Paulo Abreu - Academia.edu (original) (raw)
Papers by Paulo Abreu
Journal of Mass Spectrometry, 2012
The aggregation of cyclodextrins (CD) in aqueous solution is an old, yet still vastly unexplored ... more The aggregation of cyclodextrins (CD) in aqueous solution is an old, yet still vastly unexplored topic that has been studied at least since the 1980s. At that time, few authors took into consideration the possibility of formation of aggregates for the interpretation of thermodynamic and thermophysical properties of CDs in aqueous solution. The aggregates appear at quite low CD concentrations and seem to encompass only a small number of CD molecules. They also occur in water in the presence of hydrophobic or amphiphilic moieties, including surfactants, assuming a preassembled state with the hydrophobic chains threading through one or two CDs. After a long period in which it has been neglected, CD aggregation is now a hot topic and one far from gathering consensus. In this chapter, a timely and critical review on the phenomenon of CD aggregation and the respective supramolecular properties, including some computational rationales, will be presented. A comprehensive summary of CD aggre...
The Journal of Chemical Thermodynamics
Journal of Molecular Liquids
Cyclodextrin - A Versatile Ingredient
Journal of Molecular Liquids
Dalton Trans., 2015
Mo(vi) and W(vi) complexes with 8-HQS show differences in luminescence behaviour, attributed to t... more Mo(vi) and W(vi) complexes with 8-HQS show differences in luminescence behaviour, attributed to the relative energies of their LMCT and emitting ligand based levels.
Dalton Transactions
The Journal of Physical Chemistry A, 1997
The Journal of Physical Chemistry B, 2009
The Journal of Physical Chemistry A, 2010
In the current manuscript we present the results of a computational study on a series of chromoph... more In the current manuscript we present the results of a computational study on a series of chromophores with enhanced intrinsic hyperpolarizability. The high hyperpolarizability values of these molecules were previously reported and were achieved by making use of aromatic moieties in order to modulate the aromatic stabilization energy along the conjugated bridge between the donor and the acceptor. Calculations were performed using semiempirical, DFT, and TDDFT methods, and the results reproduce the trend determined experimentally for the first hyperpolarizability values. Several calculation schemes were used, and the best agreement was achieved when long-range Hartree-Fock exchange corrections and solvent effects are included in the DFT calculations. The long-range corrections proved to be especially important for the azobenzene derivatives, which otherwise have their hyperpolarizability overestimated considerably in the DFT calculations. The results are also analyzed within the framework of a two-level model, which correctly reproduces the trend in the hyperpolarizabilities of the molecules under study.
The Journal of Physical Chemistry A, 2000
Physical Chemistry Chemical Physics, 2000
Journal of Computational Chemistry, 2009
Factors relevant for controlling the structures determined in the local optimization of argon clu... more Factors relevant for controlling the structures determined in the local optimization of argon clusters are investigated. In particular, the role of volume and shape for the box where initial structures are generated is assessed. A thorough characterization of the optimization is also presented, based on a nearest-neighbor analysis, in clusters ranging from 30 to 55 atoms. This includes the assessment of the degree of preservation of aspects of the initial randomly generated structure in the final optimized counterpart, and the correlation between optimized energy and the number of nearest neighbors and average departure from the diatomic reference distance. The usefulness of this analysis to explore the energy landscape of atomic clusters is also highlighted.
Journal of Computational Chemistry, 2012
The efficiency of the so-called big-bang method for the optimization of atomic clusters is analys... more The efficiency of the so-called big-bang method for the optimization of atomic clusters is analysed in detail for Morse pair potentials with different ranges; here, we have used Morse potentials with four different ranges, from long- ρ = 3) to short-ranged ρ = 14) interactions. Specifically, we study the efficacy of the method in discovering low-energy structures, including the putative global minimum, as a function of the potential range and the cluster size. A new global minimum structure for long-ranged ρ = 3) Morse potential at the cluster size of n= 240 is reported. The present results are useful to assess the maximum cluster size for each type of interaction where the global minimum can be discovered with a limited number of big-bang trials.
