Enthalpy of sublimation in the study of the solid state of organic compounds. Application to erythritol and threitol

The enthalpies of sublimation of erythritol and L-threitol have been determined at 298.15 K by calorimetry. The values obtained for the two diastereomers differ from one another by 17 kJ mol(-1). An interpretation of these results is based on the decomposition of this thermodynamic property in a term coming from the intermolecular interactions of the molecules in the crystal (delta(int)H degrees) and another one related with the conformational change of the molecules on going from the crystal lattice to the most stable forms in the gas phase (delta(conf)H degrees). This last term was calculated from the values of the enthalpy of the molecules in the gas state and of the enthalpy of the isolated molecules with the crystal conformation. Both quantities were obtained by density functional theory (DFT) calculations at the B3LYP/6-311G++(d,p) level of theory. The results obtained in this study show that the most important contribution to the differences observed in the enthalpy of sublimation are the differences in the enthalpy of conformational change (13 kJ mol(-1)) rather than different intermolecular forces exhibited in the solid phase. This is explained by the lower enthalpy of threitol in the gas phase relative to erythritol, which is attributed to the higher strength of the intramolecular hydrogen bonds in the former. The comparison of the calculated infrared spectra obtained for the two compounds in the gas phase supports this interpretation.     

Aerosol and gaseous radioidine concentration in the air of Prague after the Chernobyl accident

Data on¹³¹I concentration in the atmosphere of Prague observed during the first days after the Chernobyl accident are presented. The sampling device enabling the differentiation between aerosol-fixed and gaseous form of¹³¹I is briefly described. The highest total¹³¹I concentration, 63 Bq.m^–3, was observed between 30 April and 1 May. Until 9 May the level of¹³¹I activity ranged between 14 and 1 Bq.m^–3 and then dropped below 1 Bq.m^–3. The content of gaseous¹³¹I was found to be a significant and represented on average 60–80% of its total activity.     

Isocratic networks in supercritical fluid chromatography II: Selectivities and effective plate numbers

The dependence of the selectivity α and the effective plate number, N, on temperature and pressure is shown in three-dimensional diagrams. The specific system studied was pentane as the mobile phase, unmodified silica as the stationary phase, and a set of four polycyclic hydrocarbons as the test mixture. Temperature and pressure ranges were from 25 to 300°C and from 30 to 75 bar, respectively. The plots for α and N are found to resemble those for capacity ratio, k′, and resolution, R, studied earlier with the same system [1]. A distinct maximum is observed for N in the supercritical region, as found earlier for k′ and R. This applies also to α for the homologous substrate pair naphthalene/anthracene, while the resemblance for α is less pronounced for the pair anthracene/pyrene.     

Stochastic boundary value problems: a white noise functional approach

Summary We give a program for solving stochastic boundary value problems involving functionals of (multiparameter) white noise. As an example we solve the stochastic Schrödinger equation {ie391-1} whereV is a positive, noisy potential. We represent the potentialV by a white noise functional and interpret the product of the two distribution valued processesV andu as a Wick productV u. Such an interpretation is in accordance with the usual interpretation of a white noise product in ordinary stochastic differential equations. The solutionu will not be a generalized white noise functional but can be represented as anL ¹ functional process.     

Optimization and validation of an enantioselective method for a chiral drug with eight stereo-isomers in capillary electrophoresis

Highly selective capillary electrophoresis (CE) screening methods were applied to find a satisfactory separation of a chiral drug with eight stereoisomeric compounds. The initial separation conditions were further optimized using response surface modelling by applying a Box-Behnken experimental design. This approach resulted in a rapid and efficient optimization of the buffer concentration, the concentration of two cyclodextrins, and the run voltage, in order to obtain final separation conditions of the method. Further optimization and validation of the system in terms of sensitivity and robustness resulted in a method that is suitable for quality control release purposes.     

Atomic and electronic structure of unreduced and reduced CeO2 surfaces: a first-principles study

The atomic and electronic structure of (111), (110), and (100) surfaces of ceria (CeO2) were studied using density-functional theory within the generalized gradient approximation. Both stoichiometric surfaces and surfaces with oxygen vacancies (unreduced and reduced surfaces, respectively) have been examined. It is found that the (111) surface is the most stable among the considered surfaces, followed by (110) and (100) surfaces, in agreement with experimental observations and previous theoretical results. Different features of relaxation are found for the three surfaces. While the (111) surface undergoes very small relaxation, considerably larger relaxations are found for the (110) and (100) surfaces. The formation of an oxygen vacancy is closely related to the surface structure and occurs more easily for the (110) surface than for (111). The preferred vacancy location is in the surface layer for CeO2(110) and in the subsurface layer (the second O-atomic layer) for CeO2(111). For both surfaces, the O vacancy forms more readily than in the bulk. An interesting oscillatory behavior is found for the vacancy formation energy in the upper three layers of CeO2(111). Analysis of the reduced surfaces suggests that the additional charge resulting from the formation of the oxygen vacancies is localized in the first three layers of the surface. Furthermore, they are not only trapped in the 4f states of cerium.