Facts and artifacts about aromatic stability estimation

The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G^∗∗.     

Improvement of the Derjaguin-Broekhoff-de Boer theory for the capillary condensation/evaporation of nitrogen in spherical cavities and its application for the pore size analysis of silicas with ordered cagelike mesopores

In a previous work, we proposed an improvement of the Derjaguin-Broekhoff-de Boer (DBdB) theory for capillary condensation/evaporation in open-ended cylindrical mesopores. In this paper, we report a further extension of this approach to the capillary condensation/evaporation of nitrogen in siliceous spherical cavities. The main idea of this improvement is to employ the Gibbs-Tolman-Koenig-Buff equation to predict the variation of the surface tension in spherical mesopores. In addition, the statistical film thickness (the so-called t-curve), which is evaluated accurately on the basis of adsorption isotherms measured for MCM-41 materials, is used instead of the originally proposed t-curve to take into account the excess chemical potential due to the surface forces. It is shown that the aforementioned modifications of the original DBdB theory that was refined by Ravikovitch and Neimark have significant implications for the pore size analysis of cagelike mesoporous silicas. To verify the proposed improvement of the DBdB pore size analysis (IDBdB), two series of FDU-1 samples, which are well-defined cagelike mesoporous materials (composed of siliceous spherical cavities interconnected by short necks), were used for the evaluation of the pore size distributions (PSDs). The correlation between the spinodal condensation point in the spherical pores predicted by the nonlocal density functional theory (NDFT) developed by Ravikovitch and Neimark and that predicted by the IDBdB theory is very good in the whole range of mesopores. This feature is mirrored to the realistic PSD characterized by the bimodal structure of pores computed from the IDBdB theory. As in the case of open-ended cylindrical pores, the improvement of the classical DBdB theory preserves its simplicity and simultaneously ensures a significant improvement of the pore size analysis, which is confirmed by the independent estimation of the average pore size by the NDFT and the powder X-ray diffraction method.     

Formal Mathematical Analysis of the Existence of the Common Intersection Point in Relation to Determining the Parameters Describing Ion Adsorption at the Oxide/Electrolyte Interface: Comparison of the Triple and Four-Layer Models

For most oxide/electrolyte systems potentiometric titration curves measured for different ionic strengths have a Common Intersection Point (CIP) which corresponds to the Point of Zero Charge (PZC). However, there are systems where a CIP exists but the surface charge at this point does not equal zero (PZC CIP). In this paper theoretical analysis of the systems in which the PZC and CIP do not coincide is presented. It is based on the well-known 2-pK surface charging approach and Triple Layer Model (TLM) as well as the Four Layer Model (FLM) of the electric double layer. The appropriate mathematical criterion for CIP existence was applied with detailed derivations, both for TLM and FLM. Having determined in this manner the parameter values, one can draw proper conclusions about the features of oxide/electrolyte adsorption systems, in which PZC and CIP do not coincide. The values of adsorption parameters are found by fitting simultaneously the obtained theoretical expressions to both of the experimental titration isotherms, and to the individual isotherms of electrolyte cation adsorption measured using radiometric methods.     

Carrier influence on the paramagnetic and catalytic properties of supported titanium complexes

Alumina-silica possessing various Al₂O₃/SiO₂ ratios was used as a carrier of surface titanium complexes. The systems obtained were examined as models for the investigation of the macroligand (inorganic gel) influence on the physicochemical properties of supported transition metal complexes. The titanium complexes were prepared first by the reaction of CpTiCl₃, with the hydroxyl groups of the gel and then reduced by an excess of BuLi. The influence was established of the basic gel properties on: (a) the amount of titanium(IV) and titanium(III) complexes on the alumina-silica gel surfaces; (b) the symmetry of the surface Ti^III surface complexes; (c) the electron density around the surface-Ti^III ions, ionic or covalent bonds character in a complex; and (d) the catalytic activity in olefin hydrogenation.     

Complexes of carbonyl compounds with RnAIX3n compounds : IV. The evidence for the formation of cyclic complexes chloroaluminium compounds

Reasonably stable 1 : 2 complexes are formed by carbonyl compounds with chloroaluminium compounds, usually in equilibrium with a 1: 1 complex, however with trimethylaluminium only a 1 : 1 complex is formed. In the case of complexes with an excess of MeAlCl₂, a disproportionation reaction occurs. Possible structures are discussed in light of PMR and IR spectroscopy, dipole moments and molecular weight determination. The previously proposed cyclic six-membered ring is the most reliable structure for the 1 : 2 complex.     

