Level Crossing Muon Spin Resonance Research Articles

Analysis of Avoided Level Crossing Muon Spin Resonance Spectra of Muoniated Radicals in Anisotropic Environments: Estimation of Muon Dipolar Hyperfine Parameters for Lorentzian-like Δ1 Resonances

Avoided level crossing muon spin resonance (ALC-μSR) is used to characterize muoniated free radicals. These radicals are used as probes of the local environment and reorientational motion of specific components in complex systems. The parameter that provides information about the anisotropic motion is the motionally-averaged muon dipolar-hyperfine coupling constant (Dμ‖). The ALC-μSR spectra of muoniated radicals in anisotropic environments frequently have Lorentzian-like Δ1 resonances, which makes it challenging to extract Dμ‖. In this paper, we derive a means to estimate|Dμ‖| from ALC-μSR spectra with Lorentzian-like resonances by measuring the amplitude, width, and position of the Δ1 resonance and the amplitude, width, and position of a Δ0 resonance. Numerical simulations were used to test this relationship for radicals with a wide range of muon and proton hyperfine parameters. We use this methodology to determine |Dμ‖| for the Mu adducts of the cosurfactant 2-phenylethanol in C12E4 bilayers. From this we determined the amplitude of the anisotropic reorientational motion of the cosurfactant.

Mixed-valence hydrides at [FeFe]-hydrogenase active site mimics

The number of methods to study transient paramagnetic hydrides at organometallic centres is extremely limited. The reactivity of {2Fe2S} centres with protons to produce both diamagnetic and paramagnetic systems is of central interest in developing novel catalysts for hydrogen production, inspired by the [FeFe]-hydrogenase enzymes. Here, we show how a combination of spectroscopic and electrochemical techniques is allowing access to detail of the reactivity of key species on these pathways. Electron paramagnetic resonance and infra-red spectroelectrochemical approaches have been used to observe the reduction of pre-generated diamagnetic hydrides. In contrast, avoided level crossing muon spin resonance (ALC-µSR) has been used to form the open-shell species directly and to examine the formation of short-lived intermediates in the reaction process. The combination of these techniques suggests the involvement of terminal hydrides or CO-protonation states on the pathway to the isolable bridging hydride products.

Paramagnetic probes in an organic semiconductor: μSR and DFT calculations of the Mu adducts of Alq3 and 8-hydroxyquinoline.

It has been claimed that longitudinal field muon spin relaxation (LF-μSR) experiments on the organic semiconductor (OSC) tris-(8-hydroxyquinoline)aluminum(III) (Alq3) have measured electron hopping rates of ∼1012s-1, while density functional theory (DFT) calculations suggest that electron hopping between a muoniated radical and a neighboring molecule is energetically unfavorable and that the LF-μSR experiments were probing muoniated radicals with localized spin density. We have performed avoided level crossing muon spin resonance (ALC-μSR) and transverse field muon spin rotation (TF-μSR) measurements on Alq3 and 8-hydroxyquinoline (8hq), which is meant to model the muoniated radicals present in Alq3 when they are not in an OSC. These are supplemented by benchmarked DFT calculations. The ALC-μSR and TF-μSR spectra of 8hq and Alq3 are best explained by Mu adding to all six secondary carbons of the quinolate rings with roughly equal yields and localized spin density. There is no evidence in the TF-μSR spectrum of Alq3 for the formation of radicals with muon hyperfine coupling constants of 23 or 91MHz as reported earlier by others. Our measurements support the view that there is localized spin density on the molecule to which Mu is covalently bound and the muon is not a passive probe in organic systems as it can be incorporated into radicals that have different electronic structures to the parent compounds. The muoniated radicals in Alq3 are more short-lived than in 8hq, which could be due to interactions with mobile electrons in the OSC, but with electron spin flip rates on the order of ∼107s-1.

Characterization of muoniated spin probes in cholesterol by avoided level crossing muon spin resonance

Characterization of muoniated spin probes in cholesterol by avoided level crossing muon spin resonance

Dynamics and local environment of an aromatic counterion bound to di-chain cationic surfactant bilayers studied by avoided level crossing muon spin resonance: evidence for counterion condensation.

