Showing: 10 from total: 2087 publications
1711. Thermochemical studies of three bis(O-alkyl-N-benzoylthiocarbamato)nickel(II) complexes
da Silva, MAVR ; Santos, LMNBF ; Schroder, B ; Dietze, F ; Beyer, L
in JOURNAL OF CHEMICAL THERMODYNAMICS, 2004, ISSN: 0021-9614,  Volume: 36, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation of the crystalline complexes of Ni(II) with three N-benzoylthiocarbamic-O-alkylesters, PhCONHCSOR, R = Et, n-Bu, n-Hex were determined, at T = 298.15 K, by high precision solution and reaction calorimetry. Complex -Delta(f)H(m)degrees(cr)/(kJ (.) mol(-1)) Ni(PhCONCSOEt)(2) 619.2 +/- 6.1 Ni(PhCONCSO-n-Bu)(2) 707.4 +/- 6.2 Ni(PhCONCSO-n-Hex)(2) 802.6 +/- 8.3 From the obtained results, the metal-ligand exchange enthalpies in the crystalline phase were derived, indicating that the increase of the ester-alkyl chain length in these compounds do not significantly affect the difference between the metal-to-ligand-binding enthalpy and the hydrogen-binding enthalpy, {D(M-L) - D(H-L)}.

1712. Substituent and ring effects on enthalpies of formation: 2-methyl- and 2-ethylbenzimidazoles versus benzene- and imidazole-derivatives
Jimenez, P ; Roux, MV ; Davalos, JZ ; Temprado, M ; Da Silva, MAVR ; Da Silva, MDMCR ; Amaral, LMPF ; Cabildo, P ; Claramunt, RM ; Mo, O ; Yanez, M ; Elguero, J
in MOLECULAR PHYSICS, 2004, ISSN: 0026-8976,  Volume: 102, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-methylbenzimidazole (2MeBIM) and 2-ethylbenzimidazole (2EtBIM) are reported and the results compared with those of benzimidazole itself (BIM). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311G(d,p) on B3LYP/6-31G* optimized geometries. The comparison of experimental and calculated values of benzenes, imidazoles and benzimidazoles bearing H (unsubstituted), methyl and ethyl groups shows remarkable homogeneity. The energetic group contribution transferability is not followed, but either using it or adding an empirical interaction term, it is possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole-derivatives, pyridine-derivatives, etc.) from the large amount of Delta(f)H(m)(o)(g) values available for substituted benzenes and those of the parent (pyrazole, pyridine) heterocycles.

1713. Measurement of enthalpies of sublimation by drop method in a Calvet type calorimeter: design and test of a new system
Santos, LMNBF ; Schroder, B ; Fernandes, OOP ; da Silva, MAV
in THERMOCHIMICA ACTA, 2004, ISSN: 0040-6031,  Volume: 415, 
Article,  Indexed in: crossref, scopus, wos 
Abstract A new system to measure enthalpies of sublimation by the drop method was designed, constructed and tested. The performance of the apparatus was checked by measuring the enthalpy of sublimation of ferrocene, benzoic acid and anthracene at different temperatures. The measurement of enthalpies of sublimation by the drop method is described in detail. [GRAPHICS]

1714. CALORIMETRIC STUDIES OF THE INTERACTIONS OF TWO HYBRID PEPTIDES (CECROPIN/MELITTIN WITH LIPOSOMES
Gomes, P ; Andreu, D ; Bai, G ; Bastos, M
in JOURNAL OF PEPTIDE SCIENCE, 2004, ISSN: 1075-2617,  Volume: 10, 
Abstract,  Indexed in: wos 

1715. Enthalpy of solution of alpha-cyclodextrin in water and in formamide at 298.15 K
Bastos, M ; Milheiras, S ; Bai, GY
in THERMOCHIMICA ACTA, 2004, ISSN: 0040-6031,  Volume: 420, 
Article,  Indexed in: crossref, scopus, wos 
Abstract A new dissolution microcalorimeter that can measure enthalpies of dissolution of slightly soluble solids was developed by Ingemar Wadso at Lund University, and the prototype as well as the commercial vessel were tested in our laboratory. Recently we did report the testing of the prototype and we are now extending the previous investigation to measurements with an organic solvent in the commercial vessel. The instrument performance was found to be as good with formamide as with water. The vessel was calibrated chemically (dissolution of KCl in water) and electrically, by means of a permanent and an insertion heater. The results obtained from the three methods are compared and discussed. The enthalpy of dissolution of a-cyclodextrin, dry and hydrated with six water molecules, was determined in water and in formamide. The results are discussed in terms of the difference between water and formamide as regarding dissolution and binding to the cyclodextrin molecule. 2004 Elsevier B.V. All rights reserved.

1716. In situ temperature measurement of an optical fiber submitted to electric arc discharges
Rego, G ; Santos, LMNBF ; Schroder, B ; Marques, PVS ; Santos, JL ; Salgado, HM
in IEEE PHOTONICS TECHNOLOGY LETTERS, 2004, ISSN: 1041-1135,  Volume: 16, 
Article,  Indexed in: crossref, scopus, wos 
Abstract Type S thermocouples were assembled in situ by applying high intensity electric are discharges to the contact junction of two platinum (Pt) and Pt-10% rhodium (Pt-10% Rh) wires, inserted on a silica capillary. The electrically insulated thermocouples built in this way were afterwards employed to estimate the temperature of an optical fiber subjected to arc discharges. For typical values of the arc discharge parameters used to arc-induce long-period fiber gratings (electric current I = 9 mA and arc duration t = 1 s), a capillary peak temperature value of 1420 degreesC +/- 400 degreesC was obtained by extrapolation of the experimental data for the limit situation of having a thermocouple with negligible diameter. The temperature profiles in the capillary and in an optical fiber were calculated based on a heat transfer model implemented by a finite element algorithm and fitted to the experimental temperature distribution in the Pt and Pt-10% Rh wires. The correspondent peak temperatures computed for the capillary and for the fiber were 1450 degreesC and 1320 degreesC, respectively. A good agreement between the capillary temperature values determined graphically and numerically was obtained.

