chemistry: June 2023

Thursday, 29 June 2023

Controlling chemical reactions with quantum physics

In the battle against controlling chemical reactions, researchers have reported, for the first time, the manipulation of the formation rate of urethane molecules in a solution contained inside an infrared cavity.

Controlling chemical reactions to generate new products is currently one of chemistry’s largest challenges.

Developments in this area impact numerous industries. For example, improving the production of catalysts to accelerate chemical reactions and reducing the waste generated in the manufacture of construction materials.

Manipulating the chemical reactivity of molecules

Because of this, different laboratories specialising in the field of polariton chemistry have developed experiments in optical cavities over the last ten years. These experiments aim to manipulate the chemical reactivity of molecules at room temperature, using electromagnetic fields.

Some teams have succeeded in modifying chemical reaction products in organic compounds.

However, no research team has been able to come up with a general physical mechanism to describe the phenomenon and to reproduce it to obtain the same consistent results.

Now, researchers from the University of Santiago, led by principal investigator Felipe Herrera, and the laboratory of the chemistry division of the US Naval Research Laboratory have reported the manipulation of the formation rate of urethane molecules in a solution contained inside an infrared cavity for the very first time.

The discovery proves that selectively controlling the reactivity of certain bonds in a chemical reaction at room temperature in a liquid solvent is theoretically and experimentally possible. This is through the influence of the electromagnetic field vacuum in a narrow range of infrared frequencies.

“This theoretical discovery improves our fundamental understanding of the phenomenon over other models that merely explain partial aspects of the experimental observations or simply refute the experimental evidence entirely,”

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Thursday, 22 June 2023

Atomic force microscopy probes mechanochemical kinetics

Experiments probing how force accelerates chemical reactions have provided new insights into mechanochemical kinetics. The findings by US-based scientists could inform the development of chemical manufacturing methods that are more sustainable and less wasteful than current approaches.

In mechanically activated organic chemistry, force drives the making and breaking of covalent bonds. This helps to minimise the waste and energy cost associated with organic synthesis because the reactions are run neat or with minimal solvent, and energy is provided through mechanical, rather than thermal, means.

However, as materials scientist and organic chemist Adam Braunschweig from the City University of New York explains, mechanochemistry has not yet been widely adopted, in part because of the substantial gaps that remain in our understanding of reaction kinetics during these reactions.

Aiming to provide answers, they experimentally and computationally studied the reaction kinetics of mechanically activated Diels–Alder cycloaddition reactions between four dienophiles and surface-confined diene monolayers to measure how force affects reaction rates.

The reactions were carried out on monolayers so that the complexities associated with grinding powders during milling and reactant availability were not factors in the kinetic analysis. This meant that the effects of force on the free energy of activation and reaction trajectories would be isolated.

The researchers then used elastomeric arrays containing 900 pyramidal tips to bring fluorescently labelled dienophiles into contact with monolayers of the tethered diene, anthracene, that was immobilised covalently onto the surface of a silica wafer.

‘We attached [the tips] to a piezo actuator, which just allows you to press very controllably into the surface,’ explains Braunschweig. ‘We pushed down, for one second, two seconds, three seconds, so we got the full-time course. Then, what’s great about these tips is you can tilt them with respect to a surface; so one bit touches with a different force than another bit.’

‘In a single experiment, you get the entire time course and all the forces plus each of these patterns is repeated 30 times, so you get really good, high fidelity, low-error data.’

They then used fluorescence microscopy to track the Diels–Alder adduct formation as a function of applied force and reaction time under force.

‘We discovered that the moderate mechanical energy that is applied along the right trajectory, can substantially accelerate the rate of organic reactions,’ adds Yerzhan Zholdassov , a doctoral student in Braunschweig’s lab and lead researcher on the study.

‘Moreover, different molecules have different responses to the applied mechanical energy,’ he says. ‘These findings have major implications in chemical industry, which allows us to produce chemical products without the harmful solvents and high energy input.’

Monday, 19 June 2023

This organic molecule shines bright by breaking the rules

In OLED devices, electric current excites electrons in organic molecules to a higher energy state, and the molecules fluoresce visible light as the electrons relax back to ground state.

The electronic configuration of these excited states are dictated by quantum mechanics, but one principle has stymied advances in OLED technology: Hund’s rule holds that for a given electron configuration, the state with the most unpaired electrons spinning in parallel—called the triplet state—will have a lower energy than a state in which more of these unpaired electrons have opposite spins. This higher-energy configuration is called the singlet state, and only electrons moving from the excited singlet state to the ground state fluoresce light that’s useful for OLEDs. But many of these electrons get trapped in the intermediate triplet state long enough to extinguish fluorescence and kill an OLED’s efficiency, especially when it comes to generating blue light.

