Muonium (Mu = μ+e-), which can be considered a light isotope of the H atom, has been observed for the first time in supercritical CO2 (ScCO2). It is unreactive on a time scale of a few microseconds and over a wide density range from well below to well above the CO2 critical density pc = 0.47 g/cm 3. The fraction of muon polarization in muonium, PMu, does not vary significantly at low densities but changes quickly at the highest densities, approaching zero. This density dependence is reflected in a concomitant increase observed in the lost fraction of polarization, P L, demonstrating that the dynamics of Mu formation and depolarization in ScCO2 is a direct probe of radiolysis effects in the terminal muon radiation track. In marked contrast to previous studies in hydrogen-containing solvents, C2H6 and H2O, over comparable density ranges, the diamagnetic fraction, PD, was found to be almost independent of density in CO2, attributed to the formation of the stable solvated MuCO2+ molecular ion in this hydrogen-free solvent. The differing density dependences of both the Mu and the diamagnetic fraction in CO2, in comparison with the rather similar trends seen for both in C2H2, and H2O, supports previous claims of a significant role played by proton (muon) transfer reactions in the competing processes involved in Mu formation in hydrogen-containing solvents. In addition to this being the first report of radiolysis effects accompanying energetic positive muons stopping in ScCO 2, it is the only report of end of track effects in this solvent, which has many applications in nuclear waste management and green chemistry. With a mass intermediate between that of the electron, which has provided most radiation-chemistry studies in ScCO2 to date, and the proton (or alpha-particle), implanted muons provide a unique data set, characteristic of higher LET radiation, that may be relevant to radiolysis effects induced in ScCO2 by alpha decay from heavy nuclei, for which there are no comparable studies.

The adsorption and dynamical behavior of the Mu-ethyl radical (MuC 2H 4) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5-500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC 2H 4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at S II sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ 1 resonances are symmetric and quite broad at the lower temperatures studied, but dramatically further broaden near room temperature, seen also in the TF relaxation rates, suggesting that the Mu-ethyl radical either desorbs from or hops between its binding sites at the higher temperatures. An Arrhenius estimate of the activation energy for desorption gives ∼ 20 kJ/mol, consistent with the dipolar interaction energy between the Mu-ethyl radical and an NaY cluster. The observation of such highly broadened Δ 1 ALC lines at the higher temperatures contrasts with the largely static line widths reported previously for the Mu-cyclohexadienyl radical (MuC 6H 6) in NaY. Sharper Δ 0 ALC lines for both the α and β protons of MuC 2H 4 appear near the same temperatures at which the Δ 1 lines overly broaden, and which persist to the highest temperatures (350 K). For NaY, the α proton resonances also broaden further at these temperatures. For both NaY and particularly HY, the temperature dependence of the α proton Hfc's indicates considerable distortion of the Mu-ethyl radical geometry, due to its binding to zeolite sites. Recently published calculations of binding energies and Hfc's for ethyl radicals in NaY and HY suggest a much stronger binding of the MuC 2H 4 radical than seems warranted by the data and pose as well a conundrum in comparison with earlier results for MuC 6H 6 in NaY. On the other hand, the temperature dependence of the isotropic muon Hfc's found from the T-atom model for NaY employed in these calculations is in excellent agreement with experiment. © 2007 American Chemical Society.

Reported here is the first μSR study of the muon (A(μ)) and proton (A(p)) β-hyperfine coupling constants (Hfcc) of muoniated sec-butyl radicals, formed by muonium (Mu) addition to 1-butene and to cis- and trans-2-butene. The data are compared with in vacuo spin-unrestricted MP2 and hybrid DFT/B3YLP calculations reported in the previous paper (I), which played an important part in the interpretation of the data. The T-dependences of both the (reduced) muon, A(μ)′(T), and proton, A(p)(T), Hfcc are surprisingly well explained by a simple model, in which the calculated Hfcc from paper I at energy minima of 0 and near ±120° are thermally averaged, assuming an energy dependence given by a basic 2-fold torsional potential. Fitted torsional barriers to A(μ)′(T) from this model are similar (~3 kJ/mol) for all muoniated butyl radicals, suggesting that these are dominated by ZPE effects arising from the C−Mu bond, but for A(p)(T) exhibit wide variations depending on environment. For the cis- and trans-2-butyl radicals formed from 2-butene, A(μ)′(T) exhibits clear discontinuities at bulk butene melting points, evidence for molecular interactions enhancing these muon Hfcc in the environment of the solid state, similar to that found in earlier reports for muoniated tert-butyl. In contrast, for Mu−sec-butyl formed from 1-butene, there is no such discontinuity. The muon hfcc for the trans-2-butyl radical are seemingly very well predicted by B3LYP calculations in the solid phase, but for sec-butyl from 1-butene, showing the absence of further interactions, much better agreement is found with the MP2 calculations across the whole temperature range. Examples of large proton Hfcc near 0 K are also reported, due to eclipsed C−H bonds, in like manner to C−Mu, which then also exhibit clear discontinuities in A(p)(T) at bulk melting points. The data suggest that the good agreement found between theory and experiment from the B3LYP calculations for eclipsed bonds in the solid phase may be fortuitous. For the staggered protons of the sec-butyl radicals formed, no discontinuities are seen at all in A(p)(T), also demonstrating no further effects of molecular interactions on these particular proton Hfcc.

