Engineering reactions in crystalline solids: predicting photochemical decarbonylation from calculated thermochemical parameters

A detailed thermochemical analysis of the alpha-cleavage and decarbonylation reactions of acetone and several ketodiesters was carried out with the B3LYP/6-31G* density functional method. The heats of formation of several ground-state ketones and radicals were calculated at 298 K to determine bond dissociation energies (BDE) and radical stabilization energies (RSE) as a function of substituents. Results show that the radical-stabilizing abilities of the ketone substituents play a very important role on the thermodynamics of the alpha-cleavage and decarbonylation steps. An excellent correlation between calculated values and previous experimental observations suggests that photochemical alpha-cleavage and decarbonylation in crystals should be predictable from knowledge of excitation energies and the RSE of the substituent.     

Study on resolution capacity and secondary mechanisms of three different TSK gels for organic SEC

A comparative study on the elution behavior in size-exclusion chromatography (SEC) of three polymeric TSK gel packings, named H, H(HR), and H(XL) types, is performed. The deviations of the universal calibration (u.c.) curves of seven solvent-polymer systems show evidence that the existence of secondary effects accompanying the main SEC mechanism. These secondary mechanisms are a consequence of the binary enthalpic interactions between the different components of the chromatographic system, such as polymer-solvent, polymer-gel and solvent-gel. However, in the present case, the observed deviations from a unique u.c. curve can mainly be attributed to adsorption of polymeric solutes (analytes) onto the residual moieties of the gel surfaces, (i.e., to polymer-gel interactions). These enthalpic interactions can be quantitated by the values of the distribution coefficient, Kp, which follows the order: Kp (gel H(XL)) > Kp (gel H(HR)) > Kp (gel H). Moreover, the specific resolution of the three types of packings is also estimated, and its comparison yields that Rsp (gel H(HR)) > Rsp (gel H(XL)) > Rsp (gel H), in good agreement with their increasing particle size and decreasing crosslinking degrees. Finally, the errors and inaccuracies committed on the estimation of the sample average molar masses from their respective calibration curves serves the assessment that the packing with lesser secondary mechanisms is the H-type.     

A convenient synthesis of novel pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazinones from imidazo[1,2‐a]pyridines

The imidazo[1,2‐a]pyridine system was investigated as a synthon for the building of very attractive fused triazines, a planar, angular tri‐heterocycle with potential biological activity. Thus ethyl 3‐nitroimidazo[1,2‐a]pyridine‐2‐carboxylate was treated with ammonia or with an excess of primary amines to generate the corresponding substituted nitro carboxamidoimidazopyridines. The nitro substituent in the latter products, was reduced to yield 3‐amino‐2‐carboxamidoimidazo[1,2‐a]pyridine derivatives, which in turn were treated with nitrous acid to furnish 1‐oxo‐2‐substituted pyrido(1′,2′:1,2)imidazo[5,4‐d]‐1,2,3‐triazines.     

Spectrofluorimetric Method for the Determination of Aluminum with Alizarin Red PS

 A simple and direct spectrofluorimetric method has been developed for the determination of aluminum using alizarin red PS (1,2,4-trihydroxy 9,10-anthraquinone-3-sulfonic acid). The method is based on the strong fluorescence (480/564 nm) of Al³⁺ and alizarin red. Experimental parameters such as pH, concentration of the ligand, ionic strength of the solution, reaction time and temperature were optimized in order to maximize the analytical signal. Interferences of several ions (anions and cations) were studied and evaluated. The linear range of the method extends from 3 to 100 μg L⁻¹. Limit of detection (3s_b) was 0.9 μg L⁻¹. The method was tested with a silicate certified reference material. Interferences were eliminated by a liquid extraction with cupferron. Author for correspondence. E-mail: aucelior@rdc.puc-rio.br Received September 10, 2002; accepted January 15, 2003 Published online May 5, 2003     

Conformational dynamics of a bispyridinium cyclophane

A complete study of the conformational behavior of 4,8-diaza-3(1,4),9(4,1)-dipyridina-1,6(1,4)-dibenzenacyclodecaphan-3(1),9(1)-bis(ilium) bishexafluorophosphate is described. This study allows us to conclude that the process observed by which the different chemical shifts of the pyridinium protons show coalescence at a high-temperature 1H NMR is the rotation around the C-N bond, whereas the conformational equilibrium between the four conformers is produced at low temperature.     

HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm² V^?1 s^?1 as well as good ambient stability.     

Assessment of cadmium and iron adsorption in sediment,employing a flow injection analysis system with on line filtration and detection by flame atomic absorption spectrometry and thermospray flame furnace atomic absorption spectrometry

This work presents an evaluation of iron and cadmium adsorption in sediment of the Furnas Hydroelectric Plant Reservatory located in Alfenas, Minas Gerais (Brazil). The metal determination was done employing a flow injection analysis (FIA) with an on-line filtering system. As detection techniques, flame atomic absorption spectrometry (FAAS) for iron and thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) for cadmium determinations were used. The developed methodology presented good limits of detection, being 190 μg L⁻¹ for iron and 1.36 μg L⁻¹ for cadmium, and high sampling frequency for both metals 144 and 60 readings h⁻¹ for iron and cadmium, respectively. Both metals obey the Langmuir model, with maximum adsorptive capacity of 0⋅169 mg g⁻¹ for iron and 7⋅991 mg g⁻¹ for cadmium. For iron, a pseudo-first-order kinetic model was obtained with a theoretical Q_e = 9⋅8355 mg g⁻¹ (experimental Q_e = 9⋅5432 mg  g⁻¹), while for cadmium, a pseudo-second-order kinetic model was obtained, with a theoretical Q_e = 0.3123 mg g⁻¹ (experimental Q_e = 0⋅3052 mg g⁻¹).     

A Carbene‐Rich but Carbonyl‐Poor [Ir6(IMe)8(CO)2H14]2+ Polyhydride Cluster as a Deactivation Product from Catalytic Glycerol Dehydrogenation

The title cluster, a deactivation product in the catalytic dehydrogenation of glycerol, was characterized by XRD, DFT calculations, HRMS, FTIR spectroscopy, and NMR spectroscopy. Experimental/computational studies located the 14 H ligands, and all ¹H and ¹³C{¹H} NMR resonances were assigned. The structure contains an unprecedented Ir₆H₁₄ core with two CO and eight IMe ligands.     

Mullite crystallization using fully hydrolyzed silica sol: the gelation temperature influence

Mullite is an aluminosilicate widely used as a structural material for high temperature applications. This paper studies the effect of the gelation temperature on the synthesis of two mullite precursors: polymeric and colloidal silica, using both in fully-hydrolyzed silica sol, derived from sodium silicate. The gels were synthesized using aqueous silicic acid and aluminum nitrate. Ethylene glycol was added into polymeric gels. Two gelation temperatures were used: 80 and 100 °C. In the polymeric precursor, the increasing of the gelation temperature caused an increase in the silica incorporation inside the mullite crystalline lattice at 1,000 °C, and it also generated an increase in the reaction extent at all calcination temperatures. In the colloidal precursors, these effects were more intense than in the polymeric precursors in terms of yield. Colloidal samples calcined at 1,250 °C crystallized cristobalite and alpha alumina in addition to mullite when they were previously gelled at 80 °C. On the other hand, the same sample gelled at 100 °C led to only crystallized mullite. The reaction extent increased by more than 20 % for colloidal samples gelled at 100 °C compared to colloidal samples gelled at 80 °C (calcined at 1,250 °C). This increase was due to the almost total incorporation of alumina and silica in the crystalline lattice of mullite.