Subliming the Unsublimable: How to Deposit Nanographenes

Quite sublime : Thin‐layer fabrication of unsublimable large polycyclic aromatic hydrocarbons (PAHs) by pulsed laser deposition was used to prepare samples for scanning tunneling microscopy. Giant PAHs with up to 222 carbon atoms can be visualized—a size that was previously not possible because of the lack of suitable deposition methods.

     

A thermodynamic approach to the stress-induced crystallization in cross-linked rubbers

The thermodynamic treatment of crystallization phenomena in a prestretched rubber was undertaken. Emphasis was put on defining conditions for the thermodynamic stability of the extendedor folded-chain crystal structure. The extended-chain structure is found to be stable thermodynamically at temperatures higher than the isotropic melting point of un-cross-linked polymer T in the stretched state, while the folded chain one is not. Below T, the stretch ratio of the network structure determines which crystal structure is more stable. The relation among the critical stretch ratio for the extended/folded crystalline structure transition, temperature, and molecular weight is also discussed. The crystallinity predicted by this work becomes zero at a temperature of T, the isotropic melting point of a cross-linked system. The value of T decreases with increasing cross-link density, and this is consistent with the experimental data reported in the literature.     

Experiments to determine the escape probability of photoelectrons

A novel experimental method for the assessment of the escape probability of photoelectrons as a function of depth and their mean escape depths is described and illustrated for photoelectrons leaving an aluminium oxide surface. The results are compared with those of the straight line approximation model calculations. Received: 30 July 1997 / Revised: 7 April 1998 / Accepted: 8 April 1998     

Relationship of stress to uniaxial strain in crosslinked poly(dimethylsiloxane) over the full range from large compressions to high elongations

Experiments on inflated sheets of crosslinked poly(dimethylsiloxane) covering a sixfold range of compression are combined with measurements in elongation conducted on specimens from the same sample to obtain the relationship of stress to strain over a 24-fold range in the extension ratio λ₁. With increase in λ the reduced force [f] ≡ f(λ₁ – λ)^?1 rises to a maximum near λ = 1.2–1.4, then decreases very slowly with further increase in λ. The form of the relationship of [f] to λ confirms recent theory.     

A multipolar approach to the interatomic covalent interaction energy

Interatomic exchange‐correlation energies correspond to the covalent energetic contributions to an interatomic interaction in real space theories of the chemical bond, but their widespread use is severely limited due to their computationally intensive character. In the same way as the multipolar (mp ) expansion is customary used in biomolecular modeling to approximate the classical Coulomb interaction between two charge densities and , we examine in this work the mp approach to approximate the interatomic exchange‐correlation (xc) energies of the Interacting Quantum Atoms method. We show that the full xc mp series is quickly divergent for directly bonded atoms (1–2 pairs) albeit it works reasonably well most times for 1– n (n > 2) interactions. As with conventional perturbation theory, we show numerically that the xc series is asymptotically convergent and that, a truncated xc mp approximation retaining terms up to usually gives relatively accurate results, sometimes even for directly bonded atoms. Our findings are supported by extensive numerical analyses on a variety of systems that range from several standard hydrogen bonded dimers to typically covalent or aromatic molecules. The exact algebraic relationship between the monopole‐monopole xc mp term and the inter‐atomic bond order, as measured by the delocalization index of the quantum theory of atoms in molecules, is also established. © 2017 Wiley Periodicals, Inc.     

