Aromaticity and Stability of Azaborines

The influence of the relative boron and nitrogen positions on aromaticity of the three isomeric 1,2‐, 1,3‐, and 1,4‐azaborines has been investigated by computing the extra cyclic resonance energy, NICS(0)_πzz index and by visualizing the π‐electron (de)shielding pattern as a response of the π system to a perpendicular magnetic field. The origin of the known stability trend, in which the 1,2‐/1,3‐isomer is the most/least stable, was examined by using an isomerization energy decomposition analysis. The 1,3‐arrangement of B and N atoms creates a charge separation in the π‐electron system, which was found to be responsible for the lowest stability of 1,3‐azaborine. This charge separation can, in turn, be considered as a driving force for the strongest cyclic π‐electron delocalization, making this same isomer the most aromatic. Despite the well‐known fact that the B?N bond attenuates electron delocalization due to large electronegativity difference between the atoms, the 1,4‐B,N relationship reduces aromaticity to a greater extent by making the π‐electron delocalization more one‐directional (from N to B) than cyclic. Thus, 1,4‐azaborine was found to be the least aromatic. Its lower stability with respect to the 1,2‐isomer was explained by the larger exchange repulsion.     

Thermodynamic properties of magnetic bilayers

Two models of magnetic bilayers are presented both based on the Heisenberg model. In the first case of ferromagnetically ordered ferromagnetically coupled planes of S=1 the anisotropy is of easy plane/axis type, while in the study of antiferromagnetically ordered antiferromagnetically coupled planes of S=1/2, the anisotropy is of XXZ type. Both systems are treated by Green's function method, which consistently applied within Random Phase Approximation lead to excitation energies and the system of equations for order parameters which can be solved numerically and which satisfies both Mermin-Wagner and Goldston theorem in the corresponding limit and also agrees with the Mean Field results. The basic result is that the transition temperature for magnetic dipole order parameter is unique for both planes. The results are compared with previous theoretical and experimental results.     

Powder diffraction data and mesomorphic properties of 4-butyloxyphenyl 4′-decyloxybenzoate

Unit cell parameters calculated from X-ray powder diffraction data are presented for the crystalline phase of a liquid crystal 4-butyloxyphenyl 4′-decyloxybenzoate: a = 23.098 (4) Å, b = 5.974 (6) Å, c = 12.357 (10) Å, β = 121.5283 (788)°, unit-cell volume V = 1453.56 ų. Temperature dependent X-ray diffraction data confirmed the existence of smectic A and smectic C mesophases and a more ordered, tilted crystalline smectic phase. Possibility of existence of previously reported smectic B phase as well as another crystalline phase was refuted.     

Trapping of Organophosphorus Chemical Nerve Agents in Water with Amino Acid Functionalized Baskets

We prepared eleven amino‐acid functionalized baskets and used ¹H NMR spectroscopy to quantify their affinity for entrapping dimethyl methylphosphonate (DMMP, 118 ų) in aqueous phosphate buffer at pH=7.0±0.1; note that DMMP guest is akin in size to chemical nerve agent sarin (132 ų). The binding interaction (K_a) was found to vary with the size of substituent groups at the basket′s rim. In particular, the degree of branching at the first carbon of each substituent had the greatest effect on the host‐guest interaction, as described with the Verloop′s B1 steric parameter. The branching at the remote carbons, however, did not perturb the encapsulation, which is important for guiding the design of more effective hosts and catalysts in future.     

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4′-(10-undecen-1-yloxy)-1,1′-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4′-(9-decen-1-yloxy)-1,1′-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of six mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures, a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.     

Magnetic Anisotropy of the C?C Single Bond

Anisotropic effects are broadly used in NMR spectroscopy for structure elucidation. With the development of computational methods it has become possible to quantify the effects and obtain further insight into their origin. Some classical interpretations have been questioned. Herein, we show that the classical “anisotropy cone” representing the anisotropic effect of the C? C single bond should be revised: deshielding at its side and shielding along its end are observed. Consequently, methyl, methylene, and methyne hydrogen atoms are not deshielded by C? C bonds as is conventionally explained in NMR spectroscopy textbooks. They are just less shielded than by the C? H bonds attached at the same carbon. In addition, this anisotropic effect is dependent on the environment and care should be taken when drawing conclusions based on it. For example, it differs for the staggered and eclipsed conformations of ethane in HCCH planes, as well as for cyclohexane. In fact, it is not the anisotropy of the C2? C3/C5? C6 bonds that determines the chemical shift difference of axial and equatorial protons of a rigid cyclohexane ring, but magnetic contributions from all bonds.     

New 3,4-Annelated Coumarin Derivatives: Synthesis, Antimicrobial Activity, Antioxidant Capacity, and Molecular Modeling

Summary.  The one pot reaction preparation, spectral analysis, and molecular modeling experiments on the new 3,4-annelated coumarin systems with bioactivity associated structural features are described. These provided the insight into the equilibrium of the respective tautomeric forms making possible the reconciliation of previously published spectral data with the structure assignments as well as the correction of erroneously established structures. The synthesized compounds were tested in a standard disk diffusion assay and displayed strong to moderate antimicrobial activity, with a promising new lead prototype compound (7-amino-9-hydroxy-5-oxa-7a,8,11-triazacyclopenta[b]phenanthren-6-one (5)) possessing greater activity that the antibiotics Ampicillin and Nystatin. Antioxidant capacities of the compounds, determined spectrophotometrically using a phosphomolybdenum method, were greater, and in the case of compound 5 four times the activity of α-tocopherol acetate.     

Molecular Recognition of Nerve Agents and Their Organophosphorus Surrogates: Toward Supramolecular Scavengers and Catalysts

Nerve agents are tetrahedral organophosphorus compounds (OPs) that were developed in the last century to irreversibly inhibit acetylcholinesterase (AChE) and therefore impede neurological signaling in living organisms. Exposure to OPs leads to a rapid development of symptoms from excessive salivation, nasal congestion and chest pain to convulsion and asphyxiation which if left untreated may lead to death. These potent toxins are prepared on a large scale from inexpensive staring materials, making it feasible for terrorist groups or states to use them against military and civilians. The existing antidotes provide limited protection and are difficult to apply to a large number of affected individuals. While new prophylactics are currently being developed, there is still need for therapeutics capable of both preventing and reversing the effects of OP poisoning. In this review, we describe how the science of molecular recognition can expand the pallet of tools for rapid and safe sequestration of nerve agents.     

A preliminary survey of natural radionuclides in soil and indoor radon in the town of Niš, Serbia

Journal of Radioanalytical and Nuclear Chemistry - The activity concentration of natural radionuclides in the samples of soil from the town of Ni? were measured using the gamma spectroscopy...