Naoya Aizawa, a chemist at the University of Osaka, and his colleagues wondered if they could overcome this challenge by finding exceptions to Hund’s rule where the triplet state is higher energy than the singlet state. That would clear a path for electrons languishing in a triplet state to easily move into a singlet state to emit light. Recent studies suggested that some heptazine-based molecules could do the trick.

To investigate, Aizawa and his team created computational simulations to screen 34,596 heptazine analogs for electron configurations where the triplet and singlet state energies could be inverted. Out of 5,264 promising candidates, the researchers synthesized two heptazine analogs that their calculations predicted would fluoresce blue light.

They found that a molecule called HzTFEX2 broke Hund’s rule with inverted singlet and triplet states that allow the molecule to emit blue light as predicted. When they built a prototype OLED device using HzTFEX2, the researchers calculated 85% quantum efficiency versus the maximum 25% efficiency of traditional OLEDs constrained by Hund’s rule, Aizawa says.

Cody Schlenker, a University of Washington chemist who wasn’t involved in the study, says this work shows that questioning conventional wisdom can lead to technological breakthroughs. “These inverted singlet-triplet materials will spur a lot of exciting new developments in the field” of OLEDs, he says.

Monday, 12 June 2023

The Benefits of Using Optical Sensors in Chemical Analysis

Optical sensors have revolutionized the field of chemical analysis by providing a powerful and efficient method for detecting and quantifying a wide range of analytes by measuring the light absorption, reflection, or emission from the various types of samples and providing highly accurate and precise results. This article will explore the benefits of using optical sensors in chemical analysis, its applications, and recent developments.

Optical Chemical Sensors

Optical sensors are a crucially significant group of analytical instruments that give chemical information regarding various topics, including molecular structure, microscopic imaging, and analyte concentration. Optical sensors use several signal transduction methods based on photonic attributes, including reflectivity, polarization, refractive index, fluorescence intensity, transmission, and absorbance.

Basic Components and Working

Optical chemical sensors are part of analytical systems that use optical transmission to accomplish chemical measurement by interacting with a chemical system and then converting the resultant optical signal into an electrical signal.

An optical chemical sensor has three basic components: a molecular recognition element, optoelectronic instrumentation, and optical fiber.

Molecular recognition elements that provide a response when the analyte is present are connected with optoelectronic instrumentation components using optical fibers as the integrating medium. These devices transmit information about chemical reactions by encoding it in an optical signal that moves across optical fibers.

Advantages of Optical Sensors in Chemical Analysis

Optical sensors can measure various analytes, including organic and inorganic compounds, and detect and quantify various chemical compounds, including drugs, biomolecules, explosives, and heavy metals. Optical sensors can also detect changes in the sample's physical and chemical characteristics, such as pH, temperature, and pressure, making them very flexible and suitable for various applications.

Fast and Real-time Analysis as Compared to Traditional Analytical Methods

Optical sensors allow fast and real-time analysis by providing results within seconds or minutes, significantly faster than traditional analytical methods like chromatography and spectroscopy, which is essential for many industrial and scientific applications. In addition, optical sensors can be used in situ, which means they can be deployed directly in the field, providing immediate analysis without sample preparation or transportation.

Non-Invasive and Non-Destructive Analysis

Compared to other analytical techniques, optical sensors are non-invasive and non-destructive, which means they can be used to analyze chemicals without damaging and measure them in situ without sampling or extraction. This makes optical sensors ideal for monitoring and analyzing live samples, such as cells, tissues, and organisms, without affecting their viability or function.

Moreover, optical sensors are compact portable, and scalable and can be used to analyze multiple samples simultaneously, providing cost-effective, fast, and efficient analysis, making them ideal for field and remote applications.

Recent Developments

Refractive Index (RI) Sensors

During the last two decades, refractive index (RI) sensors, including devices like resonant microcavities, photonic crystals, optical fibers, diffraction gratings, interferometers, and surface plasmon resonance instruments, have emerged as promising technologies within the larger category of optical sensors.

Based on the change in refractive index brought on by analyte attachment, the remarkable array of instruments used in optical sensors enables label-free molecular detection without the additional complication of fluorescent or enzyme tags.

Detecting Copper(II) in Drinking Water

Researchers created and tested an optical chemical sensor for detecting Copper(II) in drinking water in a study published in 2019. The researchers found that this novel sensor could detect Cu (II) at a broad concentration range after testing it with various concentrations of copper in NaCl 0.1 M solutions at different pH values. This optical, chemical sensor is ideal for low-cost, in situ, and fast detection of Cu(II) in drinking water, which would help reduce water-related health concerns.