Hole growth as a microrheological probe to measure the viscosity of polymers confined to thin films

We review recent hole growth measurements performed at elevated temperatures in freely-standing polystyrene (PS) films, using optical microscopy and a differential pressure experiment (DPE). In the hole growth experiments, which were performed at temperatures close to the bulk glass-transition temperature of PS, T = 97 °C, we find evidence for nonlinear viscoelastic effects, which markedly affect the growth of holes in freely-standing PS films. The hole radius R initially grew linearly with time t before undergoing a transition to exponential growth characterized by a growth time τ. The time scale τ₁ for the decay of the initial transient behavior prior to reaching steady state was consistent with the convective constraint release mechanism of the tube theory of entangled polymer dynamics, while the characteristic hole growth times τ of the holes were consistent with significant reductions in viscosity of over eight orders of magnitude with increasing shear strain rate due to shear thinning. DPE measurements of hole growth on very thin freely-standing films revealed that hole formation and growth occurs only at temperatures that are comparable to or greater than T, even for films for which the T_g value was reduced by many tens of degrees Celsius below the bulk value. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B:Polym Phys 44: 3011–3021, 2006     

A Chemoenzymatic Approach to Enantiomerically Pure Amines Using Dynamic Kinetic Resolution: Application to the Synthesis of Norsertraline

Dynamic transformation : A racemization catalyst and the enzyme Candida antarctica lipase B (CALB) were combined in a one‐pot dynamic kinetic resolution (DKR) of primary amines, which were transformed to their corresponding amides in up to 95 % yield and >99 % ee. This chemoenzymatic DKR was also applied to the synthesis of norsertraline (see scheme).

     

Substituent effects on the barrier to carbamate C-N rotation

Seven aryl-substituted t-butyl N-methyl-N-aryl carbamates were prepared, and in each case, the barrier to C-N bond rotation, , was determined in CDCl₃ solution using variable temperature NMR. A linear free energy relationship is observed between and the electronic stabilization effect of the substituent on the N-aryl ring. More specifically, electron donating groups increase , whereas withdrawing groups decrease . A plot against σ- was more linear (r²=0.96), than a plot against σ (r²=0.90) or σ+ (r²=0.88) and a value of ρ=1.76 was obtained at 243 K. Thus, rotation about the carbamate C-N bond in weakly polar chloroform involves a decrease in positive charge on the nitrogen.     

Physicochemical and Cytochemical Investigations on the Binding of Ethidium and Acridine Dyes to DNA and to Organelles in Living Cells

Ethidium and acridine dyes are classical model substances for studying the binding of small, pharmacologically active molecules to DNA. Intercalation between the DNA base pairs is nearly always proposed as the most important type of binding. According to our investigations, however, there is a second type of binding, which also occurs when the concentration of the bound molecules is low and will be referred to here as external or preintercalative binding. The experimental binding isotherms show that the binding constant for intercalation K_S1 is considerably smaller than that for external binding K_S2 (K_S1 > K_S2). This surprising result is not due to the binding enthalpy (ΔH ≈ ΔH) but to the binding entropy (ΔS > ΔS). Electrostatic interactions between the dye and the DNA represent the most important contribution to both types of binding; they are supplemented by hydrogen bonds and hydrophobic interactions. The behavior of a substance in living cells, however, cannot be reliably predicted from its in vitro binding to DNA. Very few substances are bound to the DNA of the nuclear chromatin in cell culture; for example, dyes often accumulate instead in the lysosomes. In some cases the dye binds specifically and very efficiently to the mitochondria of the living cell, especially to the mitochondrial membranes, the sites of oxidative phosphorylation.     

Preface to the Special Issue of Photocatalysis for Solar Fuels

As natural photosynthesis does, direct conversion of solar energy into the storable form of chemical energy is an intriguing technology for solar energy utilization. Due to increasing concerns of energy and environmental problems caused by the consumption of fossil fuels, production of chemical solar fuels (such as hydrogen, methanol or methane, etc) by artificial photosynthesis, namely photocatalytic splitting of water or reduction of CO2, has been a hot research topic in recent years.     

