more ... 10 YG Smeyers and MN Bellido, Int. J. Quantum Chem., 19 (1981) 553. 11 NL AUinger, MA Miller, FA van Catledge and JA Hirsch, J. Am. Chem. Soc., 89 (1967)4345. ... 17 KS Pitzer and DW Scott, J. Am. Chem. Soc., 65 (1943) 803. 18 KC Ingham and SJ Strickler, J. Chem. ...
Journal of Molecular Structure, 1979
This paper is a theoretical conformational analysis of 2-methylsulphonyl-l-phenylethanol, previou... more This paper is a theoretical conformational analysis of 2-methylsulphonyl-l-phenylethanol, previously studied experimentally by 'H NMR spectroscopy. CNDO/S is used in the original parametrization. The calculations predict, as being more stable, the conformation in which the methylsulphonyl group has an anti disposition with respect to the phenyl group. The different bonded and non-bonded interactions are analyzed in order to explain the stabilization factors.
Journal of Molecular Structure, 1992
Experimental evidence on the formation of very stable chelates, cis-semidione-cations, has been o... more Experimental evidence on the formation of very stable chelates, cis-semidione-cations, has been obtained by means of electron spin resonance spectroscopy. Ab initio calculations carried out in order to analyse the interaction of semidiones with different cations show qualitative agreement with experimental results.
Journal of Chemical Education, 1993
Spectrochimica Acta Part A: Molecular Spectroscopy, 1982
The charge-transfer (CT) complexes between five chloronaphthazarin derivatives and DMSO (dimethyl... more The charge-transfer (CT) complexes between five chloronaphthazarin derivatives and DMSO (dimethylsulphexide) are studied by electron spin resonance and vis.-u.v, spectroscopy. The compounds studied are 2-chloro-(l), 2,6-dichloro-(2), 2,7-dichloro-(3), 2,3,6-trichloro-(4) and tetrachloronaphthazarin ($). The radical anions are obtained by ionization of the corresponding CT complexes. The same donor molecule (DMSO) is used as ionizing solvent. The absorption CT bands are obtained using CHCI3 as solvent. Values are given for the hyperfine splittings and g factors of the radical anions, together with the Am~, and • of the electronic transitions of 1-$ and the corresponding An of the CT bands. The importance of the previous cyclic voltammetry when the radical anions or cations of quinones With electron withdrawing groups are obtained by electrolysis is emphasized.
Chemischer Informationsdienst, 1979
Spectrochimica Acta Part A: Molecular Spectroscopy, 1985
Abstract We report the chemical and in situ electrochemical generation of the 2-amino-1,4-naphtho... more Abstract We report the chemical and in situ electrochemical generation of the 2-amino-1,4-naphthosemiquinone radical anion. This radical with a complex hyperfine structure presents in an alkaline medium effects of g tensor anisotropy and dipolar interactions. Two previous papers devoted to this radical presented different hyperfine coupling constants and also different assignments of them. In this paper, we analysed the spectra obtained in both cases and made an alternative assignment of the hccs.
Journal of Molecular Structure: THEOCHEM, 1992
A potential energy function for the BeHF system has been obtained which fits published ab initio ... more A potential energy function for the BeHF system has been obtained which fits published ab initio data at a quantitative level, i.e. with an r.m.s. error about 1 kcal mol-', without any quantity of an ad hoc character, and preserves the accuracy of the ab initio points. The features of this potential have been compared with previously reported potential functions for this system. The true transition state for the Be + HF + BeF + H reaction is 32.2 kcal mall' above the reactants at R
Journal of Molecular Structure: THEOCHEM, 1998
Reaction of methoxybenzene derivatives with phenyliodine(III) bis(trifluoro-acetate), PIFA, yield... more Reaction of methoxybenzene derivatives with phenyliodine(III) bis(trifluoro-acetate), PIFA, yielded the corresponding radical cations, which were observed and characterized by ESR and visible spectroscopies. The application of this methodology to this kind of compound has allowed well-resolved ESR spectra to be obtained of the one-electron oxidized compounds, thus determining the hyperfine coupling constants of the cation radical when the solvent was 1,1,1,3,3,3-hexafluoropropanol (HFP). The requirements of the substitution pattern of the aromatic ring have been studied. In contrast, it was not possible to obtain the ESR spectra of N,N-dialkylaniline radical cations in the same reaction conditions. The ESR spectra obtained corresponded to non-identified secondary reaction products. The spectral resolution obtained by application of this method has been compared with that obtained when the radical cations were generated by in situ electrochemical oxidation. Semiempirical calculations have allowed the coupling constants to be assigned to the magnetic centers in the radical cation. The spin densities obtained from these calculations have been correlated with bibliographic data on the reactivity of radical cations under nucleophilic attack.
