Conformational Landscape and Polymorphism in 5-Acetic Acid Hydantoin.

The conformational space of 5-acetic acid hydantoin {5AAH; [2-(2,5-dioxoimidazolidin-4-yl)acetic acid]} was investigated by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of theory. A total of 13 conformers were located in the potential energy surface of the molecule, six of them bearing the carboxylic group in the cis arrangement (O═C-O-H dihedral equal to ∼0°) and the other seven possessing this group in the trans configuration (O═C-O-H dihedral equal to ∼180°). The most stable conformer (cis-I) was trapped from the gas phase into a low temperature argon matrix (10 K), and its infrared spectrum was fully assigned, also with help of results of normal coordinates' analysis based on the DFT computed vibrational data. The electronic structure of this conformer was analyzed by using the natural bond orbital (NBO) method. The investigation of the thermal properties of 5AAH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy, allowing identification of five different polymorphs. Very interestingly, in the room temperature stable polymorph the molecular units of 5AAH assume the geometry of the highest-energy conformer predicted by the calculations for the isolated molecule.

Free volume in the smectic-E phase of 4-hexyl-4' isothiocyanatobiphenyl studied by positron annihilation spectroscopy.

Positron annihilation lifetime spectroscopy has been used to study 4-hexyl-4'-isothiocyanatobiphenyl. Changes of the orthopositronium lifetime parameters with temperature have been observed for the supercooled smectic-E phase. The measurements confirm that positronium is created and annihilates in a layer of a lower electron density containing alkyl chains of molecules. The two-state bond-lattice model of glass transition explains the thermal activation of the centers where orthopositronium is created and annihilates when the glass of the smectic-E phase softens. However, the subsequent cold crystallization of the softened regions also influences the orthopositronium lifetime and intensity, which complicates the picture seen by positrons. The measurements during isothermal crystallization suggest that it progresses in two stages. The first stage can be described by the Avrami equation with the Avrami exponent close to unity, which indicates low-dimensional crystallization. Similarly to liquid n alkanes, the application of pressure is equivalent to temperature lowering with the similar equivalence relationship between pressure and temperature, which seems to confirm the structure of the smectic-E phase with sublayers containing alkyl chains in a molten state. The dependence of the orthopositronium lifetime on pressure for the smectic-E phase may be described by the bubble model where the positronium bubble is approximated with a finite square potential well with the depth of U=1.45eV.

5-Methylhydantoin: From Isolated Molecules in a Low-Temperature Argon Matrix to Solid State Polymorphs Characterization.

The molecular structure, vibrational spectra and photochemistry of 5-methylhydantoin (C4H6N2O2; 5-MH) were studied by matrix isolation infrared spectroscopy and theoretical calculations at the DFT(B3LYP)/6-311++G(d,p) theory level. The natural bond orbital (NBO) analysis approach was used to study in detail the electronic structure of the minimum energy structure of 5-MH, namely the specific characteristics of the σ and π electronic systems of the molecule and the stabilizing orbital interactions. UV irradiation of 5-MH isolated in argon matrix resulted in its photofragmentation through a single photochemical pathway, yielding isocyanic acid, ethanimine, and carbon monoxide, thus following a pattern already observed before for the parent hydantoin and 1-methylhydantoin molecules. The investigation of the thermal properties of 5-MH was undertaken by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM) and Raman spectroscopy. Four different polymorphs of 5-MH were identified. The crystal structure of one of the polymorphs, for which it was possible to grow up suitable crystals, was determined by X-ray diffraction (XRD). Two of the additional polymorphs were characterized by powder XRD, which confirmed the molecules pack in different crystallographic arrangements.

Thermally induced sigmatropic isomerization of pseudosaccharyl allylic ether.

The thermally induced sigmatropic isomerization of the pseudosaccharyl allylic ether [3-(allyloxy)-1,2-benzisothiazole 1,1-dioxide; ABID] has been investigated by a multidisciplinary approach using temperature dependent infrared spectroscopy, differential scanning calorimetry, and polarized light thermomicroscopy, complemented by theoretical methods. Migration of the allylic system from O to N occurs in the melted ABID, and the thermally obtained 2-allyl-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (ABIOD) starts to be produced at ca. 150 degrees C, in a process with an activation energy of approximately 92 kJ mol(-1). From kinetic data, a concerted [3,3'] sigmatropic mechanism is proposed. In the temperature range investigated, ABIOD was found to exhibit polymorphism. Cooling of the molten compound leads to the production of a metastable crystalline form, which upon annealing at room temperature might be transformed to the stable crystalline phase. ABID shows a single crystalline variety. Assignments were proposed for the infrared spectra of the observed neat condensed phases of the two compounds.

Thermal behaviour of human stratum corneum. A differential scanning calorimetry study at high scanning rates.

PURPOSE: To use high-speed differential scanning calorimetry (DSC) in the identification of transitions in human stratum corneum (SC). Several scanning rates (100 degrees C/min to 400 degrees C/min) are used. RESULTS: Eight transitions from 0 to 120 degrees C are detected in a significant number of samples. Most of these transitions have already been identified in previous studies, but have been labeled considering essentially that only four are present. Results also indicate some degree of reversibility for transitions occurring at temperatures above 90 degrees C. Dehydrated SC samples displayed slightly more defined transition peaks and a less frequent presence of the transitions below 50 degrees C. In turn, the delipidised SC matrix showed two major endothermic signals, centered around 55 and 100 degrees C, in conjunction with other much less marked features. CONCLUSIONS: The interpretation of DSC traces in terms of four main transition temperatures must be complemented having in mind the occurrence of other transitions, some of them at physiological temperatures. This work further suggests that transitions at temperatures above 90 degrees C may to a large degree be associated to lipids, while transition at approx. 55 degrees C is probably related to lipids covalently linked to proteins, as previously suggested.

Study of human stratum corneum and extracted lipids by thermomicroscopy and DSC.

A study on the thermal behavior of human stratum corneum and lipids is described. The use of high scanning rate DSC for both SC and extracted lipids allows the consistent determination of transition temperatures, including those of lower energy. Changes are found both at physiological and higher temperatures. There is a clear correspondence between the thermotropic behavior of these two systems. However, one of the transitions found in human SC (approximately 55 degrees C) is absent in extracted lipids and may be ascribed to those covalently-linked to corneocytes. Lipidic thermotropic behavior is clearly found above 100 degrees C, in which proteins do not play an exclusive role. Changes related to most transitions are observed directly by polarized light thermal microscopy in extracted lipids. This technique also allowed for the observation of large segregated domains in the extracted lipids. A drastic change is observed at approximately 60 degrees C, corresponding to the disruption of the lamellar structure.