Pd-Catalyzed Reduction of Aldehydes to Alcohols Using Formic Acid as the Hydrogen Donor

Facile and selective reduction of aromatic aldehydes as well as aliphatic aldehydes to alcohols was achieved using formic acid as the hydrogen donor in the presence of a catalytic amount of Pd(OAc)₂ and Cy₃P. It was found that both hydrogen atoms in the formic acid molecule can serve as the hydride source.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]     

纳米硫化钴与明胶蛋白质的原位键合作用研究

利用紫外-可见吸收光谱(UV-Vis)和傅里叶变换红外光谱(FT-IR),研究了pH值11.00时,不同温度下CoS纳米粒子与明胶蛋白质的键合作用.根据吸光度与CoS浓度的关系,由Lineweave-Burk方程计算了不同温度下CoS纳米粒子与明胶蛋白作用的键合常数K(温度为293 K时键合常数K为3.01×10³L/mol;温度为301 K时键合常数K为2.12×10³L/mol;温度为313 K时键合常数K为1.85×10³L/mol)以及对应温度下反应的热力学参数(ΔrH_m=-17.93 kJ/mol;ΔrS_m=4.93 J/(K.mol);ΔrG_m=-19.37/-19.41/-19.47kJ/mol).CoS纳米粒子与明胶蛋白之间主要靠静电力结合.研究结果为初步探索纳米颗粒与纤维状蛋白质之间相互作用的化学机制提供了必要的信息.     

Gaussian molecular orbital calculations of the barrier to internal rotation in the ethyl cation

Extended Gaussian orbital basis set calculations have been carried out on an assumed staggered and eclipsed classical geometrical configuration of C₂H. The best total energies obtained for these geometries were ?78.170692 a.u. and ?78.170674 a.u. respectively, corresponding to a barrier to internal rotation of 1.8 × 10^?5 a.u. or 11 kcal/mole. An analysis of the charge density matrix indicates that charge is distributed in these molecules in a manner consistent with the concept of hyperconjugation.     

Chemical Tools to Monitor and Manipulate the Adaptive Immune System

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Contributions to the analysis of organic xenobiotics in fish

For the residue-analysis of fish samples, a method has been developed to separate the medium polar triazines and acetamides together with lipophilic xenobiotics (organochlorine pesticides, PCBs and octachlorostyrene) from fish tissue. The detection limits of the analytes are mostly within the ppt-range (ng/kg fillet) and vary between 40 and 1050 ng/kg fillet of fish. Their recoveries range from 75 to 108%, spiked at 1.5 g/kg. The substances are extracted with a mixture of petroleum ether/ethylacetate (2:1) using a soxhlet apparatus. Subsequently, the lipids in the sample extracts are reduced to 0.4% by gel chromatography. A 5 g silica gel clean-up separates the analytes according to their polarity. First, the lipophilic compounds elute with n-hexane/ethylacetate (91:9) and afterwards the more polar compounds with n-hexane/ethylacetate (1:1). Finally, the organic extracts are concentrated to 150–300 L and the compounds are detected and quantified by GC/MS.     

Application of the Numerical Fractionation Approach to the Design of Biofunctional PEG Hydrogel Membranes

A mathematical model is described for surface‐initiated photopolymerization of PEG‐DA forming crosslinked biofunctional PEG hydrogel membranes based on the NF technique. The model includes an additional monomer with biological functionality, which is a common experimental strategy for the design of ECM mimics in tissue engineering in order to direct signaling pathways, and considers concentration‐dependent VP propagation and reaction diffusion termination. The influence of these features on the crosslink density of the soluble and gel phases, the progression through gelation, sol/gel fraction, and molecular weight distribution of biofunctional PEG hydrogel are studied using the NF model. This model may be useful for specific applications of tissue engineering.

     

Heterogeneous Photochemistry Relevant to the Troposphere: H2O2 Production during the Photochemical Reduction of NO2 to HONO on UV‐Illuminated TiO2 Surfaces

Bleached out: The detection of an oxidized co‐product, hydrogen peroxide, during an important tropospheric reaction, combined with the detection of photoaccelerated surface nitrate formation (a competing pathway), allows a likely mechanism to be proposed (see figure) which explains previously observed experimental stoichiometries.