Publications

Abstract A new mini-bomb combustion calorimeter designed at the University of Lund was improved, installed and calibrated at the University of Porto. This calorimeter is suitable for high precision combustion calorimetry with samples of mass about (10 to 40) mg. The energy equivalent of the calorimeter, = (1946.45 +/- 0.11) J center dot K-1, was obtained from 15 independent calibration experiments with benzoic acid SRM 39i. Anthracene, succinic acid, acetanilide, and 1,2,4-triazole were used as test compounds, with excellent agreement with the literature values. [GRAPHICS]

Abstract This letter explains the appropriate way to calculate the parameters in pseudo first and second order kinetics, Langmuir and Freundlich isotherm.

Abstract The energetics of the C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-, 2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as Delta(f)(ClCH2CH2OH, l) = -315.5 +/- 0.7 kJ center dot mol(-1), Delta(f)(BrCH2CH2OH, l) = -275.8 +/- 0.6 kJ center dot mol(-1), Delta(f)(ICH2CH2OH, l) = -207.3 +/- 0.7 kJ center dot mol(-1), by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies of vaporization, Delta(vap)(ClCH2CH2OH) = 48.32 +/- 0.37 kJ center dot mol(-1), Delta(vap)(BrCH2CH2OH) = 54.08 +/- 0.40 kJ center dot mol(-1), and Delta(vap)(ICH2CH2OH) = 57.03 +/- 0.20 kJ center dot mol(-1) were also obtained by Calvet-drop microcalorimetry. The condensed phase and vaporization enthalpy data lead to Delta(f)(ClCH2CH2OH, g) = -267.2 +/- 0.8 kJ center dot mol(-1), Delta(f)(BrCH2CH2OH, g) = -221.7 +/- 0.7 kJ center dot mol(-1), and Delta(f)(ICH2CH2OH, g) = -150.3 +/- 0.7 kJ center dot mol(-1). These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations were used to derive the enthalpies of formation of gaseous 2-fluoroethanol, Delta(f)(FCH2CH2OH, g) = -423.6 +/- 5.0 kJ center dot mol(-1), and of the 2-hydroxyethyl radical, Delta(f)(CH2CH2OH, g) = -28.7 +/- 8.0 kJ center dot mol(-1). The obtained thermochemical data led to the following carbon-halogen bond dissociation enthalpies: DHo(X-CH2CH2OH) = 474.4 +/- 9.4 kJ center dot mol(-1) (X = F), 359.9 +/- 8.0 kJ center dot mol(-1) (X = Cl), 305.0 +/- 8.0 kJ center dot mol(-1) (X = Br), 228.7 +/- 8.1 kJ center dot mol(-1) (X = I). These values were compared with the corresponding C-X bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCHCH2, and XC6H5. In view of this comparison the computational methods mentioned above were also used to obtain Delta(f)(FCH2COOH, g) = -594.0 +/- 5.0 kJ center dot mol(-1) from which DHo(F-CH2COOH) = 435.4 +/- 5.4 kJ center dot mol(-1). The order DHo(C-F) > DHo(C-Cl) > DHo(C-Br) > DHo(C-I) is observed for the haloalcohols and all other RX compounds. It is finally concluded that the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compounds studied in this work can be predicted by Pauling's electrostatic-covalent model.

Abstract The results are analyzed in terms of structural contribution to the standard molar enthalpies of formation. The metal-ligand exchange enthalpies in the crystalline phase show that the increase of the ester-alkyl chain length does not significantly affect the difference between the metal to ligand and the hydrogen to ligand binding enthalpies, [D(M-L) - D(H-L)].

Abstract The study of Chemical Equilibrium is a central and inevitable point for knowledge acquiring in Chemistry. However, it is here where students do present more alternative conceptions becoming more difficult for them to understand it and thus a great challenge for the teaching. As something abstract, that includes non observable events, multiple factors, advanced reasoning and complex microscopic interpretation, this is one of the topics in the Chemistry curriculum that appeals more to the use of computational simulations for its teaching. We have carried out studies using a computer programme for the teaching of chemical equilibrium which confirm the usefulness of these digital tools. Computational simulations are very useful and rich context educational resources. Webquests may be supporting material in the exploration of a computational simulation: they are a kind of an "exploration guide" and at the same time a way of creating students' nearby contexts. There are several advantages that come from the use of this work dynamic using the Web.

