Muon spin rotation (µSR) spectroscopy is a powerful technique that helps to study the behavior of materials at the atomic level. It involves using muons—subatomic particles similar to protons but with a lighter mass. When introduced into a material, muons interact with local magnetic fields, providing unique insights into the material's structure and dynamics, especially for highly reactive species such as radicals.
In a new study, a team of researchers led by Associate Professor Shigekazu Ito, from the School of Materials and Chemical Technology, Institute of Science Tokyo, Japan, utilized µSR spectroscopy to investigate the regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1. This compound is a phosphorus congener (a variant of a common chemical structure). The process of µSR spectroscopy initially involves the formation of a muonium (Mu), which is formed when a positively charged muon (µ+) captures an electron (e–). This process continues as the reaction of a muonium (Mu = [µ+e–]) with the phosphorus-containing compound, resulting in the formation of a muoniated radical at the phosphorus site. This regioselective addition is driven by the high reactivity of the phosphorus atom in the structure, which is a key feature of polyaromatic hydrocarbons. Their findings were published online in Scientific Reports on January 7, 2025.
The study revealed that muon exclusively reacts with the phosphorus atom, forming a stable yet highly reactive muoniated radical at the phosphorus site, highlighting the molecule's high reactivity. Researchers observed this interaction in detail using transverse-field µSR (TF-µSR) spectroscopy, which allowed them to directly probe the magnetic environment surrounding the radical. TF-µSR measurements indicated that even at low concentrations (0.060 M in tetrahydrofuran), the muoniation reaction occurred efficiently, producing detectable signals.
"By utilizing µSR spectroscopy, we were able to observe the regioselective muoniation process in detail, providing direct evidence of the reactive nature of phosphorus in this structure," explains Ito. "The ability to study this radical at low concentrations opens up new possibilities for investigating reactive species in various molecular systems."
Researchers used density functional theory (DFT) to study the structure and stability of the muoniated radical. Hyperfine parameters Aμ and A31P, derived from DFT, provided key insights into its electronic structure and stabilization. These calculations suggested that the structure of 12-phosphatetraphene 1 (muoniated radical) is stabilized in the flat, π-delocalized form due to the contribution of lowest possible (zero-point) energy. This stabilization prevents the formation of a thermodynamically favored saddle-type tetracyclic skeleton.
Another important observation from the study was the temperature dependencies of Aµ and A31P. As the temperature increased, both Aµ and A31P parameters decreased, suggesting a structural stabilization of the muoniated radical. These findings were supported by µSR and muon (avoided) level-crossing resonance experiments, which provided additional information on the dynamics of the muoniated radical and its structural characteristics.
"This study provides valuable insights on the dynamics and structural changes of the muoniated radical, which could influence future research into radical behavior and stabilization," says Ito. Resolving strain in the molecular framework enhances stability and reactivity, optimizing the material for practical applications like electron-spin functional materials and nucleic acid regulation. This improvement increases reliability, opening new possibilities for advanced technologies and therapeutic uses.
The regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1 is expected to have implications in the fields of material science and biology by creating electron-spin functional materials and regulatory substances for nucleic acids, respectively. Overall, this study improves the understanding of phosphorus-containing radicals and highlights the versatility of µSR spectroscopy in investigating reactive species at the atomic level.
About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to create value for and with society."
