Processive Catalysis

Nature’s enzymes are an ongoing source of inspiration for scientists. The complex processes behind their selectivity and efficiency is slowly being unraveled, and these findings have spawned many biomimetic catalysts. However, nearly all focus on the conversion of small molecular substrates. Nature itself is replete with inventive catalytic systems which modify, replicate, or decompose entire polymers, often in a processive fashion. Such processivity can, for example, enhance the rate of catalysis by clamping to the polymer substrate, which imparts a large effective molarity. Reviewed herein are the various strategies for processivity in nature’s arsenal and their properties. An overview of what has been achieved by chemists aiming to mimic one of nature’s greatest tricks is also included.     

Liquid-crystalline phthalocyanines revisited

A Commentary on the paper ”Homologous series of liquid‐crystalline metal free and copper octa‐n‐alkoxyphthalocyanines?, by J. F. van der Pol, E. Neeleman, J. W. Zwikker, R. J. M. Nolte, W. Drenth, J. Aerts, R. Visser and S. J. Picken. First published in Liquid Crystals, 6, 577‐592 (1989).     

Ion‐Pairing of Phosphonium Salts in Solution: C?H⋅⋅⋅Halogen and C?H⋅⋅⋅π Hydrogen Bonds

The ¹H NMR chemical shifts of the C(α)? H protons of arylmethyl triphenylphosphonium ions in CD₂Cl₂ solution strongly depend on the counteranions X^?. The values for the benzhydryl derivatives Ph₂CH? PPh₃⁺ X^?, for example, range from δ_H=8.25 (X^?=Cl^?) over 6.23 (X^?=BF₄^?) to 5.72 ppm (X^?=BPh₄^?). Similar, albeit weaker, counterion‐induced shifts are observed for the ortho‐protons of all aryl groups. Concentration‐dependent NMR studies show that the large shifts result from the deshielding of the protons by the anions, which decreases in the order Cl^? > Br^? ? BF₄^? > SbF₆^?. For the less bulky derivatives PhCH₂? PPh₃⁺ X^?, we also find C? H???Ph interactions between C(α)? H and a phenyl group of the BPh₄^? anion, which result in upfield NMR chemical shifts of the C(α)? H protons. These interactions could also be observed in crystals of (p‐CF₃‐C₆H₄)CH₂? PPh₃⁺ BPh₄^?. However, the dominant effects causing the counterion‐induced shifts in the NMR spectra are the C? H???X^? hydrogen bonds between the phosphonium ion and anions, in particular Cl^? or Br^?. This observation contradicts earlier interpretations which assigned these shifts predominantly to the ring current of the BPh₄^? anions. The concentration dependence of the ¹H NMR chemical shifts allowed us to determine the dissociation constants of the phosphonium salts in CD₂Cl₂ solution. The cation–anion interactions increase with the acidity of the C(α)? H protons and the basicity of the anion. The existence of C? H???X^? hydrogen bonds between the cations and anions is confirmed by quantum chemical calculations of the ion pair structures, as well as by X‐ray analyses of the crystals. The IR spectra of the Cl^? and Br^? salts in CD₂Cl₂ solution show strong red‐shifts of the C? H stretch bands. The C? H stretch bands of the tetrafluoroborate salt PhCH₂? PPh₃⁺ BF₄^? in CD₂Cl₂, however, show a blue‐shift compared to the corresponding BPh₄^? salt.     

36Cl and41Ca depth profiles in a Hiroshima granite stone and the Dosimetry System 1986

Hybrid stars composed of a strange matter core surrounded by neutron matter are investigated. We apply star models based on phenomenological equations of state (EOS) from nuclear reactions including a phase transition between the hadronic phase and the quark gluon plasma. For specific equations of state hybrid stars might exist. While the nuclear part of the EOS has only a minor influence on the properties of hybrid stars, the EOS for the quark gluon phase has a crucial impact on the existence of such objects.     

The level scheme of91Tc and theβ decay of91Ru(9/2+)

The level scheme of⁹¹Tc has been studied by means of in-beamγ-ray spectroscopy in the irradiations of⁵⁸Ni and⁵⁴Fe targets by⁴⁰Ca beams from the Munich MP tandem and the heavy ion post accelerator. A large number of positive as well as of negative parity yrast-levels with excitation energies up to 8 MeV has been established. The level scheme of⁹¹Tc has been compared with shell model claculations and a good agreement for all identified levels has been found. In parallel investigations of the residual activities of the reaction⁴⁰Ca→⁵⁴Fe, theβ decay of the (9/2⁺ ground) state of⁹¹Ru has been investigated. Aβ decay scheme of⁹¹Ru(9/2⁺) has been established. The half-life of⁹¹Ru(9/2⁺) has been measured to be 9±1 s.     

