14 research outputs found
Thermochemical Study of 1-Methylhydantoin
Using static bomb combustion calorimetry, the combustion energy of 1-methylhydantoin was obtained, from which the standard molar enthalpy of formation of the crystalline phase at T = 298.15 K of the compound studied was calculated. Through thermogravimetry, mass loss rates were measured as a function of temperature, from which the enthalpy of vaporization was calculated. Additionally, some properties of fusion were determined by differential scanning calorimetry, such as enthalpy and temperature. Adding the enthalpy of fusion to the enthalpy of vaporization, the enthalpy of sublimation of the compound was obtained at T = 298.15 K. By combining the enthalpy of formation of the compound in crystalline phase with its enthalpy of sublimation, the respective standard molar enthalpy of formation in the gas phase was calculated. On the other hand, the results obtained in the present work were compared with those of other derivatives of hydantoin, with which the effect of the change of some substituents in the base heterocyclic ring was evaluated
Experimental and Theoretical Thermochemistry of the Isomers 3- and 4‑Nitrophthalimide
This
work presents a thermochemical study of two derivatives of
phthalimide: the isomers 3-nitrophthalimide and 4-nitrophthalimide.
The enthalpies of formation for these compounds in the solid phase
were obtained by combustion calorimetry. Using ths thermogravimetry
technique, the enthalpies of vaporization were obtained. The enthalpies
of sublimation were calculated from enthalpies of fusion and vaporization.
From experimental data and by ab initio methods, the enthalpies of
formation in the gas phase were calculated. With these results, it
was possible to determine their relative stability, and it was found
that 4-nitrophthalimide is more stable than its isomer 3-nitrophthalimide.
This tendency is similar to that of 3-nitrophthalic anhydride and
4-nitrophthalic anhydride, as reported in a previous work by our research
group. The enthalpy of isomerization was also obtained, and a good
correlation with that of phthalic anhydride derivatives was found.
Finally, with the values obtained, the enthalpic difference resulting
when the imide group is substituted by an anhydride group was determined
The Intramolecular Hydrogen Bond N–H···S in 2,2′-Diaminodiphenyl Disulfide: Experimental and Computational Thermochemistry
The
intramolecular hydrogen bond of the N–H···S
type has been investigated sparingly by thermochemical and computational
methods. In order to study this interaction, the standard molar enthalpies
of formation in gaseous phase of diphenyl disulfide, 2,2′-diaminodiphenyl
disulfide and 4,4′-diaminodiphenyl disulfide at <i>T</i> = 298.15 K were determined by experimental thermochemical methods
and computational calculations. The experimental enthalpies of formation
in gas-phase were obtained from enthalpies of formation in crystalline
phase and enthalpies of sublimation. Enthalpies of formation in crystalline
phase were obtained using rotatory bomb combustion calorimetry. By
thermogravimetry, enthalpies of vaporization were obtained, and by
combining them with enthalpies of fusion, the enthalpies of sublimation
were calculated. The Gaussian-4 procedure and the atomization method
were applied to obtain enthalpies of formation in gas-phase of the
compounds under study. Theoretical and experimental values are in
good agreement. Through natural bond orbital (NBO) analysis and a
topological analysis of the electronic density, the intramolecular
hydrogen bridge (N–H···S) in the 2,2′-diaminodiphenyl
disulfide was confirmed. Finally, an enthalpic difference of 11.8
kJ·mol<sup>–1</sup> between the 2,2′-diaminodiphenyl
disulfide and 4,4′-diaminodiphenyl disulfide was found, which
is attributed to the intramolecular N–H···S
interaction
Experimental and Computational Thermochemistry of 3- and 4‑Nitrophthalic Anhydride
In order to understand the influence
that the position of the nitro
group on the aromatic ring has on the relative stability of two isomers,
the standard enthalpies of formation of 3- and 4-nitrophthalic anhydride
in the gaseous phase, at <i>T</i> = 298.15 K, were obtained
by experimental thermochemistry and theoretical studies. The standard
enthalpies of formation in the crystalline phase, at <i>T</i> = 298.15 K, were obtained by combustion calorimetry and the enthalpies
of sublimation by the Knudsen method. For the theoretical calculations,
a standard ab initio molecular orbital method at the G3 level was
used. The enthalpies of formation in the gaseous phase were obtained
from atomization and isodesmic reactions. A theoretical study of the
molecular and electronic structures of these compounds was also performed.
Differences of −9.7 kJ·mol<sup>–1</sup>, for 3-nitrophthalic
anhydride, and −2.6 kJ·mol<sup>–1</sup> for 4-nitrophthalic
anhydride, were found from a comparison between our theoretical and
experimental results
Isothermal Thermogravimetric Study for Determining Sublimation Enthalpies of Some Hydroxyflavones
The sublimation enthalpies of some
hydroxyflavones and one amineflavone
were determined with a thermogravimetric device under isothermal conditions.
These enthalpies were obtained by measuring the rate of mass loss
as a function of temperature. In this methodology, the Clausius–Clapeyron
and Langmuir equations were used. The diffusional effect of the gas
phase was included in the Langmuir equation. In order to test and
validate the experimental methodology, the sublimation enthalpy of
three standard materials, anthracene, pyrene, and benzoic acid, were
determined. The values obtained are in agreement with the data reported
and recommended in the literature. Low uncertainties were obtained
in all thermogravimetric measurements. Additionally, by differential
scanning calorimetry, the molar fraction, temperature and enthalpy
of fusion, and heat capacity of the solid phase were determined for
all of the compounds studied. The heat capacities of the gas phase
were estimated using computational methods. Isothermal thermogravimetry
was applied to study a family of flavones