Period Doubling in the Rössler System—A Computer Assisted Proof

Using rigorous numerical methods, we validate a part of the bifurcation diagram for a Poincaré map of the Rössler system (Rössler in Phys. Lett. A 57(5):397–398, 1976)—the existence of two period-doubling bifurcations and the existence of a branch of period two points connecting them. Our approach is based on the Lyapunov–Schmidt reduction and uses the C ^r -Lohner algorithm (Wilczak and Zgliczyński, available at http://www.ii.uj.edu.pl/~wilczak) to obtain rigorous bounds for the Rössler system.     

Catalytic asymmetric allylation of aldehydes using the chiral (salen)chromium(III) complexes

The enantioselective addition of allylstannanes to glyoxylates and glyoxals, as well as simple aromatic and aliphatic aldehydes, catalyzed by chiral (salen)Cr(III) complexes, has been studied. The reaction proceeded smoothly for the reactive 2-oxoaldehydes and allyltributyltin in the presence of small amounts (1-2 mol %) of (salen)Cr(III)BF₄ (1b) under mild, undemanding conditions. However, in the case of other simple aldehydes, the use of high-pressure conditions is required to obtain good yields. Classic chromium catalyst 1b, easily prepared from the commercially available chloride complex 1a, affords homoallylic alcohols usually in good yield and with enantiomeric purity of 50-79% ee. The stereochemical results are rationalized on the basis of the proposed model.     

Headgroup hydration and mobility of DOTAP/DOPC bilayers: a fluorescence solvent relaxation study

The biophysical properties of liposome surfaces are critical for interactions between lipid aggregates and macromolecules. Liposomes formed from cationic lipids, commonly used to deliver genes into cells in vitro and in vivo, are an example of such a system. We apply the fluorescence solvent relaxation technique to study the structure and dynamics of fully hydrated liquid crystalline lipid bilayers composed of mixtures of cationic dioleoyltrimethylammoniumpropane (DOTAP) and neutral dioleoylphosphatidylcholine (DOPC). Using three different naphthalene derivatives as fluorescent dyes (Patman, Laurdan and Prodan) allowed different parts of the headgroup region to be probed. Wavelength-dependent parallax quenching measurements resulted in the precise determination of Laurdan and Patman locations within the DOPC bilayer. Acrylamide quenching experiments were used to examine DOTAP-induced dye relocalization. The nonmonotonic dependence of dipolar relaxation kinetics (occurring exclusively on the nanosecond time scale) on DOTAP content in the membrane was found to exhibit a maximum mean solvent relaxation time at 30 mol % of DOTAP. Up to 30 mol %, addition of DOTAP does not influence the amount of bound water at the level of the sn(1) carbonyls, but leads to an increased packing of phospholipid headgroups. Above this concentration, elevated lipid bilayer water penetration was observed.     

State of hydrogen in idealized carbon slitlike nanopores at 77 K

The purpose of this letter is to clarify recent findings and answer to the question: "What is the state of hydrogen in carbon slitlike pores at 77 K?" For this purpose, we determined the volumetric density of hydrogen in idealized carbon pores of molecular dimension at 77 K and pressure up to 1 MPa. We used quantum corrected grand canonical Monte Carlo simulation. We recognized the highest volumetric density of confined hydrogen (around 71% of hydrogen liquid at boiling point) for effective pore width 5.6 angstroms (H* = 3.04) in the considered pressure range. Our computational results are in agreement with the calculations performed by Wang and Johnson and Rzepka et al. In contrast, we did not observe the high volumetric density of hydrogen in slitlike carbon pores exceeding the density of hydrogen liquid at the boiling point as was reported by Jagiello and Thommes. Moreover, we obtained qualitative agreement between the simulation results and some experimental findings reported by Nijkamp.     

15N NMR chemical shifts of ring substituted benzonitriles

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon.     

Simulating electron transfer attachment to a positively charged model peptide

Ab initio electronic structure methods, including stabilization method tools for handling electronically metastable states, are used to treat a model system designed to probe the electron-transfer event characterizing electron-transfer dissociation (ETD) mass spectroscopic studies of peptides. The model system consists of a cation H(3)C-(C=O)NH-CH(2)-CH(2)-NH(3)(+), containing a protonated amine site and an amide site, that undergoes collisions with a CH(3)(-) anion. Cross-sections for electron transfer from CH(3)(-) to the protonated amine site are shown to exceed those for transfer to the Coulomb-stabilized amide site by 2 orders of magnitude. Moreover, it is shown that the fates of the amine-attached and amide-attached species are similar in that both eventually lead to the same carbon-centered radical species H(3)C-((*)C-OH)NH-CH(2)-CH(2)-NH(2), although the reaction pathways by which the two species produce this radical are somewhat different. The implications for understanding peptide fragmentation patterns under ETD conditions are also discussed in light of this work's findings.     

Future of high pressure liquid chromatography: do we need porosity or do we need pressure?

Making a theoretical study supported by experiments of the kinetic advantages of increased inlet pressures versus increased external porosity using impedance plots of analysis time versus required plate number, it is found that both approaches more or less have the same effect on the kinetic performance. The need to change a given system to one with an increased inlet pressure or with an increased external porosity can best be assessed from the optimal plate number (N(opt)) of the system. When the pursued application requires a plate number that is larger than N(opt), any increase in inlet pressure and external porosity is beneficial. When the required plate number is smaller than N(opt), any increase in inlet pressure and external porosity should preferentially be accompanied by an overall reduction of the feature sizes of the support. The degree to which this feature size reduction can be realized in practice will to a large extent determine which of the two approaches will be the dominant system of the future.     

Selection of comparison criteria and experimental conditions to evaluate the kinetic performance of monolithic and packed-bed columns

The present study concerns the problem of finding appropriate experimental conditions and comparison criteria to assess the kinetic performance of LC supports with different sizes or morphologies. A general procedure, based on evaluating each support for its own optimal mobile-phase composition, is proposed. The practical elaboration of the procedure is illustrated using the specific case of a capillary LC separation of a series of polycyclic aromatic test compounds employing silica-monolith capillary columns and capillary columns packed with 6-microm porous particles. To compare the systems for their ability to yield the fastest critical-pair separation, plate-height measurements are transformed into an effective plate number kinetic plot, i.e., a plot of the extrapolated retention time divided by the square of the extrapolated effective plate number (t(R)/N(eff)(2)) versus N(eff). This type of data representation provides a direct and universal basis to compare the kinetic performance of different LC supports and it corrects for differences in retention strength arising from different phase ratios.     

Novel tetrahedral tetranickel cluster with alkylidyne ligand (NiCp)4(μ3-CR)

A novel tetrahedral (μ₃-alkylidyne)tetranickel cluster (NiCp)₄(μ₃-C(CH₂)₄CH₃) (4) was obtained in the reaction of nickelocene with potassium and 1-hexene. Compound 4 was characterised by means of MS and X-ray diffraction analysis. It crystallizes in the orthorhombic crystal system and Pna2₁ (No. 33) space group. Unit cell dimensions: a = 28.406(6) Å, b = 8.928(2) Å, c = 9.541(2) Å; Z = 4. The compound possesses 63 valence electrons, three more than the expected “magic number” for such type of clusters, and three of the four nickel atoms do not fulfil the 18VE rule. It is paramagnetic with the magnetic moment 3.54 μ_B, what corresponds to three unpaired electrons per molecule. This was confirmed by molecular orbital calculations using the density functional theory (DFT).