Avoided level crossing muon spin resonance (ALC-μSR) has been used to study the reorientational dynamics of muon-spin-labelled 2,4,6-trimethylbenzoate (246TMB-) counterions and their interaction with DODMAC (dioctadecyldimethylammonium chloride) bilayers in the Lα and Lβ liquid crystalline states. The muoniated radical anion formed by the addition of muonium to the secondary carbons of the aromatic ring of 246TMB- is used as a local spin probe. The muon and methylene proton hyperfine parameters and the electron spin relaxation rate (λe) of the muoniated spin probe were determined as a function of temperature by modelling the ALC-μSR spectra with Monte Carlo numerical simulations. The observation of a Δ1 resonance indicates that 246TMB- is undergoing anisotropic motion and doesn't reside in the aqueous layer in either the Lα and Lβ phases. The lack of an abrupt change in the hyperfine parameters or λe when the system goes from the Lβ to the Lα lamellar liquid crystalline phases suggests that 246TMB- is located at the oil-water interface rather than within the bilayer. The hyperfine parameters indicate that 246TMB- is undergoing large amplitude reorientational motion about a preferred orientation resulting from the bilayer's electric field. The interaction between 246TMB- and the bilayer decreases and the amplitude of the wobbling-in-a-cone motion increases with increasing temperature. The temperature dependence of the electron spin relaxation rate indicates the barrier to reorientation is 41.7 kJ mol-1.

Hydrogen-Atom Addition to Nucleobases in the Solid State: Characterization of the Corresponding Muoniated Radicals Using μSR.

The radicals formed by muonium (Mu) addition to four nucleobases (adenine, guanine, cytosine, and thymine) have been characterized by avoided level-crossing muon spin resonance (ALC-μSR). Mu is considered to be a light isotope of the hydrogen atom, and the muoniated radicals observed by ALC-μSR are isotopomers of the radicals initially produced by H addition to the nucleobases. The observed radicals have been assigned by considering the relative energies of the possible radicals reported in the literature and comparing the experimental muon and proton hyperfine coupling constants with values from previously reported electron paramagnetic resonance and ab initio calculations that have been scaled to account for the larger magnetic moment of the muon and its lighter mass compared with the proton. Mu addition is observed to occur only at secondary carbons of the purine rings in adenine and guanine. Mu adds to C8 and C2 of adenine with the relative amount being ∼70:30%, and Mu adds exclusively to C8 of guanine. Mu addition is predominantly to the secondary carbons of the pyrimidine ring in cytosine (C5 and C6 with relative yields ∼80:20%) with a small amount of addition at N3. Mu adds to both the secondary C6 and tertiary C5 in thymine with approximately equal yields as well as the O4 adduct being a minor product.

Radical addition to ruthenocene at low temperatures: characterization of ruthenocenyl radicals byμSR spectroscopy

The radicals formed by muonium (Mu) addition to ruthenocene at low temperature (4–200 K) have been characterized by transverse field muon spin rotation (TF-μSR) and avoided level crossing muon spin resonance (ALC-μSR) spectroscopy. The structures of the muoniated radicals have been identified by comparing the experimentally measured muon hyperfine coupling constants with values obtained from DFT calculations (UB3LYP/DGDZVP). Mu addition was observed at the ruthenium and at the cyclopentadiene (Cp) rings, both from the exterior and interior directions. Closer agreement between the DFT calculations and the experimental values are obtained if it is assumed the structures of the Mu adducts of the Cp rings are distorted due to interactions with neighbouring molecules. Changes in the ALC-μSR spectra with temperature indicated that the electron spin relaxation rate of the Cp adducts increases with temperature; however, the specific spin relaxation mechanism is unknown.

Comment on “Muon spin relaxation study of spin dynamics in poly(triarylamine)”

He et al. studied the muoniated radicals formed by muonium (Mu) addition to the low-molecular-weight polymer poly(triarylamine) (PTAA) (He et al., 2015) [1], an organic semiconductor, using avoided level crossing muon spin resonance (ALC-μSR) (McKenzie and Roduner, 2013; McKenzie, 2009) [2,3]. The goal of these experiments was to study the electron spin relaxation of a paramagnetic defect in PTAA using a local probe. Our main issue with this paper is the claim that parameters relating to the electronic structure of the muoniated radicals have been measured with greater precession than we believe is possible considering the quality of the ALC-μSR spectra presented.

Partitioning of 2-phenylethanol and limonene cosurfactants in C12E4.

