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Time: 2017-02-17

 

glue

Recently, Chemical&Engineering News (C&EN), a journal published by American chemical society, released the top ten chemical research in the world in 2014, and two research results participated by Chinese research teams are listed. Professor li yan's research team at Peking University has developed a new method for manufacturing high-purity single-walled carbon nanotubes of a specific type.

 

1. Role of different salts in Negishi coupling reaction

Since its discovery in 1977, the Nobel prize-winning Negishi coupling reaction has been widely used to splice two organic groups to produce more complex molecules, which can be antibiotics or active compounds in light-emitting diodes.

In the Negishi coupling reaction, zinc reagent is usually prepared from organometallic precursor and zinc halide. The zinc reagent transfers its organic group to palladium

catalyst

, forming palladium complexes, a process called transmetalation. The palladium complex then mediates a c-c coupling reaction between the organic group and another organic group derived from an organic halide.

This year, chemists made new discoveries about the role of salt additives in certain types of Negishi coupling reactions. Lucas c. McCann and Michael g. Organ, from the university of york, concluded after nearly a decade of research that aryl and alkyl zinc halide reagents need metal halides (such as lithium chloride) as salt additives to initiate cross-coupling reactions. However, aryl zinc reagents do not need the salt additives, and the dialkyl zinc reagents do not even work. The key, the authors explain, is to make the zinc starting reagent compatible with the right one

solvent

Polarity was matched to form zinc transmetalating species. Salt additives can facilitate this process if needed.

The discovery means that instead of relying on a standard set of reaction conditions for all types of coupling reactions, chemists can pick and choose conditions to optimize the reaction and, in some cases, not even use salt additives if a more "green" reaction is not needed.

 

2. "enhanced" DNA containing three base pairs was successfully introduced into living bacteria

The two bases in DNA -- adenine and thymine, cytosine and guanine -- are the genetic code of choice for life on earth over the course of its long evolution.

This year, Floyd e. Romesberg of the Scripps research institute and his colleagues extended the code by introducing DNA containing three base pairs (" enhanced "DNA) into living bacterial cells. The new bases, d5SICS and DNAM, interact in pairs through hydrophobic interactions, unlike natural DNA bases, which pair through hydrogen bonds.

 

3. Chiral catalysts bring new stereocomposite polymers

Geoffrey w. Coates's team at Cornell university used a chiral cobalt catalyst to copolymerize propylene oxide enantiomers with succinic anhydride to produce a semi-crystalline, three-dimensional composite of polypropylene glycol succinate, a new class of thermoplastics. The stereocomposite polymer consists of both right-handed and left-handed polymer chains and can be crystallized in a way that cannot be achieved by either right-handed or left-handed rotation alone. Polymer chemists can better control its thermal properties and biodegradability. Remember, stereocomplexes are extremely rare, with only a dozen known examples.

The team first designed a chiral cobalt catalyst and then used (R,R) or (S,S) type catalyst to copolymerize (R) or (S) type oxidized propylene with succinic anhydride to produce (R) type or (S) type polypropylene succinate. In addition to biodegradable, the three-dimensional complex is about 120 ℃, melting point of polymer than separate conformation of polymer or low density polyethylene high 40 ℃. In addition, the polymer can be rapidly crystallized from a molten state.

Potential applications of the polymer include biomedical materials and large biodegradable packaging materials.

 

Without crystallization, cryo-electron microscopy reveals the high-resolution structure of the protein "machine.

Structural biology research has reached a milestone this year. Without the traditional protein purification and crystallization process, the near atomic structure of the large ribosome subunit in the mitochondria of yeast was obtained by the research team of Venkatraman Ramakrishnan and Sjors h. w. Scheres in MRC molecular biology laboratory, Cambridge university, with A resolution of 3.2a, using only cryogenic electron microscopy.

The protein "machine" has a molecular weight of about 3 million daltons and contains 39 proteins, which are critical to the production of mitochondrial membrane proteins in yeast cells.

Next, the team plans to study ribosomes in human mitochondria.

 

5. A new method for manufacturing specific types of single-walled carbon nanotubes with high purity

With their characteristic strength, flexibility, and electrical conductivity, single-walled carbon nanotubes look like rolled up barbed wire and are promising for use in solar cells and in miniaturized electronic circuits. However, single-walled carbon nanotubes (SWNTS) will encounter a problem that cannot be solved for a long time in the production process -- the product purity is low. Carbon nanotube products are often mixtures of various diameters and chirality. Remember, chirality is the configuration of carbon atoms in a carbon nanotube, and it can affect whether a carbon nanotube behaves like a metal or a semiconductor.

This year, two teams of scientists independently published their results, finding possible solutions to the problem.

Professor li yan's team at Peking University grew single-walled carbon nanotubes that were 92 percent pure, up from 55 percent previously (Nature 2014, DOI: 10.1038/nature13434). These carbon nanotubes are chiral and have metallic properties. The key, says li, is to find the "right recipe" for the high-temperature tungsten-cobalt alloy nanocrystalline catalyst used for the growth of nanotube "seeds".

Another team of scientists in Germany and Switzerland prepared single-walled single-walled carbon nanotubes (Nature 2014, DOI: 10.1038/nature13607) using pahs as precursors. After the surface of platinum was heated, the precursor was folded into a nanotube cap. With the addition of ethanol as a carbon source, the nanotube was gradually extended, and finally a single walled carbon nanotube product without defects was obtained.

The researchers' next goal is to figure out how to scale up the synthesis and adjust the method to make pure single-walled carbon nanotubes with different sizes and chirality.

