Molecular structures and stability constants of gossypol and its aza-derivative complexes with silver(I) cations studied by potentiometric, ESI MS, NMR, and AM1d semiempirical methods

Stability constants of complexes formed by gossypol and by ten of its Schiff bases with Ag (+) cations were determined by the potentiometric method. The potentiometric and ESI MS experiments indicate the formation of AgL (+) and Ag 2L (2+) complexes between the Schiff bases G1-G7 and Ag (+) cations as well as the formation of AgL (+), Ag 2L (2+), AgL 2 (+) and Ag 3L 2 (3+) complexes between the Schiff bases G8-G10 and Ag (+) cations. The highest stability constant was found for the AgL (+) complex of G8 Schiff base and the lowest one for the AgL (+) complex of G molecule. The (13)C NMR spectra of mixtures between G and AgClO 4 as well as G1-G10 and AgClO 4 indicate that the complexation of the Ag (+) cations is exclusively realized by the aldehyde-aldehyde tautomer of gossypol and by the enamine-enamine form of gossypol Schiff bases, respectively. We show that the main coordination sites for the Ag (+) metal cations are either the oxygen or the nitrogen atoms of the amine parts of the Schiff bases of gossypol. The energetically most favorable structures of the Ag (+) complexes with gossypol (G) or with the gossypol Schiff bases (G1-G10) were calculated and visualized by the AM1d method at an semiempirical level of theory.     

Complexes of lasalocid 2-naphthylmethyl ester with monovalent metal cations studied by mass spectrometry, spectroscopic and semiempirical methods

Lasalocid acid is an ionophore able to carry protons and cations through the cell membrane. Its 2-naphthylmethyl ester (NAFB) and its complexes with Li⁺, Na⁺ and K⁺ cations were synthesized and their structures were studied by ESI-MS, ¹H NMR, ¹³C NMR, FTIR and PM5 methods. The ESI-MS spectra indicate that NAFB forms stable complexes with Li⁺, Na⁺ and K⁺ cations of exclusively 1:1 stoichiometry and that the complexation of Li⁺ cations by NAFB is favoured. The NMR and FTIR spectra indicate that the oxygen atom of the ketone group of the NAFB molecule is involved in the coordination of the cations, and the strength of this process is dependent on the type of cation. We find that the intramolecular O(3)?CH···O(2) hydrogen bond in NAFB and its complexes is partially broken in acetonitrile solution and that this process is independent of the type of cation. The PM5 semiempirical calculations allow visualisation of all structures and determination of the hydrogen bond parameters.     

Freezing in resonance structures for better packing: XeF2 becomes (XeF+)(F-) at large compression

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.     

X-ray crystallographic, FT-IR, and density functional theory studies of the salt formed between dipicrylamine and 1,5,7-triazabicyclo[4.4.0]dec-1-ene

The DPA-TBD (dipicrylamine-1,5,7-triazabicyclo[4.4.0]dec-1-ene) salt has been synthesized and characterized by FT-IR spectroscopy, X-ray single-crystal diffraction, and theoretical study. In the FT-IR spectrum of the crystalline DPA-TBD salt, an unexpected intense band at 1742 cm(-1) is present. The optimized geometry and the FT-IR spectra of the DPA salt were calculated at the B3LYP/6-31G+(d) level and give an explanation of the nature of this band as the ν(C═N═C) vibration. For comparison, the calculated IR spectra of the DPA anion, hydrogen-bonded DPA anion, and neutral DPA and TBD molecules as well as for the TBD cation are also shown. The presence of the free DPA anion in the gas phase was directly detected in the negative ion mode electrospray ionization MS spectra. The fragmentation of the DPA anion is also discussed.     

Current induced resistance change of magnetic tunnel junctions with ultra-thin MgO tunnel barriers

Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110% and an area resistance product of down to . If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supported by numerical fits to the data in order to quantify the timescales of the involved phenomena.     

Spectroscopic, semiempirical and X-ray structural study of the 2:1 complex of a cyclic diamide of o-phthalic acid with water molecule

Cyclic diamide of o-phthalic acid with 3,6-dioxa-octyl-1,8-diamine (CPhDA) was synthesised by a new method and its hydrate structure has been studied by X-ray diffraction, FT-IR, NMR and PM5 semiempirical methods. The crystal of this compound is orthorhombic, space group Pbcn, with a = 16.7033(11), b = 8.8823(5), c = 19.6182(12) and Z = 8. The IR spectrum of the crystal is consistent with the results obtained by the X-ray study and provides spectroscopic evidence for the formation of the H-bonded complex with water molecules. The calculated structure of the complex and the structural parameters are comparable with those determined by the X-ray method.     

Intramolecular hydrogen bonds with large proton polarizability in semisalts of mono- and di-N-oxides of N,N′-tetraalkyl-o-xylyldiamines

Heteroconjugated NO⁺H … N NO … H⁺N and homoconjugated NO⁺H … ON NO … H⁺ON intramolecular hydrogen bonds formed in semisalts of mono- and di-N-oxides of N,N′-tetraalkyl-o-xylyldiamines were studied by IR and NMR spectroscopy. All these hydrogen bonds show large proton polarizability. In the case of the heteroconjugated hydrogen bonds the proton transfer equilibrium shifts from compounds 1 to 3 to the left hand side since the interaction of the hydrogen bond with the solvent environment decreases in this series of compounds. With compound 1 the hydrogen bonds are slightly weaker and longer, hence the wavenumber dependence of the intensity of the continuum caused by these hydrogen bonds is slightly changed with compound 1 compared with compound 2. In the case of compound 3 the intensity of the continuum decreases because of increasing screening of the hydrogen bonds. In the series of homoconjugated hydrogen bonds, from compound 4 to 6 the intense continuum vanishes, and only the band of the 0–1 proton transition at 950 cm⁻¹ remains. The vanishing of the continuum is caused by increasing screening of the hydrogen bonds against their solvent environments by bulky groups, and thus, this change demonstrates again that the interaction of the hydrogen bond with large proton polarizabilities is a necessary prerequisite for IR continua to appear.     

Formation of hydrogen-bonded chains between a strong base with guanidine-like character and phenols with various pKa values - an FT-IR study

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-l-methyl-2H-pyrimido[1,2-a]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the pK_a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1: 1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The O … N ⁻O … H⁺N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing pK_a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N⁺H …O⁻ hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the pK_a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.     

Synthesis of new semi-synthetic dipodands and tripodands from naturally occurring polyether ionophores

A new method to synthesize novel esters of Lasalocid acid 2-5 and of Monensin A, 7-9 (semi-synthetic di- and tripodands) is described. These new compounds are characterized by spectroscopic and microbiological methods.