Avoided level-crossing muon spin resonance (ALC-μSR) has been used to study the dynamics and local environment of spin probes formed by muonium (Mu) addition to 2-phenylethanol (PEA) and limonene (1-methyl-4-(1-methylethenyl)-cyclohexene) in an aqueous dispersion of the nonionic surfactant C12E4 (tetra(ethylene glycol) n-dodecyl ether). The spin probes derived from both cosurfactants reside within the micelles in the L1 phase and the bilayers in the Lα phase rather than in the aqueous region. The local polarity measured by the different isomers of the Mu adducts of PEA suggests there is a water gradient within the micelles and bilayers. Slow rotation of the micelles broadened the Δ1 resonances with increasing temperature in the L1 phase while narrower Δ1 resonances were observed in the Lα phase due to the rapid rotation of the spin probes around a preferred axis, which was wobbling within a cone.

Rate of Molecular Transfer of Allyl Alcohol across an AOT Surfactant Layer Using Muon Spin Spectroscopy.

The transfer rate of a probe molecule across the interfacial layer of a water-in-oil (w/o) microemulsion was investigated using a combination of transverse field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR), and Monte Carlo simulations. Reverse microemulsions consist of nanometer-sized water droplets dispersed in an apolar solvent separated by a surfactant monolayer. Although the thermodynamic, static model of these systems has been well described, our understanding of their dynamics is currently incomplete. For example, what is the rate of solute transfer between the aqueous and apolar solvents, and how this is influenced by the structure of the interface? With an appropriate choice of system and probe molecule, μSR offers a unique opportunity to directly probe these interfacial transfer dynamics. Here, we have employed a well characterized w/o microemulsion stabilized by bis(2-ethylhexyl) sodium sulfosuccinate (Aerosol OT), with allyl alcohol (CH2═CH-CH2-OH, AA) as the probe. Resonances due to both muoniated radicals, CMuH2-C*H-CH2-OH and C*H2-CHMu-CH2-OH, were observed with the former being the dominant species. All resonances displayed solvent dependence, with those in the microemulsion observed as a single resonance located at intermediate magnetic fields to those present in either of the pure solvents. Observation of a single resonance is strong evidence for interfacial transfer being in the fast exchange limit. Monte Carlo calculations of the ΔM = 0 ALC resonances are consistent with the experimental data, indicating exchange rates greater than 10(9) s(-1), placing the rate of interfacial transfer at the diffusion limit.

Muon spin spectroscopy of ferrocene: characterization of muoniated ferrocenyl radicals.

Radicals formed by the reaction of muonium (Mu), a light isotope of hydrogen, with ferrocene and ferrocene-d10 have been studied with the avoided level crossing muon spin resonance (ALC-μSR) and longitudinal field muon spin relaxation (LF-μSR) techniques between 10 and 100 K. A single type of radical was observed in each compound and the muon hyperfine coupling constants (hfcc) and the muon spin relaxation rates were measured as a function of temperature. A previous report concerning the observation of Mu adducts of ferrocene (U. A. Jayasooriya et al. Chem. - Eur. J., 2007, 13, 2266-2276) appears to be incorrect. DFT calculations were performed to aid in the assignment of the ALC-μSR spectra. A tentative assignment is that the observed radicals were formed by Mu addition to the exterior of the cyclopentadienyl rings and that the structures are distorted due to interactions with neighbouring molecules. The temperature dependence of the muon hfcc can be explained assuming the population of two levels with different muon hfccs separated by 1.4 ± 0.1 kJ mol(-1). The temperature dependence of the width and amplitude of the Δ1 resonance and the muon spin relaxation rate suggests that the electron spin relaxation rate increase with temperature, but the relaxation mechanism is unknown.

