, respectively, were derived from the standard molar energies of combustion, in oxygen, to yield CO2(g) and H2O(l), at T = 298.15 K, measured by static bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure
the dependence of the vapour pressure of the solid isomers of hydroxymethylphenol with the temperature, from which the standard
molar enthalpies of sublimation were derived using the Clausius–Clapeyron equation. The results were as follows:
, for 2-, 3- and 4-hydroxymethylphenol, respectively. From these values, the standard molar enthalpies of formation of the
title compounds in their gaseous phases, at T = 298.15 K, were derived and interpreted in terms of molecular structure. Moreover, using estimated values for the heat capacity
differences between the gas and the crystal phases, the standard (p° = 0.1 MPa) molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were derived for the three hydroxymethylphenols.
The isothermal mixing of ( o + m ), ( o + p ), ( m + p )-isomers, which show small enthalpy change, can be explained in terms of differences in the intermolecular interaction between pairs of like and unlike
Authors:Teresa Maria, A. Lopes Jesus, and M. Eusébio
In this study, the thermal behavior of butanediol isomers is investigated for temperatures ranging from 103 to 303 K using
differential scanning calorimetry, complemented, when necessary, by polarized light thermal microscopy. The butanediol isomers
display quite different thermal behaviors: for 1,2- and 1,3-isomers, glass transition is the only thermal event observed;
for 1,4-butanediol, crystallization occurs on cooling even at a high scanning rate and no glass formation was detected; and
for the 2,3-isomer, glass or crystal formation is dependent on the experimental conditions employed. The glass-forming ability
of the isomers is correlated with data available on their molecular symmetry.
The destruction of butyraldehyde isomers has been studied using a silver catalyzed electrochemical oxidation technique. The
destruction rates of nand iso-butyraldehydes were measured as a function of the concentrations of butyraldehyde isomers, silver
and nitric acid in aqueous solutions, anode potential and temperature. More than 99% of n-butyraldehyde in 3M nitric acid
was destroyed within 50 minutes by electrochemical oxidation at the anode potential of 1.6V versus a reference saturated calomel
electrode (SCE). The process was found to be applicable to the destruction of butyraldehyde isomers in nitric acid solutions.
Molecular orbital calculations (EHT, IEHT, IEHT + Madelung correction) have been applied to a series of tin halides. A calculation procedure is described to derive the charge density of the electrons at the nucleus (0). The dependence of the isomer shift on covalency effects is demonstrated. The influence of the outer charge of the molecules on the isomer shift is shown. The relative change of nuclear charge radius R/R is calculated for tin.
Authors:Lorentz Jäntschi, Sorin Hodişan, Claudia Cimpoiu, Anamaria Hosu, Eugen Darvasi, and Teodor Hodişan
Modeling of thin-layer chromatographic separation of androstane isomers to find the optimum mobile phase is described in this paper. The isomers of androstane are present in a variety of samples, so achieving their optimum thin-layer chromatographic separation is very important. A mathematical model was developed and tested. The model takes into account the interaction between solvents and uses a complex function for modeling, so it provides reliable results. The proposed mathematical model gives results similar to those obtained by use of other optimization models, for example the ‘Simplex’ and ‘Prisma’ methods.
Authors:M. Kildir, Z. Morel, Z. Büyükmumcu, and H. Erten
Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of235U are compared with those calculated from the simple statistical model byMadland andEngland. The method of calculation has been extended to the isotopes having more than one isomeric state. The results may be explained according to the multi-exit-channel model of fission.
The separation and quantification of nitrophenol isomers in aqueous solutions by capillary isotachophoresis has been studied. Several electrolyte systems with different leading and terminating ions were used. The separation was carried out in a PTFE column in a device for column-coupling system.
Authors:T. Baba, T. Yamada, Y. Nishikawa, and H. Yoshida
stability of para (p--)
and ortho (o-)
isomers was investigated by CRTG and reaction kinetic analysis. The temperature
started the mass decrease of o-isomer was
about 20C lower than that of p-isomer
by CRTG. The activation energies of thermal decomposition of o-
and p-isomers were 136.9 and 153.4 kJ mol–1,
respectively. The effect of steric hindrance on heat of formation was calculated
by AM1 method using Win MOPAC3.0 for the model compound of p-
and o-isomers. The lower stability of o-isomer was the results of the steric hindrance
between the ethylene unit of aromatic ring and three alkyl chains.
Authors:Zsuzsa Király-Véghely, György Kátay, Ernő Tyihák, and Jean-Michel Merillon
A simple method is described for separation and determination of the isomers of
-resveratrol and related compounds from different types of red wine by overpressured layer chromatography (OPLC). Comparison of OPLC with TLC clearly showed the advantages of the forced-flow technique (higher theoretical plate number, good resolution, etc.) over conventional planar layer liquid chromatography. It was established that the glycosides of resveratrol isomers were always present in higher concentrations than free stilbene isomers in red wine samples. This was especially true for the Pinot Noir wine. Exploitation of the advantages of OPLC provides further possibilities of analysis and isolation of stilbene isomers from grapes and wine.