Showing: 10 from total: 2477 publications
2301. Enthalpies of combustion of di-n-propylamine, diisopropylamine, diisobutylamine, and di-sec-butylamine
daSilva, MAVR ; daSilva, MDMCR ; Gomes, MLACN ; Johnson, M ; Pilcher, G
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1997, ISSN: 0021-9614,  Volume: 29, 
Article,  Indexed in: scopus, wos 
P-001-AS7
Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation Delta(f)H(m)(o) at the temperature 298.15 K were determined from the standard molar enthalpies of combustion measured by static-bomb calorimetry for the liquids: di-n-propylamine, diisopropylamine, diisobutylamine, and di-sec-butylamine. The standard molar enthalpies of formation in the gaseous state were derived using both literature and estimated values for the enthalpies of vaporization. [GRAPHICS] The enthalpies of formation of aliphatic amines in the gaseous state are reviewed using group-additivity scheme. (C) 1997 Academic Press Limited.

2302. Standard molar enthalpies of formation of nine fluorinated beta-diketones by rotating bomb calorimetry
daSilva, MAVR ; Goncalves, JM ; Pilcher, G
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1997, ISSN: 0021-9614,  Volume: 29, 
Article,  Indexed in: scopus, wos 
P-001-BWA
Abstract The standard enthalpies of formation of the following fluorinated beta-diketones at T = 298.15 K were determined from their enthalpies of combustion measured using rotating bomb calorimetry. [GRAPHICS] With enthalpies of vaporization and sublimation, the enthalpies of formation of these beta-diketones were derived for the gaseous state in their enol and keto forms. Where applicable, the enthalpies of formation in the gaseous state correlated satisfactorily with values estimated by group additivity methods. (C) 1997 Academic Press Limited.

2303. Energetics of metal-ligand binding in copper(II) and nickel(II) complexes of two Schiff bases
daSilva, MAVR ; daSilva, MDMCR ; Monte, MJS ; Goncalves, JM ; Fernandes, EMR
in JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS, 1997, ISSN: 0300-9246, 
Article,  Indexed in: crossref, scopus, wos 
DOI: 10.1039/a607745e P-001-BMP
Abstract The standard molar enthalpies of formation, at 298.15 K, of crystalline copper(II) and nickel(II) complexes with 4,4'-ethylenedinitrilobis(pentan-2-one) (H(2)acacen) and with 1,1'-diphenyl-3,3'-ethylenedinitrilobis(butan-1-one) (H(2)bzacen), were determined from solution-reaction calorimetric measurements as, respectively, Delta(f)H(m)(o)[Cu(acacen), cr] = -385.1 +/- 3.2, Delta(f)H(m)(o)[Cu(bzacen), cr] = -203.4 +/- 5.1, Delta(f)H(m)(o)[Ni(acacen), cr] = -472.6 +/- 3.1 and Delta(f)H(m)(o)[Ni(bzacen), cr] = -292.2 +/- 5.1 kJ mol(-1). The standard molar enthalpies of sublimation of the same metal complexes, at 298.15 K, were obtained by effusion methods as: Delta(cr)(g)H(m)(o)[Cu(acacen) = 127.6 +/- 0.7, Delta(cr)(g)H(m)(o)[Ni(acacen)] = 129.1 +/- 0.9, Delta(cr)(g)H(m)(o)[Cu(bzacen)] = 205.6 +/- 3.0 and Delta(cr)(g)H(m)(o)[Ni(bzacen) = 201.7 +/- 2.8 kJ mol(-1). The differences between the mean metal-ligand and hydrogen-ligand bond-dissociation enthalpies were derived.

2304. Erratum: Thermochemical and theoretical study of some quinoxaline 1,4- dioxides and of pyrazine 1,4-dioxide (Journal of Organic Chemistry (1997) 62 (3724))
Acree W.E., J ; Powell, JR ; Tucker, SA ; Ribeiro da Silva, MDMC ; Matos, MAR ; Goncalves, JM ; Santos, LMNBF ; Morais, VMF ; Pilcher, G
in Journal of Organic Chemistry, 1997, ISSN: 0022-3263,  Volume: 62, 
Erratum,  Indexed in: scopus 

2305. Thermochemical and theoretical study of some quinoxaline 1,4-dioxides and of pyrazine 1,4-dioxide. (vol 62, pg 3724, 1997)
Acree, WE ; Powell, JR ; Tucker, SA ; daSilva, MDMCR ; Matos, MAR ; Goncalves, JM ; Santos, LMNBF ; Morais, VMF ; Pilcher, G
in JOURNAL OF ORGANIC CHEMISTRY, 1997, ISSN: 0022-3263,  Volume: 62, 
Correction,  Indexed in: wos 