Muoniated spin probes in the discotic liquid crystal HHTT: Rapid electron spin relaxation in the hexagonal columnar and isotropic phases

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-μSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

How Do Strain and Steric Interactions Affect the Reactions of Aromatic Compounds with Free Radicals? Characterization of the Radicals Formed by Muonium Addition to p-Xylene and [2.2]Paracyclophane by DFT Calculations and Muon Spin Spectroscopy

Muoniated radicals were produced by the addition of muonium (Mu) to the aromatic compound p-xylene (1) in the solid and liquid states and to the strained aromatic compound [2.2]paracyclophane (2) in the solid state. The radicals were characterized by avoided level crossing muon spin resonance spectroscopy and identified by comparing the experimentally determined muon hyperfine coupling constants with values obtained from DFT calculations. Mu was observed to add to both the secondary and tertiary carbons of 1, with the relative yield of the Mu adduct of the tertiary carbons estimated to be ∼10% in the liquid phase. The relative yield of the tertiary adduct is much higher in the solid state although this cannot be calculated exactly due to the overlap of resonances and the apparent nonuniform distribution of the radical orientations. There are three possible addition sites in 2 due to the lower symmetry of the six-membered ring compared with 1. Mu can add to the secondary carbons either from the outside of 2, generating the "exo" adduct, or from the inside, generating the "endo" adduct. The relative yields of the exo, endo, and tertiary carbon adducts are 67.1(1), 21.8(1), and 11.1(1)%, respectively. The barriers to Mu addition at the different sites of isolated molecules were determined from DFT calculations. The barriers for Mu addition to 2 are lower than the barriers for Mu addition to 1, except for addition to the "endo" position, where the unfavorable steric interactions with the second ring of 2 raise the addition barrier considerably. The measured relative yields do not reflect the distribution of products calculated using the activation energies obtained from the DFT calculations due to strong steric interactions with neighboring molecules.

Muonium--the second radioisotope of hydrogen: a remarkable and unique radiotracer in the chemical, materials, biological and environmental sciences.

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 muSR, where mu 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-muSR), avoided level crossing muon spin resonance (ALC-muSR) and longitudinal-field muon spin relaxation (LF-muSRx). 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-muSR methods. It is further shown that TF-muSR 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-muSR and LF-muSRx 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 muSR techniques for studying radicals in the gas-phase.

Molecular Dynamics in the Rod-like Liquid Crystal 4-(trans-4-Pentylcyclohexyl) Benzonitrile (PCH5) Probed by Muon Spin Resonance Spectroscopy

Abstract Two types of muoniated spin probes were produced by the addition of muonium (Mu) to the phenyl ring of the rod-like liquid crystal 4-( trans -4-pentylcyclohexyl)benzonitrile (PCH5). Avoided level crossing muon spin resonance spec-troscopy was used to characterize the muoniated spin probes and to probe dynamics of PCH5 at the molecular level. The methylene proton hyperfine coupling constant (hfcc) of one of the muoniated spin probes shift in the nematic phase due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and fiuctuations about the local director. The amplitude of these fluctuations in PCH5 is determined from the temperature dependence of the methylene proton hfcc.

Molecular Dynamics in Rod-Like Liquid Crystals Probed by Muon Spin Resonance Spectroscopy

Muoniated spin probes were produced by the addition of muonium (Mu) to two rod-like liquid crystals: N-(4-methoxybenzylidene)-4'-n-butylaniline (MBBA) and cholesteryl nonanoate (CN). Avoided level crossing muon spin resonance spectroscopy was used to characterize the muoniated spin probes and to probe dynamics at the molecular level. In MBBA Mu adds predominantly to the carbon of the bridging imine group and the muon and methylene proton hyperfine coupling constants (hfccs) of the resulting radical shift in the nematic phase due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and fluctuations about the local director. The amplitude of these fluctuations in in the nematic phase of MBBA is determined from the temperature dependence of the methylene proton hfcc. Mu adds to the double bond of the steroidal ring system of CN and the temperature dependence of the Δ(1) line width provides information about the amplitude of the fluctuations about the local director in the chiral nematic phase and the slow isotropic reorientation in the isotropic phase.

Muon spin spectroscopy of the rod-like liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB)

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy has been used to study the four cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the rod-like liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB). ALC-μSR spectra have been obtained over a wide temperature range in the isotropic (I), nematic (N), smectic-A (Sm-A) and crystalline (Cr) phases. The ALC-μSR spectra of the Mu adducts of 8CB are very similar to those of the Mu adducts of 4-n-pentyl-4'-cyanobiphenyl (5CB). Four Δ0 resonances were observed in the ALC-μSR spectra in the I, N and super-cooled Sm-A phases, from which the methylene proton hyperfine coupling constants (hfcs) of the Mu adducts of 8CB were determined as a function of temperature. The methylene proton hfcs of two of the radicals have unusual temperature dependence in the N and Sm-A phases and have smaller values than would be predicted from extrapolating the data in the isotropic phase. The shift of the hfcs is due to the dipolar hyperfine coupling, the ordering of the molecules along the applied magnetic field and the fluctuations of the molecules about the director. Δ1 resonances appear just below the Cr-Sm-A phase transition, which indicates that this transition is associated with cessation of rotation around the long molecular axes.

