Heteronuclear intermolecular resonance-assisted hydrogen bonds. The structure of pyrrole-2-carboxamide (PyCa)

The crystal and molecular structure of pyrrole-2-carboxamide (PyCa) determined by single crystal X-ray diffraction is presented. Molecular conformations of PyCa are also analyzed by FT-IR and NMR techniques. Additionally DFT calculations at the B3LYP/6-311++G(d,p) level of approximation are performed for dimers of PyCa and for related species. The existence of two tautomeric forms for the analyzed dimers differing in H-bond motifs, N-H...O or O-H...N, is studied. The geometrical and energetic features of such H-bonds show that these interactions may be classified as intermolecular resonance-assisted hydrogen bonds. Additionally the Bader theory is applied to determine and to analyze bond critical points.     

The possible covalent nature of N-H...O hydrogen bonds in formamide dimer and related systems: an ab initio study

The N-H...O hydrogen bonds are analyzed for formamide dimer and its simple fluorine derivatives representing a wide spectrum of more or less covalent interactions. The calculations were performed at the MP2/6-311++G(d,p) level of approximation. To explain the nature of such interactions, the Bader theory was also applied, and the characteristics of the bond critical points (BCPs) were analyzed: the electron density at BCP and its Laplacian, the electron energy density at BCP and its components, the potential electron energy density, and the kinetic electron energy density. These parameters are used to justify the statement that some of the interactions analyzed are partly covalent in nature. An analysis of the interaction energy components for the systems considered indicates that the covalent character of the hydrogen bond is manifested by a markedly increased contribution of the delocalization term relative to the electrostatic interaction energy. Moreover, the ratio of stabilizing the delocalization/electrostatic contributions grows linearly with the decreasing lengths of the hydrogen bond.     

High spin states above the α-decaying isomer in 211Po

Prompt and delayed γ-rays in nuclei of the ²⁰⁸Pb region produced in 450 MeV ⁷⁶Ge +²⁰⁸Pb collisions have been studied at GASP. Yrast states above the α-decaying isomer in ²¹¹Po have been located including a 0.25 μs 31/2⁻ isomer at 2135 keV and a 2 μs isomer at 4874 keV. Received: 22 September 1997     

Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

MP2/aug‐cc‐pVTZ calculations were performed on triel trifluorides, ZF₃ (Z=B, Al, Ga, In, Tl), and their complexes with N₂ and HCN species, which are acting as Lewis bases. Interaction of the ZF₃ trigonal structure with a single HCN or N₂ ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF₃ with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom. Consequently, the Z atom in ZF₃ compounds suffers from electron deficiency (hypovalency), it obeys the octet rule in ZF₃–NCH and ZF₃–N₂ moieties, and it may be considered as a hypervalent center in complexes of ZF₃ with two ligands. Much weaker interactions are observed for the boron complexes than for the other triel systems. This is because of the lower acidity of the B center relative to that of the other triel centers, and it may be the result of a stronger backbonding effect for BF₃ than for the other triel trifluorides. Interactions in aluminum complexes are characterized by meaningful electrostatic contributions, whereas for gallium complexes, the most important electron charge shift is observed as a result of complexation. Analysis of the geometry of the triel complexes and of the Z???N interactions (triel bonds) in these complexes is based on ab initio calculations as well as on the quantum theory of atoms in molecules and the natural bond orbitals method.     

Cooperativity of hydrogen and halogen bond interactions

The cooperativity effects in the Cl^?···HCCH···HF, Cl^?···ClCCH···HF and F^?···ClCCH···HF complexes are analyzed here. The results show that the formation of the hydrogen and halogen bonds is ruled by the same mechanisms and that the cooperativity enhances these interactions. The MP2(full)/6-311 ++G(d,p) calculations were performed for the above triads and the corresponding sub-units; dyads linked by the hydrogen or halogen bonds and monomers. The NEDA scheme of the decomposition of the interaction energy was applied here. It was found that for the halogen bonded systems, the most important is the polarization term of the energy of interaction while for the hydrogen bonds the charge transfer interaction energy and next the electrostatic contribution. The interaction between orbitals is also analyzed here in terms of the Natural Bond Orbitals method.     

S−H…O and O−H…O Hydrogen Bonds-Comparison of Dimers of Thiocarboxylic and Carboxylic Acids

ωB97-XD/aug-cc-pVTZ calculations were performed on dimers of selected thiocarboxylic acids and on analogous carboxylic acids. The sample of calculated thiocarboxylic acids is an extension of the Cambridge Structural Database search that contains only a few such structures. The Natural Bond Orbital (NBO) method, Symmetry-Adapted Perturbation Theory (SAPT) approach, Non-Covalent Interaction (NCI) method and Quantum Theory of Atoms in Molecules (QTAIM) were applied additionally to analyse interactions in dimers of thiocarboxylic and carboxylic acids. The insights into crystal structures as well as into results of calculations show that the formation of S−H…O hydrogen bonds between molecules of thiocarboxylic acids is steered by the same mechanisms as the formation of much stronger O−H…O hydrogen bonds in carboxylic acids. The intramolecular O−H…O and C−H…S hydrogen bonds occurring in few considered structures are also analysed.     

Subgroups of Hilbert spaces

The author was supported by the Alexander von Humboldt-Stiftung     

Application of ultra-performance columns in high-performance liquid chromatography for determination of albendazole and its metabolites in turkeys

Methods for determination of albendazole (ALB), albendazole sulfoxide (SOX) and albendazole sulfone (SON) in turkey blood plasma, using high‐performance liquid chromatography (HPLC) with fluorescence detection, were developed. Moreover, comparison of HPLC columns with ultra‐performance liquid chromatography (UPLC) columns was performed. Albendazol was administered orally in 5‐week‐old birds (n = 18) at a dose of 25 mg/kg b.w. Accuracy and precision of the developed method were satisfactory and stability studies showed acceptable variation (below 15%) in ALB, SOX and SON concentrations when the samples were stored at –75°C for 15 days. UPLC® columns gave higher peaks from typical HPLC columns retaining high quality of analysis. Pharmacokinetic analysis indicated quick elimination of ALB from turkey blood plasma. The mean residence time of SON was at least two times longer than that of SOX and four times longer than that of ALB. The elimination half‐lives for ALB, SOX and SON were 0.7 ± 0.27, 5.37 ± 6.03, 9.17 ± 5.12 h, respectively. The obtained results indicate that the described method allows for precise determination of albendazole and its metabolites in turkey plasma. Moreover, using UPLC columns in HPLC apparatus results in higher sensitivity as compared with the classical HPLC columns. Copyright © 2011 John Wiley & Sons, Ltd.