World news – African Union – New analytical approach enhances NMR signal detection in previously « invisible » regions


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November 16, 2020

Written by Sonya Fernandez, University of California – Santa Barbara

Nuclear magnetic resonance (NMR) was first widely introduced in the mid-twentieth century, and has since become an indispensable technique for examining materials down to their atoms, revealing molecular structure and other details without interfering with the material itself..

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“It is a widely used technique in chemical analysis, characterization of materials, and magnetic resonance imaging – situations in which you do an analysis that is not invasive, but with atomic and molecular detail,” said University of California, Santa Barbara Songi Han, chemistry professor. By placing a sample in a strong magnetic field and then examining it with radio waves, scientists can determine the molecular structure of a substance through the response from the vibrating nuclei in the atoms of the substance..

“However, the problem with NMR is that since it is a low-energy technology, it is not very sensitive,” Han said. « It’s very detailed, but it doesn’t get a lot of mentions. As a result, large amounts of sample material may be required relative to other techniques, and the overall weakness of the signals makes NMR less than ideal for studying complex chemical processes..

One of the remedies for this situation is dynamic nuclear polarization (DNP), which is a popular technique in which energy is « borrowed » from nearby electrons to boost the signal emitted by the nuclei..

“Electrons have much higher energy than nuclei,” Han explained.. Incorporated into specially designed « radical » particles, the polarization of these non-paired electrons is transferred to the nuclei to improve their signaling.

As the topic of DNP has gotten hot in the past decade, Han thinks we’re still scratching the surface..

“Although DNP radically altered the landscape of NMR, at the end of the day, only a handful of designer polarization agents were used,” Han said.. “A polarizing factor was used to polarize the hydrogen nuclei, but the strength of DNP is greater than that. In principle, many other sources of electron spin can polarize many other types of nuclear spin.

In a research paper published in the journal Chem, Hahn and colleagues push the boundaries of NMR by first demonstrating dynamic nuclear polarization using the transition metallic vanadium (IV). According to Hahn, their new approach – called « ultra-fine DNP spectroscopy » – provides a glimpse into the often ambiguous local chemistry around transition metals, which are important for processes such as catalysis and reductive oxidation reactions..

“We may now be able to use the endogenous minerals found in catalysts and in many other important materials,” Han said, without needing to add polarization agents – those radical molecules – to produce a stronger NMR signal..

Hahn explained that the paradox of transition metals, such as vanadium and copper, is that these atoms tend to be functional centers – places where important chemistry occurs..

“And it was very difficult to analyze these duty stations and job centers (using NMR) because they tend to become invisible,” she said.. She demonstrated that the electron spin in the transition metal tends to shorten the life of the NMR signal, causing it to disappear before it is detected..

This isn’t the first time the chemistry around transition metals has been observed, Han said, referring to studies that looked at chemical environments around gadolinium and manganese.. But the commercially available tool used in those studies presented a « very narrow view. ».

« But there are several minerals that are more important to chemistry, » she said. «  So we have developed and improved devices that enhance the frequency range from a very narrow range of a commercial instrument to a much wider range.

Through ultra-fine DNP spectroscopy, the researchers also found that the signal was actually erased within a specific region around the metal called the spin diffusion barrier, but if the nuclei are outside that region, the signal becomes visible.

“There are ways to mitigate this environment, but you need to know how and why,” Han said, adding that the paper’s lead authors, Sheetal Kumar Jane of the University of California Santa Barbara and Chung Joy Yue of Northwestern University will continue to explore and apply this new method while They pursue their academic and research life.

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Chemistry, Resonance, and Atom

World News – Australia – A new analytical approach promotes detection of NMR signal in previously « invisible » regions


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