The art of stacking: structural folding and self-assembly of branched π-conjugation assisted by O-H···O and C-H···F hydrogen bonds

An intimate interplay of O-H···O/C-H···F hydrogen bonds and π-π stacking interactions allows a phenyleneethynylene-based dendritic molecule to fold and self-assemble into two distinctively different molecular crystals as pseudopolymorphs.     

Phase transitions and critical indices of a phospholipid bilayer model

A model is described which has been used in theoretical studies of a variety of phenomena (which are briefly summarized) relating to biological membranes. It is shown that the Hamiltonian describing this model can be mapped onto an Ising Hamiltonian with a temperature dependent field. It is also shown that this field varies linearly with temperature in the critical region. Exact solutions of this model are presented and its first and second order transitions are discussed with an emphasis on obtaining its critical indices. General considerations lead to the following relations: β=1/δ, α=γ, α′=γ′, where α, β, γ, δ are the critical indices for the specific heat, magnetization, susceptibility and critical isotherm respectively (the primes denoting low temperature indices). These relations are demonstrated explicitly for the Bethe lattice with coordination numbersq=2 and 6.     

Preparation,stoichiometry and magnetic properties of Cu y Cr2Se4−z Br x spinels with 0.5≲x≲2

The chemical composition of Cu _y Cr₂Se_4?z Br _x spinels depends strongly on the preparation parameters. Spinels with 0.8?y?1.2, 0.5?x?2, and 0?z?x?0.2 have been prepared. Whereas the lattice constanta _o of these spinels differs only by less than approximately 0.6%, their Curie temperatureT _c depends sensitively on the spinel composition. For Cu_1.1Cr₂Se_3.4Br_o.46,a _o=1.0410 nm andT _c=310 K were found to compared witha ₀=1.0447 nm andT _c=84 K of CuCr₂Se₂Br₂.     

Modeling phase transition for compressible two-phase flows applied to metastable liquids

The seven-equation model for two-phase flows is a full non-equilibrium model, each phase has its own pressure, velocity, temperature, etc. A single value for each property, an equilibrium value, can be achieved by relaxation methods. This model has better features than other reduced models of equilibrium pressure for the numerical approximations in the presence of non-conservative terms. In this paper we modify this model to include the heat and mass transfer. We insert the heat and mass transfer through temperature and Gibbs free energy relaxation effects. New relaxation terms are modeled and new procedures for the instantaneous temperature and Gibbs free energy relaxation toward equilibrium is proposed. For modeling such relaxation terms, our idea is to make use of the assumptions that the mechanical properties, the pressure and the velocity, relax much faster than the thermal properties, the temperature and the Gibbs free energy, and the ratio of the Gibbs free energy relaxation time to the temperature relaxation time is extremely high. All relaxation processes are assumed to be instantaneous, i.e. the relaxation times are very close to zero. The temperature and the Gibbs free energy relaxation are used only at the interfaces. By these modifications we get a new model which is able to deal with transition fronts, evaporation fronts, where heat and mass transfer occur. These fronts appear as extra waves in the system. We use the same test problems on metastable liquids as in Saurel et al. [R. Saurel, F. Petitpas, R. Abgrall, Modeling phase transition in metastable liquids: application to cavitating and flashing flows, J. Fluid Mech. 607 (2008) 313–350]. We have almost similar results. Computed results are compared to the experimental ones of Simões-Moreira and Shepherd [J.R. Simões-Moreira, J.E. Shepherd, Evaporation waves in superheated dodecane, J. Fluid Mech. 382 (1999) 63–86]. A reasonable agreement is achieved. In addition we consider the six-equation model with a single velocity which is obtained from the seven-equation model in the asymptotic limit of zero velocity relaxation time. The same procedure for the heat and mass transfer is used with the six-equation model and a comparison is made between the results of this model with the results of the seven-equation model.     

Stereoinversion of Unactivated Alcohols by Tethered Sulfonamides

The direct, catalytic substitution of unactivated alcohols remains an undeveloped area of organic synthesis. Moreover, catalytic activation of this difficult electrophile with predictable stereo‐outcomes presents an even more formidable challenge. Described herein is a simple iron‐based catalyst system which provides the mild, direct conversion of secondary and tertiary alcohols to sulfonamides. Starting from enantioenriched alcohols, the intramolecular variant proceeds with stereoinversion to produce enantioenriched 2‐ and 2,2‐subsituted pyrrolidines and indolines, without prior derivatization of the alcohol or solvolytic conditions.     

Zinc Substitution of Cobalt in Vitamin B12: Zincobyric acid and Zincobalamin as Luminescent Structural B12‐Mimics

Replacing the central cobalt ion of vitamin B₁₂ by other metals has been a long‐held aspiration within the B₁₂‐field. Herein, we describe the synthesis from hydrogenobyric acid of zincobyric acid ( Znby ) and zincobalamin ( Znbl ), the Zn‐analogues of the natural cobalt‐corrins cobyric acid and vitamin B₁₂, respectively. The solution structures of Znby and Znbl were studied by NMR‐spectroscopy. Single crystals of Znby were produced, providing the first X‐ray crystallographic structure of a zinc corrin. The structures of Znby and of computationally generated Znbl were found to resemble the corresponding Co^II‐corrins, making such Zn‐corrins potentially useful for investigations of B₁₂‐dependent processes. The singlet excited state of Znby had a short life‐time, limited by rapid intersystem crossing to the triplet state. Znby allowed the unprecedented observation of a corrin triplet (E^T=190 kJ mol^?1) and was found to be an excellent photo‐sensitizer for ¹O₂ (Φ_Δ=0.70).     

LC–ELISA as a contribution to the assessment of matrix effects with environmental water samples in an immunoassay for estrone (E1)

Accreditation and Quality Assurance - Estrone (E1), a metabolite of the estrogenic hormones 17β-estradiol (β-E2) and 17α-estradiol (α-E2), is itself a potent estrogen which can...