Investigation of neutron deficient nuclei in the region 28<N,Z <50 with the help of heavy ion compound reactions

In-beamγ ray spectroscopy withγ singles spectra,γγ coincidences andγ ray angular distributions has been performed with several target-projectile combinations:^natCa +³²S,^natCa +⁴⁰Ca,⁵⁴Fe +¹⁴N,⁵⁸Ni +¹²C,⁵⁸Ni +¹⁴N,⁶⁶Zn +¹⁶O,⁶⁸Zn +¹⁶O. Level schemes of⁶⁶Ge,⁶⁸Ge,⁶⁹As,⁷⁰Se,⁷⁴Kr,⁸⁰Sr and⁸²Sr have been deduced. The following level energies and spin-parity assignments have been found:⁶⁶Ge: 957.4 keV, 2⁺; 1693.7; 2174.7, (4⁺); 3685.7; 4207.5;⁶⁸Ge: 1015.8, 2⁺; 1777.9, 2⁺; 2267.9, 4⁺; 2428.8, 3(⁺); 2649.1, 3^?; 3582.3, (5^?); 3649.3, (5^?); 3696.2, (6⁺); 3883.3, (6^?); 4054.4, (7^?); 4454.6; 4837.3;⁶⁹As: 98.2; 164.6;789.6;863.2; 1306;2160;2210.4;2831.5;3258.3;3263.5;3991.1;5195.7;⁷⁰Se: 945.4, 2⁺; 1600.9; 2039.4, (4⁺);⁷⁴Kr: 455.7, 2⁺; 1013.5, (4⁺); 1781.5;⁸⁰Sr: 385.4, 2⁺; 980.2, (4⁺); 1763.2, (6⁺);⁸²Sr: 573.4, 2⁺; 1328.5,(4⁺); 2229.6,(6⁺). γ ray activity spectra have been measured after the bombardment of natural Ca with³²S. The half-life of the new isotope⁶⁹Se has been found to be 27±3 sec. Recoil distance Doppler-shift measurements have been performed with the reactions^62,64Ni,⁶⁶zn(¹⁶O, 2n)^{76, 78} Kr,⁸⁰Sr. The following half-lives have been determined:⁷⁶Kr: 423.8 keV, 2⁺, 37±5 psec; 1034.2,4⁺, 5.7±1.6;⁷⁸Kr: 455.3, 2⁺, 25±3; 1120.0, 4⁺, 3.8±1;⁸⁰Sr: 385.4, 2⁺, 44±6. The energy and half-life systematics of the first excited state of even-even nuclei suggest a maximum of nuclear deformation in the region 28≦N, Z≦50 near ₃₈ ⁷⁶ Sr₃₈ or ₄₀ ⁷⁸ Zr₃₈.     

The relationship between nanoscale architecture and function in photovoltaic multichromophoric arrays as visualized by Kelvin probe force microscopy

The physicochemical properties of organic (multi)component films for optoelectronic applications depend on both the mesoscopic and nanoscale architectures within the semiconducting material. Two main classes of semiconducting materials are commonly used: polymers and (liquid) crystals of small aromatic molecules. Whereas polymers (e.g., polyphenylenevinylenes and polythiophenes) are easy to process in solution in thin and uniform layers, small molecules can form highly defined (liquid) crystals featuring high charge mobilities. Herein, we combine the two material types by employing structurally well-defined polyisocyanopeptide polymers as scaffolds to precisely arrange thousands of electron-accepting molecules, namely, perylenebis(dicarboximides) (PDIs), in defined chromophoric wires with lengths of hundreds of nanometers. The polymer backbone enforces high control over the spatial location of PDI dyes, favoring both enhanced exciton and charge transfer. When blended with an electron-donor system such as regioregular poly(3-hexylthiophene), this polymeric PDI shows a relative improvement in charge generation and diffusion with respect to monomeric, aggregated PDI. In order to correlate this enhanced behavior with respect to the architecture, atomic force microscopy investigations on the mixtures were carried out. These studies revealed that the two polymers form interpenetrated bundles having a nanophase-segregated character and featuring a high density of contact points between the two different phases. In order to visualize the relationship between the architecture and the photovoltaic efficiency, Kelvin probe force microscopy measurements were carried out on submonolayer-thick films. This technique allowed for the first time the direct visualization of the photovoltaic activity occurring in such a nanoscale phase-segregated ultrathin film with true nanoscale spatial resolution, thus making possible a study of the correlation between function and architecture with nanoscale resolution.     

Polyisocyanides derived from tripeptides of alanine

Helical polymers of isocyanotripeptides derived from alanine have been synthesized and their architectures studied in detail. The helical conformation of the polyisocyanotripeptides is stabilized by internal hydrogen-bonding arrays between the tripeptide side chains. The possibility of extending the well-defined hydrogen-bonded array, from dipeptide to tripeptide side chains, depends strongly on the stereochemistry of the constituent alanine amino acids, as has been shown by circular dichroism and IR studies. In polymers containing a weaker hydrogen-bonding array adjacent to the polymer backbone, due to steric interactions between the alanine methyl groups, a stronger second hydrogen-bonding array was present between the peptide bonds furthest away from the main chain, which is probably a result of stretching/compression of the helical-polymer conformation.