K. F. Preston

Atomic parameters for paramagnetic resonance data

The atomic parameter ψ2(0) and the isotropic hyperfine interaction A for unit spin density in the corresponding s orbital are calculated for the most abundant nuclei of the elements from helium to bismuth from the Hermann-Skillman wavefunction. Angular factors for p, d, and f orbitals are also given, and the application of the results is illustrated by the interpretation of the hyperfine intractions in the radicals PbF63− and YbH.

Radical Reactions of C60

Photochemically generated benzyl radicals react with C60 producing radical and nonradical adducts Rn C60 (R = C6H5CH2) with n = 1 to at least 15. The radical adducts with n = 3 and 5 are stable above 50�C and have been identified by electron spin resonance (ESR) spectroscopy as the allylic R3C60. (3) and cyclopentadienyl R5C60. (5) radicals. The unpaired electrons are highly localized on the C60 surface. The extraordinary stability of these radicals can be attributed to the steric protection of the surface radical sites by the surrounding benzyl substituents. Photochemically generated methyl radicals also add readily to C60. Mass spectrometric analyses show the formation of (CH3)nC60 with n = 1 to at least 34.

Collisional Deactivation of Excited Oxygen Atoms in the Photolysis of NO2 at 2288 Å

The isotopic composition of the oxygen produced from the 2288‐A photolysis at room temperature of NO2 in the presence of C18O2 has been measured as a function of the pressure of various additive gases. A simple reaction mechanism involving reactions of excited oxygen atoms is postulated to explain the results and leads to the following relative rates of electronic deactivation (including chemical reaction, where possible) of the excited atoms: CO2, 1.00; Ar, 0.01; Kr, 0.06; Xe, 0.78; N2, 0.24; N2O, 1.02; NO2, 1.62; C3H8, 4.67; and SF6, close to zero. The close correspondence of these relative rate constants to the values obtained from studies of the photolysis of N2O at 1849 A suggests that the results for the photolyses of both N2O and NO2 refer to the same species of excited atom, namely O(1D2).

Primary Processes in the Photolysis of Nitrous Oxide

From measurements of the isotopic composition of the product nitrogen arising from the photolysis at 2288, 2139, and 1849 A of gaseous N2O containing ∼ 1% 15NO, it is shown that at all three wavelengths the primary production of N atoms is unimportant, representing less than 1% of the primary production of molecular N2. Taken in conjunction with other observations made in this laboratory these results indicate that at 2139 A, and perhaps at all wavelengths in the region 1800–2300 A, the decomposition to N2+O(1D2) is the sole primary photolytic step of importance in the photolysis of N2O.

Electron spin resonance study of CH3CNSSN˙, C6H5CNSSN˙, and SNSSN˙+ free radicals

Isotropic and powder e.s.r. spectra have been recorded for CH3[graphic omitted]˙, C6H5[graphic omitted]˙, and [graphic omitted]˙+. Isotropic labelling with nitrogen-15 and sulphur-33 has been accomplished for [graphic omitted]˙+ and it has been possible to prepare 33[graphic omitted]˙+. Sulphur-33 satellites have been observed for C6H5[graphic omitted]˙. MNDO and Gaussian 76 calculations have been used to calculate the minimum-energy structures of the radicals, while INDO calculations have provided values for the hyperfine coupling constants. Unfortunately, poor agreement was obtained between the latter and the corresponding experimental values. All the radicals dimerise in solution at low temperatures and we have been able to measure the energetics of dimerisation for C6H5[graphic omitted]˙ and [graphic omitted]˙+. The dimers exist as crystalline solids which contain readily detectable amounts of the monomeric free radical.

Covalent bonding of thiophenes to Si(111) by a halogenation/thienylation route

Abstract Thienyl monolayers covalently bonded to a Si(111) surface were prepared by a wet chemical process in which a Si(111)–H surface was brominated to form Si(111)–Br and further reacted with lithiated thiophenes. Both N -bromosuccinimide and bromochloroform were found to be effective brominating reagents. The derivatized Si(111) surfaces were characterized by XPS, AES, ATR–FTIR and NEXAFS.

The proton hyperfine interaction in HC60, signature of a potential interstellar fullerene☆

Abstract HC 60 is probably present in interstellar space. Before radioastronomers can mount an effort to detect it, its proton hyperfine interaction must be determined in the laboratory with great accuracy. By reacting C 60 with H atoms we have observed the EPR spectrum of HC 60 in both liquid and solid phases. In solid neon at 4 K,the proton hyperfine interaction is slightly anisotropic, with a |=97.8 MHz and a ⊥ =91.0 MHz. The isotropic hyperfine interaction of 93 MHz agrees with that obtained in liquid benzene, and is 10% smaller than calculated from the muonium hyperfine interaction in MuC 60 . The origin of this discreapancy is discussed.

EPR spectroscopy of single crystals using a two-circle goniometer

Abstract The anisotropy of EPR spectra observed in single crystals of various symmetry classes is described. The use of a computer-assisted two-circle goniometer to assemble the hyperfine-interaction and g 2 tensors in the crystallographic axis sytem is explained.

Anisotropic hyperfine interactions of rare‐gas nuclei near trapped hydrogen atomsa)

The EPR spectra of hydrogen atoms trapped in xenon and krypton at 10 K have been reinvestigated. Analysis of the anisotropic 129Xe (enriched), and 83Kr hyperfine structure shows the H atoms are trapped in the octahedrally symmetric interstitial site (6 nearest neighbors) of the Kr and Xe matrices. For H in Kr, a simple model which considers only overlap of H and Kr orbitals in purely covalent Kr⋅⋅⋅H ’’molecules’’ accounts very well for the observed Kr hyperfine structure and the electronic g factor. To account for the corresponding magnetic constants of H in Xe, however, it is necessary to consider, along with overlap effects, the effects of transfer of electronic charge from the nearest neighbor Xe atoms to the interstitially trapped H atom, as described by admixture of a very small Xe+⋅⋅⋅H− ionic component into the covalent Xe⋅⋅⋅H ’’molecule’’. This charge transfer may also be a factor in the surprisingly large differences in the g factors and 129Xe hyperfine constants of H and D atoms trapped in Xe.

The ESR/alanine dosimeter—power dependence of the X-band spectrum

Abstract Satellite lines which accompany the central feature of the X-band ESR spectrum of α-alanine dosimeters are shown to be due to forbidden “spin-flip” transitions associated with methyl protons on nearby molecules. At microwave powers in excess of 1 mW the satellites increase in intensity relative to the central feature, and thus measurements at higher microwave powers must be based on experimentally determined calibration curves at the appropriate power levels.