Cracking Chemistry

Muons have a long history as probes to study chemistry

Muons have a long history as probes to study chemistry. Credit: Dreamstime
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Positive muons can be integrated into free radicals where they can probe the structure and dynamics. Radicals are molecules with unpaired electrons, and are frequently short-lived intermediates in chemical reactions. To be able to understand these reactions we need to determine the radical’s structure and dynamics. This is where the muon comes in, with its short lifetime it is suitable for exploring fast chemical reactions.

 “Muons have a long history of being used as exotic extrinsic probes to study chemical systems and the insight provided into these systems that are examined is invaluable. Naturally, the availability of highly spin-polarized muon beams at central facilities such as ISIS, UK; PSI, Switzerland; TRIUMF, Canada; and J-PARC, Japan have encouraged this.” Nigel. J. Clayden, University of East Anglia

“The fundamental question for chemists intending to use muons is, obviously: What chemical information can be gleaned about a system? Or more succinctly: 'Why use muons?'. The underlying rationale is that muonium, a muon which has gained an electron, can be thought of as an isotope of hydrogen and therefore, mimics its behaviour. Two properties of the muon are important in this respect: its magnetic moment and low mass. The magnetic moment means that the muon spin polarization can be used to monitor local magnetic environments in a wide range of magnetic materials from molecular magnets to superconductors. Low mass means that very large kinetic isotope effects can be expected in radical reactions involving C–H(Mu) bonds in the transition state. Similarly, any chemical system involving atomic hydrogen or a proton is, in principle, open to study. Thus, muon states in semiconductors have been much studied as models for hydrogen trapping. More importantly perhaps, muonium once formed, can react with organic molecules to give muoniated radicals.” Nigel. J. Clayden, University of East Anglia

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