Publications

Abstract <jats:p>Fullerenes are reactive as dienophiles in Diels–Alder reactions. Their distinctive molecular shape and properties result in interesting and sometimes elusive reaction patterns. Herein, to contribute to the understanding of fullerene reactivity, we evaluate the energies of reactions for Diels–Alder cycloadditions of C60, C70, and IC60MA with anthracene (Ant), by means of DFT computational analysis in vacuum and solution. The methods used showed little differentiation between the reactivity of the different fullerenes. The C70-Ant adducts where addition takes place near the edge of the fullerene were found to be the most stable regioisomers. For the IC60MA-Ant adducts, the calculated energies of reaction increase in the order: equatorial &gt; trans-3 &gt; trans-2 ≈ trans-4 ≈ trans-1 &gt; cis-3 &gt; cis-2. The change in the functional suggests the existence of stabilizing dispersive interactions between the surface of the fullerene and the addends. HOMA (harmonic oscillator model of aromaticity) analysis indicated an increase in aromaticity in the fullerene hexagons adjacent to the bonded addend. This increase is bigger in the rings of bisadduct isomers that are simultaneously adjacent to both addends, which helps explain the extra stability of the equatorial isomers. Solvation by m-xylene decreases the exothermicity of the reactions studied but has little distinguishing effect on the possible isomers. Thermal corrections reduce the exothermicity of the reactions by ~10 kJ∙mol−1.</jats:p>

Abstract This work reports a comprehensive experimental evaluation of the solid-liquid-gas phase equilibria for five representative phenylene-thiophene co-oligomers (3-ring aromatic compounds having both phenyl and thienyl units). The melting temperatures and corresponding standard molar enthalpies and entropies of fusion were measured by differential scanning calorimetry. The equilibrium vapor pressures of the crystalline solids as a function of temperature were measured by a combined Knudsen/quartz-crystal effusion method, with the consequent derivation of the standard molar enthalpies, entropies, and Gibbs energies of sublimation. The thermodynamic properties of vaporization were estimated from the fusion and sublimation data. The results were analyzed together with the literature data for the corresponding phenylene and thiophene homo-oligomers. The thermodynamic properties of fusion and sublimation exhibited a dependence on ring identity and position that cannot be adequately described by a simple group additivity reasoning. The plot of the Gibbs energy of sublimation as a function of the number of thienyl rings in the co-oligomer showed the existence of two series. Terminal 3-thienyl rings and a linear molecular shape were found to be consistent factors contributing to the stabilization of the crystal phase. The higher melting temperatures and lower volatilities of crystalline 3-thienyl compounds were tentatively explained by the ability of these rings to maximize intermolecular C-H & BULL;& BULL;& BULL;pi interactions independently of the sulfur position. The optical energy gaps, as measured by UV-vis in solution, were found to lie within the values for typical organic semiconductors (< 4 eV) and to decrease for co-oligomers containing more 2-thienyl units, following the increased ring-ring planarity of the molecules. The surface morphology of vapor-deposited thin films suggests a stronger tendency of the co-oligomers, if compared to their corresponding homo-oligomers p-terphenyl and terthiophene, to form less amorphous films.

Abstract We report a study on the energetics and structural properties of gallic (1) and ellagic (2) acids. The experimental values of standard enthalpy of formation in solid state at 298.15 K, Delta H-f(m)0 (cd) of 1 as (-985.0 +/- 2.9 kJ.mol(-1)) and 2 as (-1377.9 +/- 4.7 kJ.mol(-1)) have been determined. The vapour pressure of 1 have been measure by Knudsen effusion methodology and the derived enthalpy of sublimation, Delta H-g(cd)m(0). was combined with the Delta H-f(m)0 (cd) in order to derive its gas-phase enthalpy of formation, Delta(f) H-m(0)(1,g) = -835.7 +/- 4.0 kJ.mol(-1). Quantum chemical calculations, at DFT (M05-2X) and composite ab initio Gn (n = 3, 4) levels of theory, provided the consistency of the experimental results and a plausible estimation of Delta H-f(m)0 (g) of 2 as (-1128.6 +/- 6.4 kJ.mol(-1)), which was deduced from the isodesmic-reactions methodology. (C) 2018 Published by Elsevier Ltd.

Abstract This work aims to provide reliable heat capacities for ethylene glycol and selected oligo(ethylene glycol)s (diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, and hexaethylene glycol), which are industrially important chemicals produced on a large scale. Besides, new data extend the database needed for a better understanding of complex behavior of compounds capable of forming hydrogen bonds. Isobaric heat capacities of ethylene glycols were measured with a Tian-Calvet-type calorimeter in the temperature range of 260 to 358 K. The phase behavior was investigated with a heat-flux differential scanning calorimeter. A simple additive estimation method for liquid heat capacity of oligo(ethylene glycol)s was developed and tested through comparison with newly measured liquid heat capacities of polyethylene glycols 400 and 600.

Abstract This paper reports thermodynamic properties of phase transitions of 2,4,6-trichloro and 2,4,6-tribromo anisoles and of 2,4,6-tribromophenol. The vapor pressures of both crystalline and liquid phases (including supercooled liquid) of the three compounds were measured, respectively, in the temperature ranges T = (297.1 to 368.3) K, T = (330.7 to 391.7) K, and T = (336.5 to 401.7) K, using a static method based on capacitance diaphragm manometers. Moreover, the sublimation vapor pressures of 2,4,6-tribromophenol were also measured in the temperature interval (307.2 to 329.2) K, using a Knudsen mass-loss effusion technique. The standard molar enthalpies, entropies, and Gibbs energies of sublimation and of vaporization, at reference temperatures, were derived from the experimental results as well as the (p,T) values of the triple point of each compound. The temperatures and molar enthalpies of fusion of the three benzene derivatives were determined using differential scanning calorimetry and were compared with the values derived indirectly from the vapor pressure measurements. The thermodynamic results were discussed together with the available literature data for 2,4,6-trichlorophenol. To help rationalize the phase behavior of these substances, the crystallographic structure of 2,4,6-tribromophenol was determined by single crystal X-ray diffraction.

