Structural transitions induced by shear flow and temperature variation in a nonionic surfactant/water system

In this study, we investigate structural transitions of tetraethylene glycol monohexadecyl ether (C(16)E(4)) in D(2)O as a function of shear flow and temperature. Via a combination of rheology, rheo-small-angle neutron scattering and rheo-small-angle light scattering, we probe the structural evolution of the system with respect to shear and temperature. Multi-lamellar vesicles, planar lamellae, and a sponge phase were found to compete as a function of shear rate and temperature, with the sponge phase involving the formation of a new transient lamellar phase with a larger spacing, coexisting with the preceding lamellar phase within a narrow temperature-time range. The shear flow behavior of C(16)E(4) is also found to deviate from other nonionic surfactants with shorter alkyl chains (C(10)E(3) and C(12)E(4)), resembling to the C(16)E(7) case, of longer chain.     

Selective Dimerization of Lewis‐Acid/Base‐Stabilized Phosphanylalanes

The reaction of [{(CO)₅W}PRH₂] (R=H, Ph) with H₃Al ? NR₃ (R=Et, Me) leads to the formation of four‐membered heterocyclic compounds [({(CO)₅W}P(H)AlH ? NEt₃)₂] and [({(CO)₅W}PhPAlH ? NMe₃)₂]. Upon dissolving the solid compounds, fast equilibria between the isomers are observed on the NMR timescale. Further insight into the stability and reactivity of the isomers was gained by applying theoretical methods. DFT calculations predict that hydrogen elimination in the case of [({(CO)₅W}PhPAlH ? NMe₃)₂] may be reversible.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d ₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Molecular mechanics calculations of conjugated amides and an ab initio investigation of acrylamide and its β-amino derivative: Conformational analysis and rotational barriers

Conformations and rotational barriers in a series of conjugated primary and tertiary amides have been analyzed by a modified MM2(91) force field, which treats the amide nitrogen as part of the conjugated system by redefining the atom type for the nitrogen. Ab initio molecular orbital calculations at the MP2/6-31G* level have been performed on the stable conformers and transition structures of acrylamide and β-trans-aminoacrylamide. The results have been used, with published experimental and computational data, to generate parameters for the MM2 force field. The force field has been applied to various conjugated amides, such as reduced nicotinamide adenine dinucleotide (NADH) and NAD⁺ analogues, nicotinamide, urea, vinylogous urea derivatives, and nucleic acid bases. The fundamental difference between primary and tertiary conjugated amides with respect to both conformation and barrier is highlighted. © 1996 by John Wiley & Sons, Inc.     

Temperature quenched DODAB dispersions: fluid and solid state coexistence and complex formation with oppositely charged surfactant

Dilute dispersions of the synthetic bilayer forming double-chained cationic lipid dioctadecyldimethylammonium bromide (DODAB) were investigated. In dispersions sonicated above the chain melting temperature Tm (approximately 45 degrees C) it was found by H NMR that about 50% of the surfactant chains remained fluid when the samples were cooled to room temperature, which is 20 degrees C below Tm. In contrast, there was no sign of a fluid fraction in unsonicated samples at room temperature. The addition of the anionic surfactant sodium dodecyl sulfate (SDS) to DODAB dispersions at room temperature resulted in the formation of an essentially stoichiometric DODA-DS complex with frozen chains, as seen by titration calorimetry and H NMR experiments. For sonicated samples, turbidity experiments demonstrated that, after a fast complexation reaction, the system remains colloidally stable unless the SDS-to-DODAB mixing ratio is too close to unity. H NMR experiments also showed that in the unreacted DODAB the fraction of fluid chains remained close to 50%, indicating either that SDS reacts equally fast with fluid and frozen DODAB or that there is a relaxation of the fluid fraction after the complexation. The melting enthalpy and the melting temperature of the alkyl chains rise gradually as the mixing ratio increases. We observed with cryo-TEM that the fraction of large unilamellar vesicles was significantly larger after addition of SDS. This indicates vesicle fusion. Based on both wide- and small-angle X-ray scattering patterns, the structure of the equimolar SDS-DODAB complex at 25 degress C was proposed to be lamellar.     

A semiempirical approach to ligand‐binding affinities: Dependence on the Hamiltonian and corrections

We present a combination of semiempirical quantum‐mechanical (SQM) calculations in the conductor‐like screening model with the MM/GBSA (molecular‐mechanics with generalized Born and surface‐area solvation) method for ligand‐binding affinity calculations. We test three SQM Hamiltonians, AM1, RM1, and PM6, as well as hydrogen‐bond corrections and two different dispersion corrections. As test cases, we use the binding of seven biotin analogues to avidin, nine inhibitors to factor Xa, and nine phenol‐derivatives to ferritin. The results vary somewhat for the three test cases, but a dispersion correction is mandatory to reproduce experimental estimates. On average, AM1 with the DH2 hydrogen‐bond and dispersion corrections gives the best results, which are similar to those of standard MM/GBSA calculations for the same systems. The total time consumption is only 1.3–1.6 times larger than for MM/GBSA. © 2012 Wiley Periodicals, Inc.     

The Oxidation of ReCl5 with Oleum: Synthesis and Crystal Structure of Re2O4Cl4(SO4)

The reaction of ReCl₅ and fuming sulfuric acid (25 % SO₃) in a sealed glass tube at 200 °C led to red, needle shaped single crystals of Re₂O₄Cl₄(SO₄) (monoclinic, C2/c, a = 1501.8(2) pm, b = 1545.9(2) pm, c = 945.18(8) pm, β = 98.761(9)°, Z = 8). In the crystal structure the [ReO₂] moieties are linked by [SO₄]^2– tetrahedra to chains along the [101] direction. Each sulfate ion connects four rhenium atoms, additional two chloride ions complete the octahedral coordination sphere of each rhenium atom according to $\rm^1_\infty$ [ReO_2/1Cl_2/1(SO₄)_2/4].     

Nanoparticles: scaffolds for molecular recognition

Monolayer and mixed-monolayer protected clusters (MPCs and MMPCs) have great potential to combine molecular functionality with the intrinsic properties of nanometer-sized scaffolds. This synergy can be used to create complex functional devices, including redox-active, electronic, or magnetic storage devices, solution-based sensors, and highly efficient catalysts. This review outlines some of the recent developments in nanoscale receptors based on synthetic and nonbiological recognition elements. In these nanoparticle systems, molecular recognition is achieved by covalent attachment of receptors on the nanoparticles coupled with noncovalent interactions to target substrates. Synthetic host-guest systems, hydrogen bonding, change in redox states, pi-pi stacking, rotaxane formation, and ion recognition are the main topics covered in this review.     

Solvent-mediated self-association of a Horning-crown macrocycle

A member of a new class of novel macrocycles possessing both polyether and phenolic functionalities, forms dimers in both the solid-state and in solution when exposed to chloroform, dichloromethane or toluene, but does not self-associate in the presence of dimethyl sulfoxide.     

Spirobicyclo[2.2.2]octane derivatives: mimetics of baccatin III and paclitaxel (Taxol)

The formylated spirobyclic alcohol was computer modeled to be a mimetic of paclitaxel. In this model, the formyl group was used as a truncated paclitaxel side chain in order to reduce the computational work. Compound , carrying the paclitaxel side chain, was synthesized in six steps from optically active 1,3-diketone . Microtubule stabilization was not observed for , indicating that the model needs to be adjusted.