Applications of Optical Sensors in Chemical Analysis

Optical sensors have a wide range of applications in chemical analysis, including environmental monitoring, food safety, healthcare, and more.

Environmental Monitoring and Food Safety

Optical sensors are often used in environmental monitoring to identify pollutants, contaminants, and dangerous compounds and track soil contamination, water quality, and air quality in real time. Similarly, optical sensors provide quick, non-invasive chemical analysis of food samples to ensure they adhere to regulatory requirements and are suitable for consumption.

Detecting Biomarkers and Disease-specific Molecules

Chemical analysis via optical sensors allows the detection of biomarkers and disease-specific molecules in blood, urine, and other bodily fluids, providing early and accurate diagnosis of diseases as well as helping monitor drug efficacy and toxicity, providing real-time and personalized treatment options for patients. Optical sensors are also used in medical imaging techniques, such as optical coherence tomography (OCT) and fluorescence microscopy, to provide high-resolution and non-invasive imaging of tissues and organs.

Future Prospects

Optical sensors have been used increasingly in chemical analysis in recent years, and this trend is projected to continue. Furthermore, as optical sensors may be used to monitor and control light at the nanoscale, they are predicted to be crucial in developing next-generation technologies like quantum computing and nanophotonics, in addition to their uses in chemical analysis.

Overall, the prospects for utilizing optical sensors in chemical analysis are positive, and in the years to come, they are anticipated to have a substantial influence on various industries and fields, such as pharmaceutical, food, and environmental monitoring.

International Conference on Organic Chemistry

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Thursday, 8 June 2023

Novel molecular orbital interaction that stabilizes cathode materials for lithium-ion batteries

A large international team led by scientists from the Institute for Superconducting and Electronic Materials at the University of Wollongong has verified that the introduction of novel molecular orbital interactions can improve the structural stability of cathode materials for lithium-ion batteries.

The production of better cathode materials for high-performance lithium-ion batteries is a major challenge for the electric car industry.

In research published in Angewandte Chemie, first author Dr. Gemeng Liang, Prof Zaiping Guo and associates, used multiple capabilities at ANSTO and other techniques to provide evidence that doping a promising cathode material, spinel LiNi0.5 Mn1.5 O4 (LNMO), with germanium significantly strengthens the 4s-2p orbital interaction between oxygen and metal cations.

"The 4s-2p orbital is relatively uncommon, but we found a compound in the literature in which germanium has a valence state of + 3, enabling an electron configuration ([Ar] 3d104s1) in which 4s transition metal orbital electrons are available to interact with unpaired electrons in the oxygen 2p orbital, producing the hybrid 4s-2p orbital."

The 4s-2p orbital creates structural stability in the LNMO material, as determined using synchrotron and neutron experiments at ANSTO's Australian Synchrotron and the Australian Center for Neutron Scattering, as well as other methods.

The team used neutron and (lab-based) X-ray powder diffraction, as well as microscopy, to confirm the location of the doped germanium at the 16c and 16d crystallographic sites of the LNMO structure with Fd3 ̅m space group symmetry.

As the valence state of the germanium dopants was important to investigate, laboratory X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements at the Australian Synchrotron were carried out. They confirmed that germanium dopants have an average valence state of +3.56, with germanium at the 16c and 16d sites being +3 and +4, respectively. The results of density functional theory (DFT) calculations supported this observation.

The researchers evaluated the electrochemical performance of batteries containing LNMO and compared that with those containing LNMO with 4s-2p orbital hybridization (known as 4s-LNMO). These assessments found that doping with 2% germanium contributed to superior structural stability, as well as reduced battery voltage polarization, improved energy density, and high voltage output.

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Wednesday, 7 June 2023

A Novel Reagent for De-Electronation

Freiburg chemists have successfully converted polynuclear transition metal carbonyls into their homoleptic complex cations using common inorganic oxidants.