Mimicking photosynthesis in a computationally designed synthetic metalloprotein

While advances in protein design have made possible the construction of protein architectures with nativelike properties and predictable structures and function, there are as of yet no examples of functional, protein-based, solar energy conversion systems. This communication describes the design and characterization of an artificial reaction center (RC) protein that closely resembles the function of the natural photosynthetic RC. The synthetic protein, designed by the protein design program CORE, participates in multiple reduction/oxidation cycles with exogenous acceptors/donors following photoexcitation. The designed metalloprotein, aRC, consists of a tetrahelical bundle functionalized with two bis-histidine bound metal cofactors: a Ru(bpy)2 moiety and a heme group. Two distinct bis-histidine binding sites were engineered for each of these metal centers. Photoexcitation of aRC results in rapid ET from the RuII complex to the heme group (kET >/= 5 x 1010 s-1) yielding a long-lived (70 ns) charge-separated state (CSS), RuIII/FeII. This long-lived CSS participates in subsequent ET reactions with exogenous donors and acceptors in multiple photocycles, thus mimicking the basic function of native photosynthetic RCs. This study illustrates the successful design and construction of a protein-based functional charge separation device using a combination of automated computational protein design and knowledge of the engineering principles of biological electron tunneling extracted from natural electron-transfer systems. To our knowledge, this represents the first example of a functional protein-based artificial reaction center.     

Carbon-13 kinetic isotope effect of the decarbonylation of oxalic acid

Carbon-13 intramolecular kinetic isotope effects in the decarbonylation of oxalic acid dihydrate of natural isotopic composition by SO₃ and by fuming sulphuric acid at room temperature and decarbonylation of oxalic acid dihydrate by 100% H₃PO₄ in the temperature interval 80–150°C have been determined. The obtained isotopic and kinetic results have been compared with the earlier¹³C experimental and theoretical studies in other solvents.     

Investigations of aromatic polyesters with polynaphthalene systems in the main chain—VI: NMR studies of the structure of the aromatic copolyesters

Nuclear magnetic resonance spectra of several kinds of terephthalic copolyesters of 4,4′dihydroxydinaphthyl 1,1′ and 2,2 bis(4-hydroxyphenyl) propane were recorded. Proton signals of the terephthalic acid unit corresponding to heterolinkages and homolinkages could be observed in the range from 8.23 to 8.53 ppm. The average sequence lengths and the degrees of randomness in the copolyesters were calculated from the intensities of these signals.     

The synthesis of 8‐hydroxyquinazoline derivatives and their acid‐base interactions

The reaction of N‐(2‐R¹‐oxyphenyl)benzimidoyl chlorides with cyanamide derivatives in the presence of titanium tetrachloride as a catalyst has yielded some new 4‐amino‐8‐R¹‐oxy‐2‐phenylquinazolines. pK_a values have been determined for these compounds and the influence of substituents at the basicity of the parent system has been discussed. Some investigations on the methyl‐quinazolinyl ether cleavage in different medium have been done yielding the appropriate hydroxyquinazoline derivatives. In those cases when the deprotection of 4‐amino‐8‐methoxy‐2‐phenylquinazoline was carried in aqueous acidic solutions, the formation of the hydrolysis products 3,4‐dihydro‐2‐phenyl‐4‐quinazolone derivatives was observed as well.     

Comparative Evaluation of CE and HPLC for Determination of Cotinine in Human Urine

Two rapid and popular methods—capillary electrophoresis (CE) and high-performance liquid chromatography (HPLC) have been compared for analysis of cotinine in human urine. Cotinine was analyzed in less than 7 min, with detection limits of 5 and 3.2 ng mL⁻¹ for CE and HPLC, respectively. The performance of the methods was evaluated in terms of sensitivity, specificity, precision, accuracy, and limits of detection and quantification. Calibration plots were linear in the range 50–4,000 ng mL⁻¹, at least, and mean recoveries were satisfactory for both techniques. The methods were successfully used for quantification of cotinine in urine.