A new analytical approach improves detection of nuclear magnetic resonance signal in previously “invisible” regions

First introduced into widespread use in the mid-20th century, nuclear magnetic resonance (NMR) has become an indispensable technique for examining materials down to their atoms, revealing molecular structure and other details without interfering with the material itself.

“It is a widely used technique in chemical analysis, material characterization, magnetic resonance imaging, situations where non-invasive analysis is performed, but with atomic and molecular detail,” said Songi Han, UC chemistry professor. Santa Barbara. By placing a sample in a strong magnetic field and then probing it with radio waves, scientists can determine the molecular structure of the material from the response of the oscillating nuclei in the material’s atoms.

“However, the problem with NMR was that because it’s a low-energy technique, it’s not very sensitive,” Han said. “It’s very detailed, but you don’t get much signal.” As a result, large amounts of sample material may be required compared to other techniques and the general weakness of the signals makes NMR less than ideal for studying complex chemical processes.

A remedy for this situation lies in dynamic nuclear polarization (DNP), a popular technique in which energy is “borrowed” from nearby electrons to enhance the signal emanating from the nuclei.

“Electrons have much higher energy than nuclei,” Han explained. Built into specially designed “radical” molecules, the polarization of these unpaired electrons is transferred to the nuclei to enhance their signal.

However, DNP’s hot topic has become in the past decade, Han thinks we’re still scratching the surface.

“While DNP radically changed the NMR landscape, at the end of the day, only a handful of design polarizing agents were used,” Han said. “A polarizing agent has been used to polarize hydrogen nuclei, but the power of DNP is greater than that. In principle, many other sources of electron spin can polarize many other types of nuclear spins.”

In an article published in the journal Chem, Han and colleagues push the boundaries of NMR with the first demonstration of dynamic nuclear polarization using the transition metal vanadium (IV). According to Han, their new approach, dubbed “hyperfine DNP spectroscopy,” offers a look at the typically dark local chemistry around transition metals, which are important for processes such as catalysis and redox reactions.

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

The irony of transition metals like vanadium and copper, Han explained, is that those atoms tend to be functional centers, places where important chemistry takes place.

“And those exact centers of action and functional centers have been very difficult to analyze (with NMR) because they tend to become invisible,” he said. Electrons spinning in the transition metal tend to shorten the duration of the NMR signal, he explained, causing them to disappear before they can be detected.

This wouldn’t be the first time the chemistry around transition metals has been observed, Han said, pointing to studies that looked at the chemical environments around gadolinium and manganese. But the commercially available tool used in those studies offered “a very narrow view”.

“But there are many other metals that are much more important to chemistry,” he said. “So we developed and optimized instrumentation that improves the range of frequencies from the very narrow range of a commercial instrument to a much wider range.”

With their hyperfine DNP spectroscopy the researchers also found that the signal is actually canceled within a certain region around the metal called the spin diffusion barrier, but if the nuclei are located outside that zone the signal becomes visible. .

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