Chasing cations
ChemSci Pick of the Week
Scientists have studied transient, hard-to-capture reaction intermediates, using an enhanced form of NMR spectroscopy.
Zeolites are a type of mineral made from elements such as aluminium and silicon – they have a porous structure that gives them some special uses in chemical reactions. For example, they can accommodate other atoms or molecules in their pores. This makes them very useful as catalysts in chemical reactions, because they can hold molecules in the right positions to react.
An international team of scientists – from China, the UK, the US, France and Belgium – have used advanced spectroscopy techniques to study some of these reactions. They are particularly interested in learning more about reaction intermediates – that is the species that exists fleetingly in between the starting chemical and the final product. They have focused their study on carbocation intermediates, which are molecules containing a carbon atom that is positively charged.
Studying carbocation intermediates is challenging because they don’t exist for very long, and there are never many of them in the reaction at once. This is a problem because most of the techniques that chemists use to study molecules take some time, and require a certain quantity and concentration of the molecule to be present.
Frédéric Blanc, from the University of Liverpool, along with his international team, have attempted to study carbocation intermediates in zeolites using a technique called nuclear magnetic resonance (NMR) spectroscopy. This technique is very similar to MRI, which is often used to make medical diagnoses.
Frédéric explains: "Molecules are too small to be observed by the naked eye. Chemists rely on the detection of these molecules to make progress and often use a technique called NMR to do so. Unfortunately, this technique is very slow and requires a lot of time (minutes or hours) to detect the NMR signal. The problem is exacerbated when the quantity of chemicals is very small."
The team has overcome this problem using a fairly recently redeveloped technique called dynamic nuclear polarization (DNP). DNP works by boosting the signal obtained from NMR – by several orders of magnitude – by using a higher electron polarisation. This means that the carbocation intermediates can be detected, even when there are very few of them. Not only that, but it can be done in seconds.
"I have always been fascinated by chemical structures and ways to see invisible matter," says Frédéric. "We can now not only see this invisible matter but we can do so very quickly."
The majority of both basic and fine chemicals – as well as polymers that serve as raw materials for consumer goods – are produced by catalysis processes. By improving the way we study these processes, the team hopes to help other scientists develop better, cheaper and quicker processes for making useful chemicals.
This article is free to read in our open access, flagship journal Chemical Science: Dong Xiao et al., Chem. Sci., 2018, Advance Article. DOI: 10.1039/C8SC03848A. You can access all of our ChemSci Picks in this article collection.
ChemSci Pick of the Week
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