Novel analytical approach detects nuclear magnetic resonance signal in earlier ‘invisible’ regions

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First introduced into widespread use in the mid-20th century, nuclear magnetic resonance (NMR) has since become an indispensable technique for examining the material beneath its atoms, to reveal molecular structure and other details. Without interfering with the content.

Professor Songai Han of UC Santa Barbara Chemistry said, “This is a widely used technique in chemical analysis, material characterization, MRI situations, in which you perform a non-invasive analysis, but of nuclear and molecular details.” with.” By placing a sample in a strong magnetic field and then examining it with radio waves, scientists can determine the molecular structure of the material’s atoms by reacting to oscillations in the nucleus.

“However, the problem with NMR has been that because it is such a low-energy technology, it is not very sensitive,” Han said. “It’s very detailed, but you don’t get a lot of clues.” As a result, a large amount of sample material may be required relative to other techniques, and the general weakness of the signals makes NMR less than ideal for the study of complex chemical processes.

One measure of this situation lies in dynamic atomic polarization (DNP), a popular technique in which “borrowed” from nearby electrons to amplify the signal emanating from the nucleus.

“Electrons have a lot more energy than nuclei,” Han explained. Built into specially designed “radical” molecules, the polarization of these unpublished electrons is transferred to the nucleus to improve their signal.

A topic similar to DNP has heated up over the last decade, however, Han thinks we’re still scratching the surface.

“Despite the DNP fundamentally changing the landscape of NMR, at the end of the day, only a handful of designer polarization agents have been used,” Han said. “A polarizing agent has been used to polarize hydrogen nuclei, but the power of DNP is higher than this. In theory, many other sources of electron spin can polarize many other types of nuclear spin.”

In a paper published in the journal Chem, Han and his colleagues pushed the boundaries of NMR with the first demonstration of dynamic atomic polarization using the transition metal vanadium (IV). According to Han, his new approach — dubbed “hyperfine DNP spectroscopy” —is a glimpse into the generally ambiguous chemistry around transition metals, which are important for processes such as catalysts and reduction-oxidation reactions. .

“We may now be able to use endogenous metals that are present in catalysts and many other important materials,” Han said, without polarizing agents – to add those radical molecules – to produce a strong NMR signal .

Ironically with transition metals such as vanadium and copper, Han explained, those atoms have functional centers – places that have significant chemistry.

“And those exact action centers and functional centers have become very difficult to analyze (with NMR) because they become invisible,” she said. Electrons in the transition metal rotate to shorten the lifetime of NMR signals, he explained, before they can be detected and disappear.

This will not be the first time that chemistry has been seen around transition metals, Han said, pointing to studies that looked at the chemical environment surrounding gadolinium and manganese. But commercially available tools used in those studies offered a “very narrow approach”.

“But there are many more metals that are much more important for chemistry,” she said. “So we have developed and optimized instrumentation that extends the frequency range from the very narrow scope of a commercial instrument to a much wider range.”

Researchers with their hyperfine DNP spectroscopy also found that the signal is called spin diffusion inhibition in a certain region around the metal, but the signal is visible if the nucleus is located outside that region.

“There are ways to lighten that environment, but you need to know how and why,” said Han, the paper’s co-lead author, Sheetal Kumar Jain of UC Santa Barbara and Chung-Jui Yu of Northwestern University Stay tuned and apply this new method as they pursue their academic and research careers.

The new computational model stands for making nuclear magnetic resonance an even more powerful tool for researchers

more information:
Sheetal Kumar Jain et al. Dynamic atomic polarization with vanadium (IV) metal centers, Chem (2020). DOI: 10.1016 / j.chempr.2020.10.021

Journal Information:

Provided by University of California – Santa Barbara

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