Journal of Chemical Theory and Computation, 2009
Journal of Chemical Information and Modeling, 2010
We extend the scope of a recent method for superimposing two molecules ( J. Chem. Phys. 2009, 131... more We extend the scope of a recent method for superimposing two molecules ( J. Chem. Phys. 2009, 131, 124126-1-124126-10 ) to include the identification of chiral structures. This methodology is tested by applying it to several organic molecules and water clusters that were subjected to geometry optimization. The accuracy of four simpler, non-superimposing approaches is then analyzed by comparing pairs of structures for argon and water clusters. The structures considered in this work were obtained by a Markovian walk in the coordinate space. First, a random geometry is generated, and then, the iterative application of a mutation operator ensures the creation of increasingly dissimilar structures. The discriminating power of the non-superimposing approaches is tested by comparing the corresponding dissimilarity measures with the root-mean-square distance obtained from the superimposing method. Finally, we showcase the application of those methods to characterize the diversity of solutions in global geometry optimization by evolutionary algorithms.
Journal of Chemical & Engineering Data, 2010
Dalton Transactions, 2011
Multinuclear ((1)H, (13)C) magnetic resonance spectroscopy, DFT calculations and luminescence tec... more Multinuclear ((1)H, (13)C) magnetic resonance spectroscopy, DFT calculations and luminescence techniques have been used to study 8-hydroxyquinoline-5-sulfonate (8-HQS) and its complexes with Zn(ii), in aqueous solution. The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to obtain a detailed understanding of the complexation between the Zn(2+) ion and 8-HQS. In addition to a complete assignment of the (1)H and (13)C NMR signals of 8-HQS, a full speciation study has been performed. Over the concentration region studied, Zn(2+) metal ion forms only one significant complex species with 8-HQS in aqueous solution in the pH range 6-8. Job's method shows that this species has a 1:2 (metal:ligand) stoichiometry. The geometry around the metal centre, according to structural optimization using DFT calculations, is suggested to be square bipyramidal, with two coordinated water molecules mutually trans, and the remaining positions occupied by the donor groups of the two coordinated 8-HQS ligands. On binding to Zn(ii), 8-HQS shows a marked fluorescence compared with the weakly-luminescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive fluorescent sensor to detect Zn(2+) metal ion in surface waters, biological fluids, etc. Based on results of the structural studies, suggestions are made of ways for enhancing fluorescence sensitivity.
Journal of Mass Spectrometry, 2012
The aggregation of cyclodextrins (CD) in aqueous solution is an old, yet still vastly unexplored ... more The aggregation of cyclodextrins (CD) in aqueous solution is an old, yet still vastly unexplored topic that has been studied at least since the 1980s. At that time, few authors took into consideration the possibility of formation of aggregates for the interpretation of thermodynamic and thermophysical properties of CDs in aqueous solution. The aggregates appear at quite low CD concentrations and seem to encompass only a small number of CD molecules. They also occur in water in the presence of hydrophobic or amphiphilic moieties, including surfactants, assuming a preassembled state with the hydrophobic chains threading through one or two CDs. After a long period in which it has been neglected, CD aggregation is now a hot topic and one far from gathering consensus. In this chapter, a timely and critical review on the phenomenon of CD aggregation and the respective supramolecular properties, including some computational rationales, will be presented. A comprehensive summary of CD aggre...
The Journal of Chemical Thermodynamics
Journal of Molecular Liquids
Cyclodextrin - A Versatile Ingredient
Journal of Molecular Liquids
Dalton Trans., 2015
Mo(vi) and W(vi) complexes with 8-HQS show differences in luminescence behaviour, attributed to t... more Mo(vi) and W(vi) complexes with 8-HQS show differences in luminescence behaviour, attributed to the relative energies of their LMCT and emitting ligand based levels.
Dalton Transactions
The Journal of Physical Chemistry A, 1997
The Journal of Physical Chemistry B, 2009
The Journal of Physical Chemistry A, 2010
In the current manuscript we present the results of a computational study on a series of chromoph... more In the current manuscript we present the results of a computational study on a series of chromophores with enhanced intrinsic hyperpolarizability. The high hyperpolarizability values of these molecules were previously reported and were achieved by making use of aromatic moieties in order to modulate the aromatic stabilization energy along the conjugated bridge between the donor and the acceptor. Calculations were performed using semiempirical, DFT, and TDDFT methods, and the results reproduce the trend determined experimentally for the first hyperpolarizability values. Several calculation schemes were used, and the best agreement was achieved when long-range Hartree-Fock exchange corrections and solvent effects are included in the DFT calculations. The long-range corrections proved to be especially important for the azobenzene derivatives, which otherwise have their hyperpolarizability overestimated considerably in the DFT calculations. The results are also analyzed within the framework of a two-level model, which correctly reproduces the trend in the hyperpolarizabilities of the molecules under study.