Journal of the Chemical Society, Perkin Transactions 2, 1982
Twelve naphthazarin derivatives were reduced electrochemically giving rise to the corresponding s... more Twelve naphthazarin derivatives were reduced electrochemically giving rise to the corresponding semiquinone anion radicals. Their e.s.r. spectra were recorded in situ and their hyperfine coupling constants and g factor were evaluated. For this purpose, SCF INDO MO calculations were performed in order to obtain an approximate value of the different coupling constants. The structure of these radicals was also deduced from these calculations, showing a strong linear O–H–O hydrogen bond, free rotation of the methyl group, and the absence of hyperfine interactions in the methoxy-group.
Spectrochimica Acta Part A: Molecular Spectroscopy, 1988
Abstract The radical anions of several methyl methoxy benzoates are obtained by an in situ electr... more Abstract The radical anions of several methyl methoxy benzoates are obtained by an in situ electrochemical method. ESR spectra of these radicals and u.v. spectra of their parent compounds are analyzed in order to study the change of conformation in the ortho substituted esters, when these are reduced. The effects of the substituents on the hyperfine coupling constants (HCCs) are analyzed by means of an additivity model.
Spectrochimica Acta Part A: Molecular Spectroscopy, 1987
Abstract The hyperfine coupling constants and g factor of the mono-, di- and 2,4,6-trimethylbenzo... more Abstract The hyperfine coupling constants and g factor of the mono-, di- and 2,4,6-trimethylbenzoate radical anions are analyzed in terms of a planar conformation (in contrast to ortho-substituted benzoic acids). Theoretical studies support this conclusion.
Journal of the Chemical Society, Perkin Transactions 2, 1990
A mechanism for glucose degradation in alkaline media has been proposed based upon ESR spectrosco... more A mechanism for glucose degradation in alkaline media has been proposed based upon ESR spectroscopy studies. This degradative process has been carried out with aqueous Bu4NOH in dimethyl sulphoxide (DMSO) at room temperature. Several radicals with semidione-like structures are generated during this process, the presence of an α-hydroxycarbonyl group in the molecule being a crucial requisite for the formation of such radicals. Cleavage of the C(3)–C(4) bond, due to a base catalysed retroaldolization is the first step of the degradation. The important role of DMSO in this process as a methylating agent has also been demonstrated. The corresponding hyperfine splittings (hfs) and g-factors of the radicals detected are given.
Ultrasonics Sonochemistry, 2001
The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform ... more The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform in the presence of oxygen, at 20 and 475 kHz ultrasound frequencies has been investigated by the ESR-spin trapping technique. 5,5-Dimethyl-1-pyrroline-Noxide (DMPO), 3,3,5,5-tetramethylpyrroline-N-oxide (TMPO) and N-tert-butyl-a-phenyl nitrone (PBN) were able to trap superoxide radical anion, generated as the result of the sonication of the organic media. The addition of superoxide dismutase (SOD) resulted in a dramatic decrease of the ESR signal intensity of the superoxide radical adduct. In addition, the thermolysis of the liquids under ultrasound was shown by ESR detection of the spin adducts of the radicals formed by homolytic fragmentation. Occasionally, the nature of the detected spin adduct was dependent on the sonication time or on the frequency of the ultrasonic radiation. Experiments carried out in the presence of 2-methyl-2-nitrosopropane (MNP) resulted in the detection of radicals originating from thermal decomposition of the spin trap, showing its lability under ultrasonic radiation.
The Journal of Physical Chemistry B, 2008
The temperature dependence of the cw-EPR spectra corresponding to the propagating radical respons... more The temperature dependence of the cw-EPR spectra corresponding to the propagating radical responsible for the polymerization of methacrylic monomers has been simulated by an integrated computational approach that determines the structural and magnetic parameters (via quantum mechanical calculations) and diffusive properties (via hydrodynamic based methods), which represent the overall input of the stochastic Liouville equation that yields the spectrum. The system has been modeled as a rotator with only one relaxation process, the rotation around the C alpha-C beta bond. The simulations clearly indicate that the change of the spectral shape with the temperature is essentially related to the internal flexibility of the radical end.