2306. Standard enthalpies of formation of 2-thenoyltrifluoroacetone and monothio-2-thenoyltrifluoro acetone by rotating-bomb calorimetry
daSilva, MAVR ; Santos, LMNBF ; Pilcher, G
in JOURNAL OF CHEMICAL THERMODYNAMICS, 1997, ISSN: 0021-9614,  Volume: 29, 
Article,  Indexed in: scopus, wos 
P-001-B51
Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline 1,1,1-trifluoro-4-(2-thienyl)-4-hydroxy-3-buten-2-one, HTTFA, and 1,1,1-trifluoro-4-(2-thienyl)-4-mercapto-3-buten-2-one, HTTFAS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K were measured by microcalorimetry, with the following results: [GRAPHICS] The results demonstrate an increase in stabilization energy of approximate to (38.7 +/- 5.0) kJ.mol(-1) in HTTFA compared with thiophene and that in these molecules, the intramolecular hydrogen bond energy (O-H...O), appears approximate to 20 kJ.mol(-1) larger than for (S-H...O). (C) 1997 Academic Press Limited.

2307. Electrochemical study of aqueous-organic gel micro-interfaces
Silva, F ; Sousa, MJ ; Pereira, CM
in ELECTROCHIMICA ACTA, 1997, ISSN: 0013-4686,  Volume: 42, 
Article,  Indexed in: crossref, scopus, wos 
Abstract Microhole arrays created by laser photoablation of thin polymer films were used to support an organic gel phase prepared by addition of 1,3:2,4-dibenzylidene sorbitol (DBS) to a solution of o-nitrophenyloctyl ether (NPOE) and an appropriate supporting electrolyte. The preparation and properties of the organic gel phase are described and the NPOE-DBS gel is assessed. The polarized microhole supported organic gel interface is shown to be capable of use as an amperometric detector for ions. (C) 1997 Published by Elsevier Science Ltd.

2308. The complexation of Cu(II) by anthropogenic fulvic acids extracted from composted urban and livestock wastes
daSilva, JCGE ; Machado, AASC ; Pinto, MSSDS
in JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH PART B-PESTICIDES FOOD CONTAMINANTS AND AGRICULTURAL WASTES, 1997, ISSN: 0360-1234,  Volume: 32, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The fulvic acid (fua) fractions of two samples of composted solid wastes [urban (urfua) and livestock (lsfua) wastes], commercialized to be used in agriculture as organic correctives or fertilizers, were analyzed for their affinity towards Cu(II) at pH=6. Molecular fluorescence spectroscopy (synchronous mode) was used to monitor the quenching caused by the complexation upon addition of Cu(II) to fua. Spectral data were preprocessed by a chemometric self-modeling mixture analysis method (SIMPLISMA) to detect the number of different types of fluorescent binding sites that exist in each fua, their spectra and the corresponding quenching profiles [fluorescence intensity as function of the total Cu(II) concentration]. From the analysis of the quenching profiles, the amount of binding sites (C-L) and the corresponding conditional stability constants (K') were calculated. Both fua samples have approximately C-L = 0.21 mmol/g and the logarithms of K' are 4.21(3) and 4.51(8), respectively for urfua and lsfua. The differences detected between these fua samples and those extracted from natural soils can be attributed mainly to the relatively small humification extent of the present anthropogenic fua samples.

2309. Procedure for the classification of fulvic acids and similar substances based on the variation with pH of their synchronous fluorescence spectra
daSilva, JCGE ; Machado, AASC
in ANALYST, 1997, ISSN: 0003-2654,  Volume: 122, 
Article,  Indexed in: crossref, scopus, wos 
DOI: 10.1039/a703652c P-001-AB5
Abstract A method for the classification of fulvic acids (FA) and similar substances Is presented, It consists of three steps: (i) for each sample, sets of synchronous molecular fluorescence spect ra as the pH is changed are acquired; (ii) the spectral set is reduced to a small number of spectra and concentration profiles of the detected components by evolving factor analysis with a gradient concentration window (EFA-GCW) are obtained; (iii) the spectra of the components of the samples are subjected to pattern recognition analysis by cluster analysis and multi-dimensional scaling. The procedure was tested with 13 samples of FA of natural and anthropogenic origin. Classification rules as a function of the origin and acid-base properties of the samples, in the form of figures with spectra and tables with spectral characteristics, were obtained. The classification according to the origin of the FA samples is useful for the evaluation of the 'extent of humification of FA, The following information on the acid-base properties of the fluorescent fractions of FA was obtained: only FA extracted from soil have fluorescent fractions with relatively strong acid character (pK(a) approximate to 3.2); all FA extracted from different origins have similar fluorescent structures with relatively weak acid character (pK(a) approximate to 6.2); in addition, FA extracted from low humified materials (leaf litter and composted wastes) have another set of similar fluorescent structures with weak acid character (pK(a) = 9.2).

2310. Variation of the stability of complexes of Al(III) with a fulvic acid extracted from a humic cambisol soil in the pH range three to five
Esteves da Silva, JCG ; Machado, AASC ; Ramos, MA ; Arce, F ; Rey, F
in Environmental Toxicology and Chemistry, 1997, ISSN: 0730-7268,  Volume: 16, 
Article,  Indexed in: crossref