Brian C. Webster

Muonium substituted organic free radicals in liquids. Muonium—electron hyperfine, coupling constants of alkyl and allyl radicals

Abstract Muonium substituted free radicals are observed by muon spin rotation when positive muons are stopped in liquid olefins and dienes. From muon precession the isotropic muon—electron hyperfine coupling constants have been determined for 44 radicals and analyzed to yield the radical structures. Primary, from mono-olefins, and mainly allylic radicals from conjugated dienes. They are formed by addition of the light hydrogen isotope muonium (Mu = μ + the relations between coupling constants and radical structures follow the principles known for the analogous H substituted radicals. Further, the regi to that of H atoms.

Muon spin rotation spectra for muonium isotopically substituted ethyl radicals

The temperature dependence of the β-hyperfine coupling constant is reported for muonic radicals formed in ethene CH2CH2, [2H1]ethene CH2CHD, [2H3]ethene CHDCD2 and [2H4]ethene CD2CD2. These studies are complemented by e.p.r. observations upon the radical CH2DĊH2.The barrier to internal rotation for the radical CHD2ĊD2 is discussed.

The barriers to internal rotation for muonic-substituted ethyl radicals

By fitting of the observed temperature dependence of the β-hyperfine muon–electron interaction to a theoretical model for the muonic radicals CH2MuĊH2, CHDMuĊH2, CHDMuĊD2 and CD2MuĊD2 values for the barrier height V2 restricting internal rotation are calculated. A decomposition of the barrier into pair interactions indicates the isotope effect upon the barrier to be in the sequence VDD2 < VDH2 < VHH2 < VMuD2 < VMuH2. On this basis the barriers to internal rotation for the radicals CH2MuĊD2 and CD2MuĊH2 are predicted to be of the order 3076 and 3186 J mol–1, respectively.

Vibrational behaviour of the carbon–muonium bond in the muonated ethyl radical

Using a variation-perturbation approach, harmonic and anharmonic corrections to some stationary point properties of the ethyl radical are computed for deuterium and muonium isotopomers. The anharmonic corrections in the muonated radical are found to be highly significant. The fundamental frequency for the C—Mu bond is calculated as 7877 cm–1. The effect upon the muon–electron β-hyperfine coupling constant of excitation of the muonated radical to higher vibrational states is investigated.

Muonium radical formation in elemental sulphur

Abstract The formation of a molecular radical following muon implantation in sulphur S 8 is established experimentally by level crossing resonance. The position of the resonance determines the hyperfine constant as 233 ± 5 MHz. Of various possible assignments evaluated by ab initio molecular orbital calculations, the most promising appears to be the muonic sulfanyl radical, ṠMu.

Experimental and theoretical studies of the charge-density distribution in 3,3,6,6-tetramethyl-S-tetrathiane

A multipole model of the charge-density distribution in a ring containing C—S and S—S bonds has been obtained using high-resolution X-ray diffraction data from a single crystal of 3,3,6,6-tetramethyl-S-tetrathiane. Analysis of the bond critical points in both experimental and ab initio densities suggests a sulfur–sulfur bond order only slightly greater than unity. For isolated molecules, ab initio geometry optimisations predict a small energy stabilisation of the chair conformer relative to the twist-boat conformer observed in the crystalline state.

On the interpretation of muon spin rotation and level-crossing resonance observations on the muonic ethyl radical

Abstract Analysis of the muon spin rotation (μSR) data for the muonic ethyl radical CH 2 MuĊH 2 yields β-hyperfine coupling constants in good agreement with the recent level-crossing resonance (LCR) measurements. Other coupling constants are predicted and await confirmation by LCR. The role of hyperconjugation in the ethyl radical is assessed by an ab initio molecular-orbital calculation.

Paramagnetic muonium states in elemental sulfur

Zero-point vibrational corrections are calculated by abinitio molecular orbital methods for the energy and the reduced hyperfine tensor of the muonated sulfanyl radical SMu 2Π. Only a small α-isotope effect is found for the reduced isotropic muon–electron hyperfine coupling constant of SMu relative to SH. Interstitial muonium in S8 is also investigated. The results are discussed in the light of recent observations on muonium in elemental sulfur.

Catalytic properties of γ-alumina. Ab initio molecular orbital studies of clusters of chlorinated γ-alumina

Quantum-chemical studies of the surface-site geometries for aluminium–oxo (hydroxo) and chloro aluminium–oxo (hydroxo) cluster models derived from the spinel structure of γ-alumina are reported. The ab initio MO calculations which use an SV-321G basis indicate that a stabilisation of the LUMO energy occurs with a narrowing of the energy gap, Egap′, by the substitution of chlorine for oxygen or hydroxy groups in γ-alumina cluster models containing tetrahedral aluminium environments. The studies also show that a hierarchy of potential Bronsted and Lewis character is exhibited with Altet—O—Altet > Altet—O—Aloct > Aloct—O—Aloct′, indicative of a relationship between morpho-electronic factors and Lewis acidity at surface adsorption sites. The Bronsted acid character of the cluster is generated by the location of a near-neighbour tetrahedral aluminium environment. Substitution of chlorine for oxygen into the cluster model enhances the Bronsted acid character for the aluminium–oxo (hydroxo) cluster. The presence of octahedral aluminium–oxo (hydroxo) environments reduces both the Bronsted and Lewis acid character, respectively, for an aluminium–oxo (hydroxo) cluster.

Vibrationally averaged β-hyperfine coupling constants for the muonium-substituted ethyl radical

A variation-perturbation approach is applied to compute isotope-dependent zero-point vibrational corrections to the β-hyperfine coupling constant of the ethyl radical. The corrections increase the coupling by ca. 5, 7 and 23% for the deuterium-, protium- and muonium-substituted radicals, respectively. The large vibrational correction to the muon β-hyperfine coupling constant could explain the ‘residual’ isotope effect observed experimentally.