Muonium–the Second Radioisotope of Hydrogen: A Remarkable and Unique Radiotracer in the Chemical, Materials, Biological and Environmental Sciences

Abstract

Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of μSR, where μ stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR) and longitudinal-field muon spin relaxation (LF-μSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-μSR methods. It is further shown that TF-μSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair. Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-μSR and LF-μSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of μSR techniques for studying radicals in the gas-phase.

Muons in chemistry

“…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. Many excellent reviews of the applications of muons in chemistry are available [1,2] and this Comment is not intended to repeat these, but rather to highlight those developments with a promising future. Nor is it the intention to consider what can be loosely termed questions of condensed matter physics, since these are ably described by other papers in this series as well in a recent book [3].…”

Two-component density functional theory for muonic molecules: Inclusion of the electron-positive muon correlation functional

“…1,2 In fact, because of its similarity to proton, muon's mass is ∼1/9th the proton's mass, 2 muon is sometimes called the light radioisotope of hydrogen. [3][4][5] If instead of muon, a muonium atom, Mu, composed of a muon and an electron, binds to a molecule the resulting muonic radical may react with other molecules opening a whole area called muonic chemistry. [6][7][8][9][10][11][12][13][14] These radicals are studied by a special spectroscopy called the muon spin resonance spectroscopy, µSR, [15][16][17][18][19][20][21][22] which may yield unique information on the binding site of muon in organic, organosilicon and biochemical molecule.…”

Muonium Addition to a peri‐Trifluoromethylated 9‐Phosphaanthracene Producing a High‐Energy Paramagnetic π‐Conjugated Fused Heterocycle

“…The H-atom surrogate can add to the unsaturated molecular units and provide the corresponding transient paramagnetic species. [15,16] Our previous mSR study on the 1,3-diphosphacyclobutane-2,4-diyl of open-shell singlet phosphorus heterocycle successfully characterized muonium addition to the 4-membered phosphorus heterocycle affording a metastable Pheterocyclic radical, suggesting usefulness of mSR for chemistry of transient radical species generated from phosphorus heterocycles. [17] As for the low-coordination structures involving phosphorus, muonium adducts of a phosphaalkene bearing P = C double bond were successfully characterized.…”

“…Muon spin rotation/resonance (mSR) spectroscopy is a powerful analytical method for radical reactions of organic molecules. [14,15] The positive muon (m + ) of light proton isotope (Mass = 0.1134 amu) for materials study is prepared by using a high-energy beam of proton from accelerator facilities via a nuclear reaction affording pi mesons (p + ). The muons obtained in accelerator factories are inherently spin-polarized.…”

“…Addition of muonium at the quaternary skeletal carbons in 1 is unlikely, and muonium addition to the meso-mesityl group should show muon hfc over 400 MHz. [14,15] To confirm 1Mu as well as to understand its structural aspects, we next carried out theoretical calculations. We carried out DFT structural optimizations for 1Mu under the condition of UwB97XD/6-311G(d,p).…”