Perhaloanthracene radical cations de-electronize trimetal dodecacarbonyls under carbon monoxide pressure to form the first clustered transition metal cations. Image Credit: research group
The research group of Malte Sellin, Christian Friedmann, and Professor Dr. Ingo Krossing from the Institute of Inorganic and Analytical Chemistry, as well as Maximilian Mayländer and Sabine Richert from the Institute of Physical Chemistry at the University of Freiburg, were included in the study.
The study demonstrates that a nitrosonium salt can transform an anthracene derivative with a half-step possibility of 1.42 Volts against Fc0/+ to the radical de-electronating salt.
De-Electronator Made From Commercial Chemical
Chemists at the University of Freiburg have been looking for a way to ionize substrates without triggering undesired side reactions in order to gain access to the hitherto almost unknown class of clustered transition metal carbonyl cations.
During ionization, a neutral molecule loses one or more electrons. Consequently, a positively charged molecule known as a cation is established. A so-called innocent de-electronator is an ionizing agent that only accepts electrons from the substrate and exhibits no other unpleasant reactivities.
Since the only innocent de-electronator known to date, a perfluorinated ammoniumyl cation, requires laborious and time-consuming synthesis, the Freiburg researchers devised a substitute that is produced directly from a commonly produced chemical: A nitrosonium salt can transform the anthracene derivative, which has a half-step possibility of 1.42 volts vs. Fc0/+, to the radical de-electronator salt.
“The de-electronating salt allows us to remove electrons from the system while preserving the structure. So it’s particularly mild and creates systems that we haven’t been able to represent before. In the long term, these could help us to produce better catalysts,” Krossing explains.
The Perhalogenated Anthracene De-Electronator is Putative
To begin, the researchers attempted to synthesize the desired transition metal carbonyl cations by reacting trimetal dodecacarbonyls with a silver salt as an oxidant. The expected result was also not obtained when the trimetal dodecacarbonyls were directly reacted with nitrosyl cations.
“However, if the nitrosyl cation is reacted in advance with a perhalogenated anthracene derivative, then the resulting acene radical cation de-electronates the trimetal dodecacarbonyls under carbon monoxide atmosphere and leads to the desired salts,” Sellin explains.
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Tuesday, 6 June 2023

Electrophotocatalysis widens the scope of carbonyl olefination

A new electrophotocatalytic method broadens the scope of the classic carbonyl olefination reaction. The approach works with a wider array of starting materials and produces less waste than traditional methods like the Wittig reaction.

Olefins are important building blocks for organic synthesis. But established methods of accessing these compounds from cheap carbonyl feedstocks often require strong bases, which limit the reactions’ functional group tolerance. Now, Tristan Lambert and graduate student Keri Steiniger from Cornell University, US, have instead employed electrophotocatalysis to access a greater range of olefins while simplifying the waste stream.

The process couples electrochemical and electrophotocatalytic oxidations to generate highly reactive intermediates in a controlled manner. The process enables readily available alkenes, themselves acting as olefination agents, to react with both aldehydes and ketones. The main by-products are nitrogen and hydrogen gases.

A powerful photooxidant catalyst allows the transformation to take place over a competing carbon–carbon bond-forming reaction, explains Lambert. ‘This project began by asking whether our TAC electrophotocatalyst could induce the oxidative denitrogenation of diazenes but with a product selectivity different than what was typical for such reactions,’ he says. ‘We speculated that controlling the rate of back electron transfer might enable conversion to olefins rather than cyclopropanes. That turned out to be true and the key, we believe, is due to the redox characteristics of the TAC.’

First, a diazo compound is electrochemically generated from an organic carbonyl and a hydrazine. A cyclic diazene then results from cycloaddition with the chosen alkene. The loss of nitrogen gas via electrophotocatalysis results in a distonic radical cation before reduction to the final olefin product. ‘It looks like a bit of a Rube Goldberg setup when drawn out on paper, but remarkably it all works together pretty well,’ adds Lambert.

Thierry Ollevier, an expert in homogeneous catalysis from Université Laval in Quebec, Canada, praises the approach. ‘With nitrogen and hydrogen as the main by-products, this new method has a major advantage over standard olefination methods [that usually lead] to problematic waste,’ he says. ‘Using in situ generated diazoalkanes paves the way to other applications involving subsequent reactions of unstabilised diazoalkanes. Scaling up can be envisioned.’

As an alternative to Wittig olefination, the new strategy offers broad applicability for synthesis of materials, pharmaceuticals and natural products. ‘This innovative method offers a real alternative to classical olefination, especially to make alkene targets that include base-sensitive functionality like alkyl halides or acetates,’ notes William Unsworth, an expert in synthetic organic chemistry from the University of York, UK. ‘The ever-growing prominence of electrochemical synthesis means that the method should be well used in organic synthesis in industry and academia.’

Friday, 2 June 2023

A Proton, An Electron—And Thou

I’ve argued ad nauseam that battery-electric propulsion is poorly suited for most full-scale aircraft applications. To deliver a certain amount of power for a certain length of time requires a weight in batteries 15 times that of conventional fuels. Batteries may serve for flights at low speeds over short distances, such as an hour of student instruction, but they don’t lend themselves to passenger- or cargo-carrying trips of many hundreds of miles at hundreds of miles an hour. For that, you need fuel.