 

6. Afm images of hydrogen bond interactions are questionable

In 2013, a team from China reported in the journal Science on atomic force microscopy (AFM) images of hydrogen bond interactions, showing the electron density of the hydrogen bond that connects 8-hydroxyquinoline molecules (Science 2013, DOI: 10.1126/science.1242603).

But, according to the Finnish and Dutch scientists research results published in this year, Phys. Rev. Lett. 2014, DOI: 10.1103 / physrevlett. 113.186102), the team acquired images, hydrogen bonds may not be real, but the tip of atomic force microscopy and the potential energy surface interactions between molecules.

"We're not saying there are no hydrogen bonds," the scientists said. "we're just showing what happens when there are no bonds, which you can compare." The scientists used atomic force microscopy to study a tetramer of a double (p-pyridine) acetylene molecule held together by an intermolecular c-h · N hydrogen bond, bringing two nitrogen atoms on separate molecules closer to 3A. The two nitrogen atoms should not have any bonding interactions, but the afm image shows a bond between the two atoms.

 

7.IrO4+ becomes a molecule of *** with an oxidation state of +9

According to the report, since 2009, professor zhou mingfei of fudan university has cooperated with professor Riedel of free university of Berlin, Germany, to carry out experimental preparation of high-oxidation valence compounds and research on their chemical properties. In 2009 through the metal iridium atoms and oxygen molecule reaction method for the first time in cryogenic gases matrix was prepared by four iridium oxide neutral molecules, infrared absorption spectrum experiment combined with the theory of quantum chemistry calculation showed that the molecular of iridium (with the valence electron configuration d1) in VIII valence, shows that besides the ruthenium and osmium and xenon three elements, iridium elements can form VIII valence compounds.

Based on this work, they propose that iridium could be in the IX valence state if the d electron of the neutral iridium tetroxide molecule is further ionized to produce iridium tetroxide positive ion. In order to validate this view from the experiment, Zhou Mingfei group with pulse laser sputtering - supersonic molecular beam loading technology under the condition of the gas phase was prepared by four iridium oxide ion, the team recently established cascade time of flight mass spectrometry - infrared photolysis from spectroscopy successfully received four iridium oxide gas phase ion infrared vibration spectrum, for the first time confirmed that the gas phase four iridium oxide ions with regular tetrahedron structure, in which the iridium in IX valence, which determines the IX valence compounds in experiment

 

8. Two new discoveries of graphene: it is degradable and can transmit protons

One of graphene's properties, its chemical stability, was called into question this year. A study shows that when reduced graphene oxide (RGO) ACTS as a supporting layer in the catalytic reactions and electrons

equipment

When in use, the material can be decomposed. This study demonstrated that RGO, as a support layer for titanium dioxide nanoparticles, unexpectedly disintegrates when exposed to ultraviolet light (chem.mater. 2014, DOI: 10.1021/cm5026552). Hydroxyl radicals are generated on the surface of these photocatalytic nanoparticles, which oxidize and attack RGO, leading to RGO fragmentation and the formation of pahs. If exposed to ultraviolet light, these pahs eventually break down completely into carbon dioxide and water.

Another study by scientists at the university of Manchester, United Kingdom, found that pure single graphene was surprisingly good at transmitting protons (Nature 2014, DOI: 10.1038/nature14015). The discovery could be used in fuel cells, which require thin proton conducting membranes.

 

9. Perovskite materials facilitate the research of efficient and low-cost solar cells

Commercial solar cells made from semiconductor materials such as high-purity silicon are about 25 per cent efficient at converting sunlight into electricity, but they are expensive. In the past, lower-cost batteries, such as those based on polymers or quantum dots, have been consistently less efficient at converting solar energy, at about 10 percent.

Perovskite-type solar cells have been making rapid progress since 2012. In February, C&EN reported that the best perovskite solar cells at the time had about 16 percent conversion efficiency; Earlier this month, the national renewable energy laboratory confirmed that a solar cell from the Korea Research Institute of Chemical Technology had a conversion efficiency of 20.1 percent.

In addition, scientists at northwestern university this year demonstrated that the CH3NH3 SnI3 could be used to make perovskite batteries. The (CH3NH3)SnI3 is an air-sensitive, lead-free material that conforms to ABX3 stoichiometry and is generally incompatible with other solar cell components. This material is lead-free and addresses concerns about lead toxicity (nat.photonics 2014, DOI:

10.1038 / nphoton. 2014.82).

Researchers at Oxford University have found that adding an insulating polymer embedded with carbon nanotubes increases the resistance of perovskite solar cells to humidity and thermal degradation (Nano Lett. 2014, DOI: 10.1021/nl501982b).

 

Computational chemical modeling helps to discover new products and new reaction routes

Scientists at Stanford university have developed a new computational chemical system called ab initio nanoreactor to help develop new reaction pathways and new chemical products.

The method USES ab initio molecular dynamics accelerated by a graphics processing unit (video card) to simulate chemical reactions. In the simulation, the nanoreactor identified some products that had been discovered through experimental means, and also identified some products that had not yet been discovered. The reason these products have not been discovered is usually because chemists cannot achieve the high temperatures and pressures required to prepare them in the laboratory.

The scientists used the system to simulate acetylene polymerization and the formation of biomolecules and other complex products from simple compounds that existed on early earth, similar to those used in the classic 1953 urey-miller experiment. The computational system mixes and compresses compounds in a virtual environment, using quantum mechanics to simulate bond breaking, bond formation, and molecular rearrangement, and to determine the reaction mechanism by tracking the minimum energy path between the reactants and the products.