The Journal of Physical Chemistry A, 2005
, an adequate computational protocol for the calculation of isotropic hyperfine coupling constant... more , an adequate computational protocol for the calculation of isotropic hyperfine coupling constants (hfcc's) was proposed. The main conclusion concerns the reliability of the scheme B3LYP/TZVP//B3LYP/ 6-31G* in the predictions of hfcc's with low computational cost. In the present study, we gain insight into the behavior of the above functional/basis set scheme on nuclei of the third row, for which few systematic studies have been carried out up to the present date. The systems studied are neutral, cationic, anionic, localized, and conjugated radicals, containing 29 Si, 31 P, and 33 S nuclei. After carrying out a regression analysis, we conclude that density functional theory (DFT) predictions on the hfcc's of the third-row nuclei are reliable for B3LYP/TZVP by using an optimized geometry with B3LYP/6-31G* combination. By comparison with other much more computationally demanding schemes, namely, B3LYP/cc-pVTZ and B3LYP/cc-pVQZ, we conclude that the B3LYP functional in conjunction with the TZVP basis set is the most useful computational protocol for the assignment of experimental hfcc's, not only for nuclei of first and second rows, but also for those of the third row.
Polymer, 2001
The kinetics of the radical polymerization of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate... more The kinetics of the radical polymerization of 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate was studied in benzene and dioxane/hexane solutions at different conditions of temperature, monomer and initiator concentrations by using ultraviolet and electron paramagnetic resonance spectroscopic techniques. The values of the ratios between the propagation rate constant and the square root of the termination rate constant (kp/kt1/2) were determined by UV spectroscopy by
Phosphorus, Sulfur, and Silicon and the Related Elements, 2005
Abstract
Macromolecules, 2007
The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studi... more The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studied. The experimental two-step method consists of a short initial photoinduced polymerization period and subsequent register of the monomer concentration evolution in the dark. After switching off the light, the radical concentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerization reaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units). By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values of the termination rate coefficients, 〈k t 〉, scarcely exceeding 10 000 L mol-1 s-1 , and radical concentrations at the beginning of the postpolymerization about 10-8 mol L-1 are determined. The reliability of these fitting results is checked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecular weight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomer and to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of the instantaneous chain length distribution. The extremely low 〈k t 〉 values obtained by the analysis suggest that the power-law exponents which describe the chain-length dependency of k t should be higher than the unity for long radicals even in dilute solution (i.e., at low monomer conversions).
Journal of Molecular Structure, 1982
... 10 YG Smeyers and MN Bellido, Int. J. Quantum Chem., 19 (1981) 553. 11 NL AUinger, MA Miller,... more ... 10 YG Smeyers and MN Bellido, Int. J. Quantum Chem., 19 (1981) 553. 11 NL AUinger, MA Miller, FA van Catledge and JA Hirsch, J. Am. Chem. Soc., 89 (1967)4345. ... 17 KS Pitzer and DW Scott, J. Am. Chem. Soc., 65 (1943) 803. 18 KC Ingham and SJ Strickler, J. Chem. ...
Journal of Molecular Structure, 1979
This paper is a theoretical conformational analysis of 2-methylsulphonyl-l-phenylethanol, previou... more This paper is a theoretical conformational analysis of 2-methylsulphonyl-l-phenylethanol, previously studied experimentally by 'H NMR spectroscopy. CNDO/S is used in the original parametrization. The calculations predict, as being more stable, the conformation in which the methylsulphonyl group has an anti disposition with respect to the phenyl group. The different bonded and non-bonded interactions are analyzed in order to explain the stabilization factors.
Journal of Molecular Structure, 1992
Experimental evidence on the formation of very stable chelates, cis-semidione-cations, has been o... more Experimental evidence on the formation of very stable chelates, cis-semidione-cations, has been obtained by means of electron spin resonance spectroscopy. Ab initio calculations carried out in order to analyse the interaction of semidiones with different cations show qualitative agreement with experimental results.
Journal of Chemical Education, 1993