Brynmor Mile

Studies of radical–molecule reactions using a rotating cryostat. Reactions of hydrogen atoms with organic substrates at 77 K

The rotating cryostat has been used in conjunction with electron spin resonance (e.s.r.) spectroscopy to study directly the reactions of hydrogen and deuterium atoms with a range of organic substrates in the solid state at 77 K. In every case the primary radicals formed by the reaction were trapped and could be observed by e.s.r. For ethylene, allene, ketene and acetaldehyde, free radicals formed by a secondary reaction were also observed.No primary abstraction reactions were observed for any of the alkenes studied, but hydrogen abstraction occurred almost exclusively for acetaldehyde and allyl alcohol, and both abstraction and addition reactions were observed for substituted benzenes.The relative amounts of addition to the two ends of the double bond in a series of asymmetric alkenes were measured and are in qualitative agreement with predictions based on quantum mechanical calculations, though for pent-2-ene the experimental selectivity was much greater than expected. A small secondary inverse deuterium isotope effect was observed for the addition of hydrogen atoms to 1,1-dideuteroethylene.For the substituted benzenes preferential addition to the ortho-position of the ring occurred, with much less addition to the para-position and virtually none at the meta-position.

Electron spin resonance allied with spin traps to detect and identify atoms and radicals on Pd‐supported catalyst surfaces in solution

Spin trapping of radical species from the surface of the hydrogenation catalyst Pduf8ff BaSO4 was examined using the spin trap N‐tert‐butyl‐α‐phenylnitrone (PBN). Hydrogen (and deuterium) atoms can be trapped from the surface of the palladium under toluene solution provided that H2 (or D2) is bubbled through the solution in the presence of the PBN. However, no atoms can be trapped from the surface of the reduced catalyst system in the absence the bubbling dihydrogen. The hydrogen‐trap signal is removed by the presence of the alkenes ethene and cyclohexene, and there was no evidence for the formation of derivative radical‐trap signals. For trans‐2‐hexene a diminished hydrogen trap signal was observed, together with some evidence for another radical, either an alkoxyl or a secondary alkyl species. Copyright

ESR studies of rhodium atoms deposited from thermal sources into hydrocarbon matrices at 77 K

Thermally generated rhodium atoms have been trapped in an adamantane matrix at 77 K and the resulting X- and K-band ESR spectra investigated over a range of temperatures. All three g-shifts of the observed orthorhombic spectrum are well resolved and the spectrum is assigned to neutral Rh atoms trapped in asymmetric matrix sites. The spectra were well simulated with the following parameters: g1u2006=u20062.761, g2u2006=u20062.572, g3u2006=u20061.940 and A1u2006=u2006305 MHz, A2u2006=u2006284 MHz, A3u2006=u2006244 MHz. These are used to estimate spin density contributions of 0.55 (4d), 0.22 (5s) and 0.23 (5p) toward the ground state electronic configuration of the trapped atoms.

The first ENDOR spectrum of naked metal clusters: 7Li3, 6Li 7Li2 and 7Li3(H2O)x in an adamantane matrix

Abstract The electron double resonance (ENDOR) spectra of naked and hydrated neutral homonuclear metal clusters are reported. 7 Li 3 , 6 Li 7 Li 2 , and 7 Li 3 (H 2 O) x clusters have been prepared in an adamantane matrix using the rotating cryostat variant of the matrix isolation technique. The centre of the ENDOR doublet for the major cluster, 7 Li 3 , occurs at 46.15 MHz giving an isotropic hyperfine interaction (hfi) of 92.30 MHz which is close to that of 92.70 MHz determined previously from the EPR spectrum of Li 3 . Weaker ENDOR transitions from 6 Li 7 Li 2 give hfis of 37.1 MHz for 6 Li and 51.84 MHz for 7 Li. These are consistent with the values for 7 Li 3 if account is taken of the statistical factor of two favouring the location of the single 6 Li nucleus in 6 Li 7 Li 2 at the terminal positions of the obtuse angled structures in the minima of the ‘Mexican hat’ hypersurface of the cluster. A weaker doublet, centred at 36.59 MHz is assigned to a hydrated cluster complex 7 Li 3 (H 2 O) x . The reduction in the hfi value compared with that in the neutral cluster 7 Li 3 indicates a 20% transfer of unpaired electron spin density to the coordinated water molecules. A complex series of doublets centred at the proton frequency (14.31 MHz) indicate the presence of a number of hydrated Li 3 clusters. These first observations of the ENDOR transitions of neutral trimeric metal clusters suggest that it may be possible to observe the spectra of other, larger and more complex, clusters.

EPR spectrum of an acute-angled Na3 cluster in an adamantane matrix

The isotropic EPR spectrum of an acute-angled Na3 cluster associated with water in an adamantane matrix has the EPR parameters g= 2.001, aNa(1)= 17.96 mT and aNa(2)= 1.25 mT.

