J. R. Morton

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.

Microwave Spectrum and Structure of Propynal (H–C≡C–CHO)

The moments of inertia IB0 and IC0 of fifteen isotopic species of propynal have been determined from the frequencies of certain lines in the type a spectra. With the aid of type b transitions, IA0 and the inertial defects Δ have been obtained for some of the isotopic species. Using equations relating the atomic coordinates to the change in the moments of inertia on isotopic substitution, three independent sets of molecular parameters have been obtained (two for C=O). The agreement between the three sets of data is within the probable experimental error. The mean values of the molecular parameters are as follows: rs(C–H) (acet.) = 1.0553A, rs(C–H) (ald.) = 1.1064A, rs(C≡C) = 1.2089A, rs(C–C) = 1.4446A, rs(C=O) = 1.2150A, ≰CCO = 123°47′, ≰CCC = 178°24′, ≰CCH (ald.) = 113°54′, ≰CCH (acet.) = 180°0′.The error in the bond lengths is estimated to be ±0.001 A, and in the angles ±10′. A slight deviation from linearity (1°36′±10′) was apparent in the carbon chain.The moments of inertia IB0 and IC0 of fifteen isotopic species of propynal have been determined from the frequencies of certain lines in the type a spectra. With the aid of type b transitions, IA0 and the inertial defects Δ have been obtained for some of the isotopic species. Using equations relating the atomic coordinates to the change in the moments of inertia on isotopic substitution, three independent sets of molecular parameters have been obtained (two for C=O). The agreement between the three sets of data is within the probable experimental error. The mean values of the molecular parameters are as follows: rs(C–H) (acet.) = 1.0553A, rs(C–H) (ald.) = 1.1064A, rs(C≡C) = 1.2089A, rs(C–C) = 1.4446A, rs(C=O) = 1.2150A, ≰CCO = 123°47′, ≰CCC = 178°24′, ≰CCH (ald.) = 113°54′, ≰CCH (acet.) = 180°0′.The error in the bond lengths is estimated to be ±0.001 A, and in the angles ±10′. A slight deviation from linearity (1°36′±10′) was apparent in the carbon chain.

Electron Spin Resonance Spectrum of XeF in γ‐Irradiated Xenon Tetrafluoride

The radical XeF has been detected by means of electron spin resonance in a single crystal of XeF4 γ‐irradiated at 77°K. The following hyperfine interaction constants were obtained for magnetic field directions parallel and perpendicular to the Xe–F bond:F∥19=2649 Mc, F⊥19=540 Mc, Xe∥129=2368 Mc, and Xe⊥129=1224 Mc. The unpaired electron occupies a σ orbital, and it was shown from parameters derived from the respective atomic wavefunctions that the orbital is predominantly F 2p and Xe 5p in character. The experimentally determined g values for XeF were g∥=1.9740, g⊥=2.1251. Departures from the free‐spin value are interpreted in terms of interaction between the orbital ground state and excited states of the molecule.

Paramagnetic Resonance Spectra of O− Trapped in Alkali Iodide Crystals

The impurity ion O− has been introduced into alkali iodide crystals and detected at 4°K by EPR spectroscopy. A quantitative understanding of the g factor and 17O hyperfine interaction was obtained with the aid of equations derived in a recent analysis of the analogous S− impurity.

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.

Electronic Structure of Sulfate, Thiosulfate, and Related Ions. II. ESR Spectra of SO4− and S2O3−

The radical ions SO4− and S2O3−, trapped in γ‐irradiated K2SO4 and Na2S2O3·5H2O crystals, respectively, have been identified by their ESR spectra. In the preceding paper molecular orbital calculations were described the results of which predict a ···1t163t262a1, 2A1 (symmetry Td) configuration for SO4− and a ···5e45a1, 2A1 (symmetry C3v) configuration for S2O3−. The ESR data show that in the K2SO4 crystal the symmetry of SO4− is no higher than C2v and so correlation with the predicted configuration was inconclusive. In the case of S2O3−, however, the spectral parameters possessed axial symmetry, the ···e4a1, 2A1 configuration being confirmed by the nature of the hyperfine and g tensors.

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.

Paramagnetic Resonance Spectrum of O− Trapped in KCl, RbCl, and KBr

The impurity ion O− has been introduced into KCl, RbCl, and KBr and detected at 4°K by EPR spectroscopy. The center has orthorhombic symmetry. A quantitative understanding of the g factor and of the O17 hyperfine structure was obtained with the aid of equations derived in a recent analysis of the an‐alogous S− impurity. The hyperfine splitting was described by the atomic parameters ψ2(0), 〈r−3〉l, and 〈r−3〉s. The present results indicate that in an earlier investigation of O− trapped in alkali iodide crystals erroneous values were obtained for these parameters.