The Journal of Physical Chemistry A, 2000
Physical Chemistry Chemical Physics, 2000
Journal of Computational Chemistry, 2009
Factors relevant for controlling the structures determined in the local optimization of argon clu... more Factors relevant for controlling the structures determined in the local optimization of argon clusters are investigated. In particular, the role of volume and shape for the box where initial structures are generated is assessed. A thorough characterization of the optimization is also presented, based on a nearest-neighbor analysis, in clusters ranging from 30 to 55 atoms. This includes the assessment of the degree of preservation of aspects of the initial randomly generated structure in the final optimized counterpart, and the correlation between optimized energy and the number of nearest neighbors and average departure from the diatomic reference distance. The usefulness of this analysis to explore the energy landscape of atomic clusters is also highlighted.
Journal of Computational Chemistry, 2012
The efficiency of the so-called big-bang method for the optimization of atomic clusters is analys... more The efficiency of the so-called big-bang method for the optimization of atomic clusters is analysed in detail for Morse pair potentials with different ranges; here, we have used Morse potentials with four different ranges, from long- ρ = 3) to short-ranged ρ = 14) interactions. Specifically, we study the efficacy of the method in discovering low-energy structures, including the putative global minimum, as a function of the potential range and the cluster size. A new global minimum structure for long-ranged ρ = 3) Morse potential at the cluster size of n= 240 is reported. The present results are useful to assess the maximum cluster size for each type of interaction where the global minimum can be discovered with a limited number of big-bang trials.
Journal of Chemical Theory and Computation, 2009
Journal of Chemical Information and Modeling, 2010
We extend the scope of a recent method for superimposing two molecules ( J. Chem. Phys. 2009, 131... more We extend the scope of a recent method for superimposing two molecules ( J. Chem. Phys. 2009, 131, 124126-1-124126-10 ) to include the identification of chiral structures. This methodology is tested by applying it to several organic molecules and water clusters that were subjected to geometry optimization. The accuracy of four simpler, non-superimposing approaches is then analyzed by comparing pairs of structures for argon and water clusters. The structures considered in this work were obtained by a Markovian walk in the coordinate space. First, a random geometry is generated, and then, the iterative application of a mutation operator ensures the creation of increasingly dissimilar structures. The discriminating power of the non-superimposing approaches is tested by comparing the corresponding dissimilarity measures with the root-mean-square distance obtained from the superimposing method. Finally, we showcase the application of those methods to characterize the diversity of solutions in global geometry optimization by evolutionary algorithms.
Journal of Chemical & Engineering Data, 2010
Dalton Transactions, 2011
Multinuclear ((1)H, (13)C) magnetic resonance spectroscopy, DFT calculations and luminescence tec... more Multinuclear ((1)H, (13)C) magnetic resonance spectroscopy, DFT calculations and luminescence techniques have been used to study 8-hydroxyquinoline-5-sulfonate (8-HQS) and its complexes with Zn(ii), in aqueous solution. The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to obtain a detailed understanding of the complexation between the Zn(2+) ion and 8-HQS. In addition to a complete assignment of the (1)H and (13)C NMR signals of 8-HQS, a full speciation study has been performed. Over the concentration region studied, Zn(2+) metal ion forms only one significant complex species with 8-HQS in aqueous solution in the pH range 6-8. Job's method shows that this species has a 1:2 (metal:ligand) stoichiometry. The geometry around the metal centre, according to structural optimization using DFT calculations, is suggested to be square bipyramidal, with two coordinated water molecules mutually trans, and the remaining positions occupied by the donor groups of the two coordinated 8-HQS ligands. On binding to Zn(ii), 8-HQS shows a marked fluorescence compared with the weakly-luminescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive fluorescent sensor to detect Zn(2+) metal ion in surface waters, biological fluids, etc. Based on results of the structural studies, suggestions are made of ways for enhancing fluorescence sensitivity.