Magnetic phase transition in KGd(WO4)2 double tungstate

Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO₄)₂ have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.     

High-Yield Synthesis of New Ni–Ru Carbido–Carbonyl Clusters

Four new bimetallic Ni–Ru mixed-metal carbido–carbonyl clusters of a general formula [NiRu₅C(CO)₁₄ L₂], where L₂ = dppe (2), dppp (5), (PPh₃)₂ (8) and (PBu₃)₂ (13) were synthesised following the general synthetic strategy using penta- and hexaruthenium carbido carbonyl dianions in the reactions with corresponding dichloro complexes of Ni(II). The molecular and crystal structures of 2, 5, 7, 8, and 13 were determined by single crystal X-ray analysis. The mixed-metal core is octahedral with the P atoms coordinating to the Ru atoms.     

Correlated ab initio study of nucleic acid bases and their tautomers in the gas phase,in a microhydrated environment and in aqueous solution. guanine: surprising stabilization of rare tautomers in aqueous solution

Altogether eight keto and enol tautomers of guanine were studied theoretically in the gas phase, in a microhydrated environment (1 and 2 water molecules) and in bulk water. The structures of isolated, as well as mono- and dihydrated tautomers were determined by means of the RI-MP2 method using the extended TZVPP (5s3p2d1f/3s2p1d) basis set. The relative energies of isolated tautomers included the correction to higher correlation energy terms evaluated at the CCSD(T)/aug-cc-pVDZ level. The relative enthalpies at 0 K and relative free energies at 298 K were based on the above-mentioned relative energies and zero-point vibration energies, temperature-dependent enthalpy terms and entropies evaluated at the MP2/6-31G level. The keto form having hydrogen atom at N7 is the global minimum while the canonical form having hydrogen atom at N9 represents the first local minimum at all theoretical levels in vacuo and in the presence of 1 and 2 water molecules. All three unusual rare tautomers having hydrogens at N3 and N7, at N3 and N9, and also at N9 and N7 are systematically considerably less stable and can be hardly detected in the gas phase. The theoretical predictions fully agree with existing theoretical as well as experimental results. The effect of bulk solvent on the relative stability of guanine tautomers was studied by self-consistent reaction field and molecular dynamics free energy calculations using the thermodynamic integration method. Bulk solvent, surprisingly, strongly favored these three rare tautomers over all remaining low-energy tautomers and probably only these forms can exist in water phase. The global minimum (tautomer with hydrogens at N3 and N7) is by 13 kcal/mol more stable than the canonical form (3rd local minimum). Addition of one or two water molecules does not change the relative stability order of isolated guanine tautomers but the respective trend clearly supports the surprising stabilization of three rare forms.     

Functionalisation of the template-free and template-structured silica films synthesised on glass substrates by sol–gel technique

Possibility of the post-synthesis functionalisation of the template-free and template-structured silica films of ca. 200 nm thickness on glass slides was evaluated. The films were prepared by dip-coating from TEOS sol–gel precursor in the absence or presence of CTAB template. It has been found out that the template-structured silica films can be functionalised with Ag nanoparticles via [Ag(NH₃)₂]NO₃ ion-exchange or with adsorbed Methylene Blue (MB) cationic dye due to the presence of the well-organised mesopores after template removal. In contrast, only the external geometric surface of the template-free silica films appeared to be accessible for modifier molecules. Possibility of functionalisation of the multi-layered template-structured silica film depends on the sequence of dip-coating and calcination steps upon their preparation. When preparation includes reiteration of dip-coating and calcination steps, only the latest top silica film appears to be accessible to modifier molecules. When preparation includes successive dip-coating cycles accomplished by calcination of the final multi-layered film, all pores appear to be accessible since their formation occurs via simultaneous removal of the template molecules over the whole thickness of the multi-layered template-structured silica film.     

Polyurethane-Based Extended Release Form of Piroxicam

Summary: The anti-inflammatory effect of piroxicam (Px) immobilized on a polyurethane (PU) matrix was studied in vivo by measuring levels of t-methylimidazole acetic acid (t-MeImAA) in urine of experimental animals. Released into body from the polymeric matrix, Px affected inflammation process, significantly reducing the t-MeImAA levels. The retention of Px within the PU/drug system – largely due to physical modification of the polymer – resulted in durable therapeutic action of the system.     

Hyperconjugation effect on diene reactivity in 1-methyltetrazolo[5,1-a]isoindole-derived amides and thioamides

Optimized synthesis of 1-methyltetrazolo[5,1-a]isoindole-derived amides and thioamides was elaborated. Based on ¹³C NMR spectroscopy and X-Ray diffraction studies data, it was proposed that zwitterionic resonance structures contributed significantly to the structure of these compounds. Geometry optimization was performed in vacuo using m06-2x/cc-pvtz method taking into account polarizing effect of environment (PCM model) and specific intermolecular interactions. Electronic density distribution in these molecules was analyzed using NBO method. Using Δ(HOMO_diene–LUMO_dienophile) for amides and thioamides in vacuo and in a protic solvent, possibility of Diels–Alder reaction was evaluated. Energies of π-π conjugation and n→σ* hyperconjugation for the amide derivative were estimated in vacuo, aprotic and protic solvents.     

Reactions of N-sulfonyl-1,2,3-thiadiazole-4-carbimidamides with sulfonyl chlorides: regiospecific synthesis and structure of 2-sulfonyl-1,2,3-triazoles

Chemistry of Heterocyclic Compounds - Sulfonylation of 1,2,3-thiadiazolecarboxylic acid amidines in the presence of a base was found to be accompanied by the Cornforth rearrangement of the...     

Self-condensation of β-(isoxazol-5-yl) enamines under treatment with acetyl chloride and acids. Synthesis of novel 1,3-diisoxazolyl-1,3-dieneamines and 1,3,5-triisoxazolyl benzenes

Two directions for self-condensation of β-(isoxazol-5-yl) enamines under treatment with either acetyl chloride or acids were found leading to new 1,3-diisoxazolyl-1,3-dieneamines and 1,3,5-triisoxazolyl benzenes. The effect of solvent, acid, temperature and the reaction time on the ratio of reaction products were investigated. Trans-E-cis configuration of prepared 1,3-diisoxazolyl-1,3-dieneamines was unambiguously confirmed by 2D NMR spectra and X-ray analysis. A new mechanism of 1,3-diisoxazolyl-1,3-dieneamines formation was proposed.