Publications

Abstract The energetic study of 4-nitro-2,1,3-benzothiadiazole has been developed using experimental techniques together with computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpy of formation of crystalline 4-nitro-2,1,3-benzothiadiazole (181.9 +/- 2.3 kJ . mol(-1)) was determined from the experimental standard molar energy of combustion -(3574.3 +/- 1.3) kJ . mol(-1), in oxygen, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The standard (p degrees = 0.1 MPa) molar enthalpy of sublimation, at T = 298.15 K, (101.8 +/- 4.3) kJ . mol(-1), was determined by a direct method, using the vacuum drop micro-calorimetric technique. From the latter value and from the enthalpy of formation of the solid, it was calculated the standard (p degrees = 0.1 MPa) enthalpy of formation of gaseous 4-nitro-2,1,3-benzothiadiazole as (283.7 +/- 4.9) kJ . mol(-1). Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation 4-nitro-2,1,3-benzothiadiazole. The ab initio results are in good agreement with the experimental data.

Abstract The present work reports the values of the standard (p degrees = 0.1 MPa) molar energies of combustion for the liquid isomers methyl picolinate and methyl isonicotinate and for the crystalline isomer methyl nicotinate measured by static bomb calorimetry. The molar enthalpies of vaporization of the three isomers at T = 298.15 K as well as the molar enthalpies of sublimation of two of them (methyl picolinate and methyl nicotinate) were derived from vapor pressure measurements at different temperatures using a static method. The molar enthalpies of vaporization at T = 298.15 K of methyl picolinate and methyl isonicotinate were also measured using Calvet microcalorimetry. The thermal behavior of the compounds was studied by differential scanning calorimetry, and the temperatures and the molar enthalpies of fusion were determined using this technique. The experimental results were used to derive the gaseous standard molar enthalpies of formation, at 298.15 K, of the three pyridinecarboxylic acid methyl ester isomers and the triple points of the ortho and meta isomers.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline picolinamide (2-NH2COPy), nicotinamide (3-NH2COPy), isonicotinamide (4-NH2COPy), nicotinamide N-oxide (3-NH2COPyNO), and isonicotinamide N-oxide (4-NH2COPyNO) were measured by static-bomb calorimetry. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three pyridinecarboxamide isomers were measured by microcalorimetry and the standard molar enthalpies of sublimation for the two pyridinecarboxamide N-oxide compounds were measured by a mass-loss effusion technique. From the enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies D-m(o) of the (N+-O-) covalent bonds were derived. Comparison has been made with D-m(o) (N-O) values in pyridine N-oxide derivatives. [GRAPHICS] (C) 2001 Academic Press.

Abstract The adsorption behaviour of n-hexanol on a Au (111) single-crystal electrode from aqueous 0.2 mol dm(-3) NaClO4 was studied on the basis of dynamic capacity measurements and of capacitive charge measurements by chronocoulometry. The charge density, sigma(M), as a function of the applied potential, E, and of the n-hexanol concentration was analysed thermodynamically, yielding the Gibbs surface excess, Gamma, and the Gibbs energy of adsorption, Delta(ads)G degrees, of n-hexanol as a function of potential and concentration. The results of the thermodynamic analysis of the adsorption data at the potential of maximum adsorption are discussed in comparison with the literature data for the adsorption of this compound on Hg [1,2] and on silver single-crystals [3,4]. The results obtained show that the adsorption process could be described by a Frumkin isotherm, the parameters of which were determined and discussed. The values of the standard Gibbs energy of adsorption suggest physical adsorption of the alcohol, without the existence of specific interactions of the adsorbed molecules with gold. Differences and inconsistencies for the results obtained from the analysis of charge and capacity data are discussed in terms of the experimental technique.

Abstract The behaviour of several high Miller index crystal surfaces of gold is examined by cyclic voltammetry in 0.01 mol dm(-3) perchloric acid solutions. The potentials of zero charge, E(sigma=0), and their temperature coefficients, dE(sigma=0)/dT for the stepped faces (332), (771), (320) and (410) are determined from a single minimum of the differential capacity curves. The results are compared with those existing for other stepped and low-index crystal faces in order to establish the relevance of the crystallographic structure in the double layer parameters. The influence of the step density and step orientation is discussed. The entropies of formation, S*, are calculated and discussed in terms of the gold-water interactions.

Abstract The adsorption of mannitol and sorbitol on Au(111) has been followed by differential capacity measurements. Results obtained for the adsorption of these polyalcohols in dilute solutions of HClO4 and NaClO4 showed that the adsorption process could be described by a Frumkin isotherm, the parameters of which were determined and discussed. The values of the standard Gibbs energy of adsorption suggest physical adsorption of mannitol and sorbitol, without the existence of specific interactions of the adsorbed molecules with gold. The values of the interaction parameters are negative in both electrolytes, indicating a net attractive interaction in the adsorbed layer. There are noticeable differences between the values of the standard Gibbs energy of adsorption and of the interaction parameter for the two polyols, which have been interpreted in terms of the conformational properties of the two isomers in the adsorbed layer.

Abstract The behaviour of Au(210) in HNO3 and NaNO3 solutions was studied with cyclic voltammetry and differential capacity measurements as a function of concentration and temperature. The effect of concentration on both j-E profiles and C(E) curves suggests that for low concentrations and around zero charge potential, NO3- adsorption is similar to ClO4- adsorption on Au(210). For higher concentrations, the adsorption of NO3- ions at positive charges seems to occur to a greater extent than ClO4- ions. The entropy of formation of the electrochemical interface was derived from the effect of temperature in HNO3 solution. It was found to be qualitatively similar to what was observed in HClO4, solution.

Abstract Potentials of zero charge, E(sigma=0), and the temperature coefficients of E(sigma=0) for stepped surfaces of sold single crystals (511), (533), (331) and (311) were obtained from the minimum of the differential capacity curve in dilute HClO4 solutions. The results are compared with those obtained for the low index faces and dilute in terms of the effect of the surface density of steps and the step orientation. The possible relevance of the effect of steps on the interaction of water with gold surfaces is discussed.

Abstract The minimum in the differential capacity of (111), (100), (110), (210) and (311) single-crystal gold faces in contact with dilute solutions of HClO4 as a function of the temperature was used to estimate the temperature coefficient of the potential of zero charge of these interfaces. The results are discussed in terms of the interaction of water with the metallic surfaces and the role played by the surface atomic arrangement. The strength of Au (hkl)-water interactions are proposed to be in the order Au (111) > Au (311) > Au (100) > Au (110) > Au (210) ≈ Hg. © 1990.