Muon spin spectroscopy of the discotic liquid crystal HAT6

Avoided level crossing muon spin resonance (ALC-muSR) has been used to study the cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the discotic liquid crystal HAT6 (2,3,6,7,10,11-hexahexyloxytriphenylene) in the crystalline (Cr) phase, the hexagonal columnar mesophase (Col(h)) and isotropic (I) phase. In the Cr phase unpaired electron spin density can be transferred from the radical to neighboring HAT6 molecules depending on the overlap of their pi-systems and hence on the relative orientation of the triphenylene rings. The two Delta(1) resonances in the ALC-muSR spectra of the Cr phase indicate that the neighboring HAT6 molecules have two preferred orientations with respect to the radical: one which results in negligible spin density transfer and a second where 17% of the unpaired spin density is transferred. The ALC-muSR spectra in Col(h) and I phases are substantially different from those of the Cr phase in that there are two narrow resonances superimposed on an extremely broad and intense resonance. The narrow resonances are due to highly mobile radicals located in the aliphatic region between the columns and the broad resonance is due to radicals incorporated within the columns of HAT6 molecules. The large width and amplitude of this resonance indicates that the radicals within the columns are undergoing rapid electron spin relaxation but the mechanism that causes this relaxation is unknown.

Muon Spin Spectroscopy of the Nematic Liquid Crystal 4-n-Pentyl-4′-cyanobiphenyl (5CB)

Avoided level crossing muon spin resonance (ALC-microSR) spectroscopy has been used to study the four cyclohexadienyl-type radicals produced by the addition of muonium (Mu) to the rodlike liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). ALC-microSR spectra have been obtained over a wide temperature range in the isotropic, nematic, and crystalline phases. Four Delta0 resonances were observed in the ALC-microSR spectra, from which the methylene proton hyperfine coupling constants (hfcs) of the Mu adducts of 5CB were determined as a function of temperature. The methylene proton hfcs of two of the radicals have unusual temperature dependence in the nematic phase and have smaller values than would be predicted from extrapolating the data in the isotropic phase. We have used the Maier-Saupe theory for rodlike liquid crystals to explain the temperature dependence of the methylene proton hfcs, which results from the ordering of the 5CB molecules, the alignment of the molecules with the external magnetic field, and fluctuations that average the anisotropic hyperfine coupling constants. There are no Delta1 resonances in the ALC-microSR spectra of the nematic phase due to the radicals rotating rapidly around the long molecular axis and fluctuations about the local director. The Delta0 resonances broaden substantially as the temperature is lowered due to the slowing down of the fluctuations, which have an average activation energy of approximately 15.9 kJ mol(-1). Cooling the sample below 275 K stopped the rotation around the long molecular axis and led to the appearance of Delta1 resonances.

Local ordering in quadrupolar liquids observed by the hyperfine couplings of the cyclohexadienyl radical

Hyperfine coupling constants (hfcs) are a sensitive means of examining the electronic structure of a radical in solution and monitoring how this is altered through solvent interactions. The hfcs of the cyclohexadienyl radical's methylene proton and muon are altered by the partial ordering of the quadrupole moments of the neighboring solvent molecules around the dipole of the muoniated cyclohexadienyl radical. The methylene hfcs of the muoniated cyclohexadienyl radical were measured by avoided level crossing muon spin resonance (ALC-μSR) in a series of benzene/cyclohexane mixtures. The methylene hfcs were found to be directly proportional to the concentration of solvent quadrupole moments, which contradicts the model of Vujosevic et al. [J. Phys. Chem. B 111 (2007) 199] where the hfc shift was predicted to be proportional to the quadrupole concentration to the 43 power. In this work the model of Vujosevic et al. has been extended and an analytical method for calculating the internal field was developed. This extended model reproduces the observed linear relationship between the hfcs and the quadrupole concentration. It was found that the diameter of solvents and solute molecules is of great importance.