Among potential aviation fuels that are not hydrocarbons like gasoline and ethanol, hydrogen is currently the most promising. Seated at the head of the periodic table, hydrogen is the lightest element, a minimalist atom consisting only of a proton and an electron. It is not scarce: three-quarters of the mass of the universe is hydrogen. Hydrogen contains lots of energy—about three times as much, per pound, as gasoline. In that respect, it’s at the opposite end of the spectrum from batteries and far ahead of any hydrocarbon fuel.

Unfortunately, its density at sea level pressure is 1/8000 that of gasoline, and, since energy is proportional to mass, not volume, it must be stored in condensed form to be a practical fuel. It can be cooled to an extremely low temperature and stored as a liquid, but the liquid form has significant disadvantages, notably that it must be taken aboard shortly before flight and the tanks have to be extraordinarily well insulated. The approach used by car manufacturers, and likely to be adopted by builders of hydrogen powerplants for airplanes, is to store gaseous hydrogen under high pressure in carbon-fiber tanks. With the high energy content of the compressed hydrogen somewhat compensating for its low density, cars like Toyota’s Mirai and Hyundai’s NEXO are able to get reasonable range (around 300 miles) out of 700-bar (10,000-psi) tanks that fit unobtrusively within the chassis of the vehicle.

Hydrogen can fuel reciprocating and turbine engines, but that avenue, while it has familiarity to recommend it, is not the one currently getting the most attention. Instead, hydrogen would be used to produce electricity to drive electric motors.

The apparatus that makes electricity out of hydrogen is called a fuel cell. Instead of storing an electrical potential as a battery does, a fuel cell generates one as it goes along. Various kinds of fuels and cycles are possible. A hydrogen fuel cell works, in broad outline, by separating the hydrogen molecules’ positively-charged protons from their negatively-changed electrons with a chemical catalyst. A filtering membrane, permeable to protons but not to electrons, divides the particles into two groups with opposite electrical charges. The particles are finally reunited through an external electrical circuit—this is where the useful work is done—and combined with ambient oxygen to produce water.

One of the fuel cell’s great advantages over the internal combustion engine is that it has no moving parts and therefore very high reliability—always a desirable thing in aviation. Another, on general environmental grounds, is that its exhaust consists principally of water. Advocates of hydrogen power tend, however, to gloss over the fact that hydrogen is not found lying around on earth, like coal or oil, waiting to be scraped up or pumped out. It exists mainly in chemical compounds, notably water, from which it must be forcibly liberated before it can be used. The creation of hydrogen gas requires energy, and most of that energy comes, today at least, from fossil-fueled powerplants.

The electrical potential across a single fuel cell is small—currently less than one volt—and so for practi-cal applications multiple cells, combined in a “stack,”are connected in series and parallel to produce a desired voltage and power output. A powertrain consisting of such a stack, an electric motor, and the required power controller is comparable in size and weight to a conventional reciprocating or turbine engine. It will almost certainly be cheaper to manufacture and have a much longer service life. It is more efficient, so it rejects less waste heat and entails less cooling-related drag. Electric motors are compact and permit slender, perfectly streamlined nacelles and cowlings. They are also highly reliable. Furthermore, because of the high energy content of hydrogen and the superior efficiency of the fuel cell, less fuel—a fifth as much by weight—is required for a given mission.

So far, so good. Now we come to the hard part, which is entirely related to the inconveniently low density of hydrogen. The happy coincidence that wings appropriately sized for flight have hitherto provided the internal volume for an appropriate supply of fossil fuel disappears when the fuel is hydrogen. Even assuming storage at 700 bar, the hydrogen required to go a given distance at a given speed takes up about five times as much space as conventional fuels would. And you can’t put compressed hydrogen into just any unused compartment; you have to store it in strong spherical or cylindrical containers with fancy impermeable liners,and these have to be placed somewhere near the center of gravity, or evenly distributed around it. The weight saved in fuel mass, luckily, may cancel some or all of the added burden of special tanks.

Several firms, including Boeing and Airbus, have developed fuel-cell prototypes in the past. A California company, ZeroAvia, has replaced one engine on each of two twin-engine Dornier 228s with 600 kW (800 hp) hydrogen-electric powertrains; one is currently being tested in England and the other in California. ZeroAvia has also announced a program with Textron to convert a Cessna Caravan to fuel cell power. Other conversion projects have been announced by other firms, but as far as I know they are not so far advanced as ZeroAvia’s are.

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