Abstract A monitoring program of pesticides was implemented in the "ZV1" vulnerable zone (Directive 91/676/EEC) in Portugal, in order to assess the impact of intensive horticulture practices on groundwater contamination. The monitoring network comprised 23 sampling points sampled every 3 months during a 2-year period. Forty-two pesticides belonging to varied chemical families, including current pesticides, persistent organic pollutants (POPs), and degradation products, were analyzed by solid-phase microextraction (SPME) and gas chromatography with electron-capture detection-thermoionic specific detection (GC-ECD-TSD) or mass spectrometry (GC-MS). Statistical treatment was performed by descriptive analysis followed by chemometric multivariate analysis. The latter included cluster analysis, linear discriminant analysis, and factor analysis. Twenty-two pesticides were quantitated, 20 pesticides were not detected, and metalaxyl, benalaxyl, quinalphos, pirimicarb, and prometryn were only qualitatively detected. The most frequently detected pesticides (% of samples analyzed) were lindane (53%), pendimethalin (49%), endosulfan sulfate (44%), and endosulfan (38%) while those that most frequently exceeded the 0.1 mu g L-1 European Union (EU) limit were pendimethalin (13%), endosulfan (12%), endosulfan sulfate (11%), and atrazine (9%). 45% of the samples exceeded the EU limit for individual pesticides while 27% exceeded the limit set to the sum of pesticides (0.5 mu g L-1). Principal component analysis revealed five principal components that were attributed to environmental/agrochemical managing factors. The broad range of pesticides investigated combined with the intensive character of the local agriculture contributed to the diversity of pesticides that were detected. However, the frequency of pesticides above the EU regulatory limit is comparable to that found in the literature concerning different Portuguese and European regions.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation in the condensed phase of seven isomers of fluoromethylaniline were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g), N-2(g) and HF center dot 10H(2)O(l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation of these compounds, also at T = 298.15 K, were determined using Calvet microcalorimetry, while the enthalpies of fusion of the solid compounds were determined by differential scanning calorimetry. The standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived from the former two experimental quantities. G3MP2//B3LYP calculations were performed for all possible fluoromethylanilines allowing the estimation of data for the isomers that were not studied experimentally. The Cox scheme was applied with two different approaches for the estimation of the standard molar enthalpies of formation of all the isomers studied, and this led to the conclusion that the literature values for the enthalpies of formation of the meta and para isomers of methylaniline seem to be not reliable. Further G3MP2//B3LYPs calculations on the methylaniline isomers yielded new values for the standard molar enthalpies of formation of the isomers of methylaniline, which have been tested under the Cox scheme, resulting in better estimates.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the condensed phase of all the fluoroanilines, with the exception of the 2,3,5-trifluoroaniline compound, were derived from the standard molar energies of combustion in oxygen at T = 298.15 K, measured by rotating bomb combustion calorimetry. Calvet high-temperature vacuum sublimation experiments were performed to measure their enthalpies of vaporization or sublimation. These experiments allowed the determination of the standard molar enthalpies of formation in the gaseous phase and at T = 298.15 K. These values are also compared with estimates based on G3MP2B3 and BP86/6-31+G(d) computations, which have been extended also to the fluoroaniline that was not studied experimentally. The results are in close agreement with a mean deviation of similar to 3 kJ center dot mol(-1). The largest difference between experimental and G3MP2B3 values is found for the pentafluoroaniline (-7.0 kJ center dot mol(-1)). For the three monofluoroanilines, the composite approach has been used also to compute gas-phase acidities, electron and proton affinities, ionization enthalpies and N-H bond dissociation enthalpies. The computed values compare well with available experimental results supporting the new computed data.

Abstract Predicting tissue and environmental distribution of chemicals is of major importance for environmental and life sciences. Most of the molecular descriptors used in computational prediction of chemicals partition behavior consider molecular structure but ignore the nature of the partition system. Consequently, computational models derived up-to-date are restricted to the specific system under study. Here, a free energy-based descriptor (del G(k)) is introduced, which circumvent this problem. Based on Delta G(k), we developed for the first time a single linear classification model to predict the partition behavior of a broad number of structurally diverse drugs and other chemicals (1300) for 38 different partition systems of biological and environmental significance. The model presented training/predicting set accuracies of 91.79/88.92%. Parametrical assumptions were checked. Desirability analysis was used to explore the levels of the predictors that produce the most desirable partition properties. Finally, inversion of the partition direction for each one of the 38 partition systems evidences that our models correctly classified 89.08% of compounds with an uncertainty of only +/- 0.17% independently of the direction of the partition process used to seek the model. Other 10 different classification models (linear, neural networks, and genetic algorithms) were also tested for the same purposes. None of these computational models favorably compare with respect to the linear model indicating that our approach capture the main aspects that govern chemicals partition in different systems. (C) 2007 Wiley Periodicals, Inc.