A first complete μSR study of the T dependences of the (reduced) muon, Aμ′(T), and proton, Ap(T), β-hyperfine coupling constants (hfcc) of the muoniated t-butyl radical is reported in the faujasitic zeolites NaY and USY, and the results are compared with similar data and with early EPR results in condensed bulk phases. The results are also compared with single-molecule UMP2 and DFT/B3LYP calculations in the bulk and in an NaY zeolite fragment of Si and O atoms with both OH- and H-capping. Muon hfcc are reported for the first time for the Mu-isobutyl radical in the bulk phase and are also compared with theory and with EPR data. The present results for the muon and proton hfcc of Mu-t-butyl in the bulk complement earlier work published elsewhere at higher temperatures but are extended here down to 5 K to facilitate comparisons with in vacuo theory at 0 K. Good fits to the data for both Aμ′(T) and Ap(T) for Mu-t-butyl are found from the calculated hfcc in both the bulk and in NaY, assuming a Boltzmann-weighted energy dependence given by a simple twofold torsional potential, providing an estimate of the barrier to internal rotation. In contrast to the bulk data, there is no clear discontinuity seen in Aμ′(T) for Mu-t-butyl in NaY or USY at the melting point of isobutene, demonstrating the dominance of single-molecule guest-host interactions in the faujasite supercage. In contrast to the Aμ′(T) dependence in the bulk, there is no discontinuity seen for either of the proton hfcc, Ap,CH3(T) or Ap,CH2Mu(T), at the melting point, which also exhibit similar behavior in NaY, suggesting that its observation in the bulk for only the muon hfcc arises from a specific effect of the intermolecular interactions on the vibrational averaging of the muon hfcc. The measured muon hfcc for Mu-t-butyl in NaY fall below those in the bulk at low temperatures, indicating some transfer of electron spin density to the Na cation, which is confirmed by specific additional level-crossing resonances not observed in USY. The Na nuclear hfcc, ANa(T), follow a similar trend with temperature as Aμ′(T), with an estimate of the Na spin density at 0 K that also agrees well with theory. © 2013 American Chemical Society.

The study of metal nanoparticles (NPs) and gold NPs (AuNPs), in particular, has been a subject of considerable interest in recent years. The present paper reports on the formation of the muoniated cyclohexadienyl radical in the Mu + C6H6→ MuĊ6H6addition reaction in the same 8 and 10 nm AuNP samples, encapsulated in SBA-15 mesoporous silica, where the spin-relaxation rates λMuwere recently reported on [FlemingJ. Phys. Chem. C, 2019, 123, 27628, paper “P1”], but the final state was not identified in that study. The formation of this MuĊ6H6radical and its interactions with the AuNP surfaces is investigated herein by the technique of avoided level crossing (ALC) resonance spectroscopy, for the same range of Bz loadings as studied earlier. The positions of the level crossings, giving the hyperfine coupling constants (hfcc) for the C-Mu and C-H bonds at the “ipso” position where Mu adds to the ring, are essentially the same as those seen in solid Bz and in bare silica, and hence these hfccdo notdistinguish site locations for the surface-adsorbed benzene. Theamplitudesof these resonances, which are a measure of the fraction of Mu forming the free-radical final state, do, however, provide such a distinction, being much less for Bz adsorbed in the AuNP/silica samples than in the bare silica. This loss in amplitude is attributed to competitive reaction channels, one forming the MuĊ6H6free radical interacting with the AuNPs and the other due to formation ofdiamagneticfinal states. An important signpost as to the nature of these competitive reaction channels is provided by the measuredwidthsof the ALC resonances seen. These are surprisingly narrower in the presence of the AuNPs than for the bare silica, implying that the MuĊ6H6radical isnotin direct contact with the AuNPs. These differing possibilities and contrasting mechanisms are discussed.