A series of thiyl radicals prepared by the photolysis of the parent disulfide have been trapped using 5,5-dimethyl-I -pyrroline N-oxide (DMPO) and 2,4,6-tri-tert-butylnitrosobenzene (BNB). The EPR spectra of the resulting spin adducts show proton hyperfine interactions from the 6-protons of the alkyl group of the thiyl, thus allowing definitive radical assignments to be made. For thiyl BNB spin adducts, a characteristic nitrogen coupling constant is observed. For secondary and tertiary thiyls, two adducts arise from addition to the nitrogen and oxygen of the nitroso group. The extent of reaction at the oxygen increases with increasing disulfide concentration. This is attributed to the formation of a complex between the thiyl and the parent disulfide, whose bulk favours its addition to the less sterically hindered oxygen position. The rate constant for the reaction of Pr'S' radicals with (PriS)2 to form the complex is 240 times lower than that for reaction with BNB. Direct addition of Pr'S' to the nitrogen of BNB is 11.5 times more efficient than to oxygen.

Small proton hyperfine interactions (ca. 0.1 mT) are observed from the &protons of the alkyl group of the parent thiyl radical in spin adducts from 5,5-dimethyl-3,4-dihydropyrrole N-oxide.Thiyl radicals are important mediators in the reactions of sulfur antioxidants in hydrocarbons1 and in many biological systems where sulfur compounds reduce free radical-induced tissue i n j ~r y . 2 . ~ They may also be the cause of arteriosclerosis in homocysteinuria.3 Unfortunately their large g-factor anisotropy, arising from the near-degeneracy of two n-type orbitals at the sulfur centre, causes such extensive line broadening as to make the radicals unobservable directly by electron spin resonnance (ESR) spectroscopy in the liquid state.5 Recourse has to be made to spin traps which convert the thiyl radicals to 'stable' N-oxide spin adducts which are readily detected by ESR.6 However, there is the disadvantage that only primary radical types rather than specific radical structures can usually be determined for most spin adducts, especially those from nitrone spin traps, since no hyperfine interactions with magnetic nuclei in the primary radical are observed. Here we show that the thiyl spin adducts of the nitrone spin trap 5,5-dimethyl-3,4-dihydropyrrole N-oxide (DMPO) are exceptional in that they do show resolvable coupling to &protons of the alkyl group attached to the sulfur and hence allow more definitive primary radical assignment to be made.

A rotating cryostat has been used to generate several new naked metal clusters whose structures and reactions have been studied by EPR spectroscopy. Most alkali-metal trimers adopt an obtuse triangular geometry with a 2B2 ground state in CZv symmetry. A new acute Na, cluster was formed by interaction with water, Na,(H,O), where x 3 I . The first EPR spectrum of a mixed alkali-metal trimer, Na,Li, has been recorded. The fluxionality of both Na, and Li, in an adamantane matrix is lost and a static form results with the Li located at the apical position. The first ENDOR (electron nuclear double resonance) spectrum of a naked metal cluster, Li,, has been recorded. It gives more accurate paramagnetic parameters and shows conclusively that the cluster is a fluxional, pseudo-rotating Jahn-Teller molecule. The ENDOR spectra of Li,(H,O), complexes were also observed. A new coinage metal trimer, Au2Ag, has been characterised by EPR spectroscopy and has the silver in the apical position in a static obtuse structure. Reactions of the alkali-metal and coinage-metal trimers with simple molecules such as water and ethene resulted in a change in geometry from an obtuse or fluxional molecule to an acute static entity. The flexible nature of clusters is discussed in the context of modelling the chemical events occurring at the active metallic sites of heterogeneous catalysts. Experimental Principle of the rotating cryostat techniqueIn the matrix-isolation technique reactive and tethered in specific sites in the matrix species are trapped so that they cannot AXIS OF ROTATION DIRECTION OF ROTATION M A m SURFACEOF BANDCOVERED WITH REACTANT 'METAL 7 Basis of the presentation given at Dalton Discussion No. 1, 3rd-5th

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