Abstract Oxidative treatment of Multi-Walled Carbon Nanotubes (MWCNT's) was done by chemical functionalization by using the mixture acid, which is a mixture of sulfuric acid (H2SO4) and nitric acid (HNO3). Functionalization was governed by four parameters namely mixture acid concentration, temperature(T), time of heating(t) and the amount of MWCNTs used. After functionalization, functionalized MWCNT's were then diluted in dimethylformamide (DMF) to analyse the percentage of soluble MWCNT's. Also, by increasing the time of functionalization, it was observed that overall yield decreases but the percentage of functionalized product inside the yield quantity remains the same. Material characterization was also carried out at several steps to validate this theory. Chemical functionalization of MWCNT's is generally significant for the manufacturing of polymerbased nanocomposites. Oxidative treatment enhances the dispersion and interfacial bonding within the epoxy matrix. In this research work, a bio-based epoxy resin was selected for the manufacturing of nanocomposite samples with various concentrations of pristine and functionalized MWCNTs. Mechanical and electrical characterization was finally carried out to increase the knowledge on the interaction of MWCNT's with the selected green epoxy matrix system and their influence on the original properties of the resin. (C) 2019 The Authors. Published by Elsevier B.V.

Abstract Herein, we present experimental evidence that protic ionic liquids (PILs), derived from 1:1 liquid mixtures of the organic superbases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with carboxylic acids, form azeotropic mixtures with acid/base molar fractions different from 1:1. The ability of the carboxylic acids to form strong hydrogen bonds with the PIL ion pair leads to an azeotropic composition richer in the acid component. The results show that the azeotropic composition is ruled by the extent of acid-base equilibrium and the relative volatility of the neutral species in the PIL medium. The PILs show marked negative deviations from Raoult's Law with the stronger superbase (DBU) leading to an azeotropic composition closer to the equimolar 1:1 ratio.

Abstract The synthesis and reactivity of 2-[(1E,3E)-4-arylbuta1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels-Alder (DA) reactions with several electron-poor and electron-rich dienophiles under microwave irradiation was studied. The optimized reaction conditions were achieved with N-methylmaleimide as the dienophile and Sc(OTf)3 (OTf = triflate) as a Lewis acid under microwave-assisted and solvent-free conditions. The Lewis acid improved the reaction yields as it prevented the adducts obtained from undergoing a second DA reaction; thus, the for-mation of a bisadduct was avoided. The alpha,beta,gamma,delta-diene of the starting chromones was the most reactive, and the computational results confirmed the experimental findings. Theoretical calculations also provided a rationale for the unexpected lack of reactivity shown by some dienophiles. The adducts prepared were dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); however, the aza adducts were sensitive to the highly energetic reaction conditions necessary for the aromatization.

Abstract A comprehensive thermodynamic study of the whole ortho-polyphenylbenzenes series from biphenyl (n = 1) to hexaphenylbenzene (n = 6) is presented. Combustion calorimetry and phase equilibria measurements for 1,2,3,4-tetraphenylbenzene (n = 4) and pentaphenylbenzene (n = 5) together with literature data were used to understand and quantify the constraint effect of ortho-substitution on the molecular energetics and phase stability of polyaromatic compounds. All of the derived thermodynamic properties (enthalpy of sublimation, entropy of sublimation, and gas phase molecular energetics) show a marked trend shift at n = 4 to 5, which is related to the change of the degree of molecular flexibility after 1,2,3,4-tetraphenylbenzene (n = 4). The greater intramolecular constraint in the more crowded members of the series (n = 5 and 6) leads to a significant change in the molecular properties and cohesive energy. The trend shift in the molecular properties is related with the decrease in molecular flexibility, which leads to lower molecular entropy and destabilization of the intramolecular interaction potential due to the increased hindrance in a confined molecular space.

Abstract Methylation at the C2 position of 1,3-disubstituted imidazolium-based ionic liquids (ILs) is one of the structural features that has gained attention due to its drastic impact on thermophysical and transport properties. Several hypotheses have been proposed to explain this effect but there is still much discrepancy. Aiming for the rationalization of the effects of these structural features on the properties of imidazolium ILs, we present a thermodynamic and computational study of two methylated ILs at the C2 position of imidazolium, [C-1(4) C-2(1) (3)C(1)im][NTf2] and [C-1(3) C-2(1) (3)C(1)im][NTf2]. The phase behaviour (glass transition and vaporization equilibrium) and computational studies of the anion rotation around the cation and ion pair interaction energies for both ILs were explored. The results have shown that C2-methylation has no impact on the enthalpy of vaporization. However, it decreases the entropy of vaporization, which is a consequence of the change in the ion pair dynamics that affects both the liquid and gas phases. In addition, the more hindered dynamics of the ion pair are also reflected in the increase in the glass transition temperature, Tg. The entropic contribution of anion-around-cation rotation in the imidazolium [NTf2] ILs was quantified experimentally by the comparative analysis of the entropy of vaporization, and computationally by the calculation of the entropies of hindered internal rotation. The global results exclude the existence of significant H-bonding in the C2-protonated (non-methylated) ILs and explain the C2-methylation effect in terms of reduced entropy of the ion pair in the liquid and gas phases. In light of these results, the C2-methylation effect is intrinsically entropic and originates from the more hindered anion-around-cation rotation as a consequence of the substitution of the -H with a bulkier -CH3 group.