1717. Thermodynamics of the interaction between a hydrophobically modified polyelectrolyte and sodium dodecyl sulfate in aqueous solution
Bai, GY ; Santos, LMNBF ; Nichifor, M ; Lopes, A ; Bastos, M
in JOURNAL OF PHYSICAL CHEMISTRY B, 2004, ISSN: 1520-6106,  Volume: 108, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The thermodynamics of the interaction between a hydrophobically modified cationic polyelectrolyte and an anionic surfactant (sodium dodecyl sulfate, SDS) has been investigated by microcalorimetry, conductivity, and UV-vis spectrophotometry. The polyelectrolyte employed was a newly synthesized polymer (D40OCT30) based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with a degree of substitution (DS) of 28.1%. The interaction between D40OCT30 and SDS was found to be very strong because of the introduction of ionic and hydrophobic moieties on the backbone of the dextran polymer. The aggregation concentration of polyelectrolyte-SDS complex (CAC(complex)) was derived from the curves of variation of the observed enthalpy, solution conductivity, and optical dispersion with SDS concentrations. The results show that these values obtained from different methods are coincident and increase with D40OCT30 concentration. A mechanism of interaction is proposed and discussed in detail in the text. The total interaction enthalpies were derived from the observed enthalpy curves. The results indicate that the total interaction process is entropy-driven. From the calorimetric and turbidity measurements, the partial phase diagram that describes the dependence of the phase boundary on polymer alkyl side chain concentration is also deduced.

1718. Self-aggregation of amphiphilic cationic polyelectrolytes based on polysaccharides
Nichifor, M ; Lopes, S ; Bastos, M ; Lopes, A
in JOURNAL OF PHYSICAL CHEMISTRY B, 2004, ISSN: 1520-6106,  Volume: 108, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The self-aggregation of polyelectrolytes having N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendant groups (alkyl = octyl, dodecyl, or cetyl) randomly distributed along a polysaccharide backbone (dextran) was studied by steady-state fluorescence techniques using several free fluorescent probes or pyrene-labeled polymers and by viscometry. The onset, offset, and highest values of the fluorescence response of N-phenylnaphthylamine (NPN), pyrene (Py), and 1,6-diphenyl-3,5,6-hexatriene (DPH) were corroborated with NPN and DPH anisotropy and quenching experiments to describe the dynamic of hydrophobic microdomain formation and microdomain characteristics. The start of the aggregation process (critical aggregation concentration, cac) and the microdomain characteristics such as polarity, microviscosity, size, and number strongly depend on the alkyl chain length and the degree of substitution with cationic pendant groups. Fluorescence experiments with pyrene-labeled polymers and viscosity data suggest that microdomains are mainly formed by intramolecular hydrophobic associations, except for the polymers carrying octyl groups, where some intermolecular associations were revealed.

1719. Standard molar enthalpies of formation of two crystalline bis[N-(diethylaminothiocarbonyl)benzamidinato]nickel(II) complexes
Ribeiro Da Silva, MAV ; Santos, LMNBF ; Schroder, B
in Journal of Chemical Thermodynamics, 2004, ISSN: 0021-9614,  Volume: 36, 
Article,  Indexed in: crossref, scopus 
Abstract The standard (p°=0.1 MPa) molar enthalpies of formation of the crystalline complexes bis[N-(N″,N″-diethylaminothiocarbonyl) benzamidinato]nickel(II), {Ni(datb)2}, and bis[N-(N″,N″- diethylaminothiocarbonyl)-N′-phenylbenzamidinato]nickel(II), {Ni(datpb)2}, were determined, at T=298.15 K, by high precision solution-reaction calorimetry. ΔfHm°(cr)/ (kJ·mol-1)Ni(datb)26.5 ± 7.9Ni(datpb) 2285.1 ± 11.3 From the obtained results, the metal-ligand exchange enthalpies in the crystalline phase were derived. The enthalpy of a hypothetical metal-ligand exchange reaction in the crystalline phase was derived, thus allowing a discussion of the energetics of complexation in comparison with known crystal-structural parameters.

1720. Standard molar enthalpy of formation of mono thiodibenzoylmethane by rotating-bomb calorimetry
Da Silva, MAVR ; Santos, LMNBF
in JOURNAL OF CHEMICAL THERMODYNAMICS, 2004, ISSN: 0021-9614,  Volume: 36, 
Article,  Indexed in: crossref, scopus, wos 
Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation of crystalline thianthrene and monothiodibenzoylmethane, HDBMS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K of dibenzoylmethane (HDBMO) and monothiodibenzoylmethane (HDBMS) were measured by microcalorimetry. [GRAPHIC] From the present results it could be shown that, in these molecules, the intramolecular hydrogen bond energy (O-H...O), is ca. 20 kJ . mol(-1) larger than for (S-H...O).