An ESR study of the reaction of silver atoms with B2F4 under matrix isolation conditions

Diboron tetrafluoride has been found by ESR to complex with silver atoms when the species are deposited together in hydrocarbon matrices at 77 K in a rotating cryostat. The major feature in the ESR spectra of the deposit in adamantane and cyclohexane matrices (Ag ∶ B2F4 n∶ matrix ratio of ∼1 ∶ 5 ∶ 500) was an almost isotropic set of two doublets of 13 (or possibly 15)-line multiplets. The two sets of doublets arise from the almost equally abundant 107Ag and 109Ag isotopes with I n= 1/2. The multiplets on each silver line indicate hyperfine interaction with either four equivalent 11B nuclei (I n= 3/2) or two equivalent 11B nuclei and four equivalent 19F nuclei. Multiplets from the less abundant 10B isotope (19.8%, I n= 3) were not resolvable. The almost isotropic multiplets are best simulated as axial spectra nwith |A∥|(11B) = 17.5 MHz and |A⊥|(11B) = 14.1 MHz and Aiso(107Ag) = 1371.0 MHz. The complex is either [Ag(B2F4)2] or [Ag(B2F4)]. Assuming that A∥ and A⊥ are both positive, the total spin density is calculated to be 0.85 in [Ag(B2F4)2] and 0.80 in [Ag(B2F4)]. The most likely structures together with density function calculations on these two possibilities are discussed.

Disproportionation–combination reactions of cyclohexyl radicals at low temperature in the solid state

The disproportionation–combination ratio of cyclohexyl radicals has been measured at low temperatures (135–195 K) in a variety of solid phases, cyclohexane, cyclo-octane, benzene, camphane and water. The radicals were generated by carrying out the reaction C6H11Br + Na·→C6H11·+ NaBr, using the rotating cryostat technique. When allowance is made for the occurrence of side and secondary reactions the values of the ratio kd/kc were found to lie in the range 0.44 to 1.1 depending on the temperature and solid used. These values are close to those found in liquid cyclohexane (1.1) and in the gas phase (0.5) at ambient temperatures. The fact that the ratio is only slightly influenced by marked changes in reaction conditions suggests that though the transition states must be different, these differences are very small as far as the energies and the volumes of the states are concerned.

The use of the tertiary alkyl tetraoxide–peroxyl equilibrium, ROOOOR ⇌ 2RO2˙, as a clean source of tertiary alkyl peroxyls

Below 155 K there is a dynamic equilibrium between tertiary alkylperoxyls and their tetraoxide combination products. The system can be recycled repeatedly between 113 and 155 K without loss of radicals. The stationary peroxyl concentration depends solely on the temperature and the initial tetraoxide level. The equilibrium provides a clean source of peroxyls, since no other radicals such as alkoxyls are present. We have used the equilibrium to determine the mechanisms, and to estimate the rate constants for the reactions of tertiary alkylperoxyls with antioxidant and spin trap substrates. The method depends on rapidly mixing cold solutions of the substrates and solutions containing equilibrium concentrations of tertiary peroxyls. The mixing takes place in situ within an EPR cavity and the decay of the peroxyl signal is monitored continuously nby EPR. The decay profiles have been kinetically analysed at three levels of approximation with the most accurate rate parameters being obtained by kinetic modelling of the total reaction scheme.The rate parameters for the reaction of peroxyls and the antioxidant, IONOL, are in good agreement with those determined by other kinetic EPR methods (KEPR). Studies of the reaction with the much used spin trap DMPO have shown that a rapid removal of peroxyls does not result in a paramagnetic mono-radical spin adduct, but yields a diamagnetic product probably containing two peroxyls added sequentially or concertedly to the spin trap.

The chemistry and the geometric and electronic structures of small naked metal clusters prepared using a rotating cryostat and studied by electron paramagnetic resonance

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 C2v symmetry. A new acute Na3 cluster was formed by interaction with water, Na3(H2O)x where x 1. The first EPR spectrum of a mixed alkali-metal trimer, Na2Li, has been recorded. The fluxionality of both Na3 and Li3 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, Li3, 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 Li3(H2O)x 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.

The ESR spectrum and structure of a heteronuclear trimeric cluster, Au2Ag, prepared using a rotating cryostat

Abstract The ESR spectrum of the mixed coinage metal cluster Au2Ag in a perdeuteriobenzene (C6D6) matrix prepared using a rotating cryostat is reported. The cluster has the magnetic parameters g = 1.9292 ± 0.0001, a(2Au197) = 39.7±0.05 mT and a(1Ag(107+109)) = 1.3 ± 0.05 mT. These indicate a static obtuse triangular structure with a 2B2 ground state in C2v symmetry, with two gold nuclei at identical terminal positions.