Ffrancon Williams

Hydrogen atom abstraction by methyl radicals in methanol glasses at 15–100 K: evidence for a limiting rate constant below 40 K by quantum-mechanical tunneling☆

Abstract Rate constants for hydrogen atom abstraction by methyl radicals in methanol glasses have been measured from 100 to 15 K. The Arrhenius plot is nonlinear and the reaction rate constant appears to reach a limiting value below 40 K. The results are discussed in terms of simple models for quantum-mechanical tunneling in the solid state at low temperatures. Assuming that the methyl group rotation in methanol brings about a merging of the energy level distribution at the potential barrier, the observation of temperature-independent rate constants below 40 K may be attributable to a freezing out of this rotation such that tunneling occurs only from the zero-point vibrational level.

ESR spectra and structure of the CF3Cl−, CF3Br−, and CF3I− radical anions

Abstract The anisotropic ESR spectra of the CF 3 X − radical anions (X = Cl, Br, I) have been observed following γ irradiation at 77 K of solid solutions containing up to 5 mole % of the CF 3 X parent compound in tetramethylsilane (TMS), neopentane, and 2-methyltetrahydrofuran. The resolution of the line components in the TMS matrix allowed the spectra to be analyzed in detail, the parallel and perpendicular features showing clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit ESR parameters obtained. Confirmation of the CF 3 X − identification was achieved through chemical studies which showed that a similar ESR spectrum was generated by electron attachment to the parent molecule in a photoionization experiment. Also, the spectrum of the CF 3 radical was observed to grow in during the decay of the CF 3 X − spectrum in neopentane above 100 K. The spin density distributions calculated from the ESR parameters of these congeneric radical anions suggest that the unpaired electron resides in an a 1 (σ * ) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C 3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p orbitals of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.

Kinetics of Ionic Processes in the Radiolysis of Liquids. II. Proton Transfer from Cyclohexane Ions to Ethanol

The extent to which positive ions produced by the radiolysis of cyclohexane undergo proton transfer to a dissolved scavenger before geminate recombination has been studied by an isotopic method. Ethanol‐d (C2H5OD) has been used, and the yield of HD is related to the yield of scavengable ions. The yield is strongly dependent on ethanol concentration over most of the range but at high concentration (>1M) a limiting value is reached corresponding to an estimated total yield of 2.6 ion/100 eV. The dependence of the yield on scavenger concentration suggests that the distribution of recombination times for ion pairs lies in the range between 10−7 and 3×10−11 sec corresponding to initial separation distances between 280 and 19 A, respectively. A median separation distance of 50 A is indicated. The results are consistent with the earlier supposition that the electron is trapped as a negative entity with molecular dimensions. Some implications of this model are examined.

ESR spectra and structure of the n-butane and n-hexane radical cations

Abstract Well-resolved ESR spectra of the n -butane and n -hexane radical cations have been generated, each spectrum showing a 1:2:1 triplet and a detailed substructure. The results are interpreted in terms of an a g singly occupied MO ( σ C—C, δC–H ax ) which is delocalized over the extended carbon chain (C 2h symmetry) and two axial hydrogens.

ESR evidence for the formation of the ring-opened cation CH2OCH2± from ethylene oxide

Abstract ESR results show that the radical cation formed from ethylene oxide in the solid state is the ring-opened 2-oxa-trimethylene cation with a symmetrical (C2ν) planar structure similar to that of the isoelectronic allyl radical. In contrast, the trimethylene oxide radical cation retains the ring structure of the parent molecule and its ESR parameters are characteristic of an oxygen-centred species.

Electron spin resonance spectrum of the perfluorocyclobutane radical anion. [77/sup 0/K; gamma radiation; A/sub F/ value of 148G]

The ESR spectrum was presented for the perfluorocyclobutane radical anion prepared by ..gamma.. radiation at 77/sup 0/K of a solid solution of 5 mole percent of perfluorocyclobutane in neopentane. Studies were performed using photobleaching experiments by exposing the sample to unfiltered light from a tungsten lamp. (DDA)

The ESR spectrum and structure of the dimer radical cation of dimethyl selenide (Me2Se-SeMe+2) in a γ-irradiated single crystal

Abstract The dimer radical cation of dimethyl selenide has been identified in a γ-irradiated single crystal of the parent compound, its main (evenSe) ESR spectrum showing hyperfine coupling to twelve equivalent hydrogens. Also, the relative intensity of the naturally abundant77Se satellite spectrum is in agreement with a hyperfine interaction involving two anisotropically equivalent selenium atoms. The principal values and direction cosines of the77Se hyperfine andg tensors have been determined and are shown to be satisfactorily interpreted in terms of a centrosymmetric structure, the unpaired electron occupying an antibonding (σ*) orbital built almost entirely from the pσ orbitals of the two selenium atoms. Analysis of theg tensor components indicates that the CSeC planes are perpendicular to the direction of the Se-Se bond. It is also shown that the isotropic1H splitting andg factor obtained previously from solution ESR studies of the dimethyl sulphide dimer cation are consistent with the structure advanced here for the selenium dimer. The results seem to rule out the possibility of any significant d orbital participation in the bonding.

Electron spin resonance spectrum of ((MeO)/sub 3/B. B(OMe)/sub 3/)/sup -/. A novel sigma radical with a one-electron bond. [Gamma radiation]

The esr spectrum of crystalline B(OMe)/sub 3/, gamma irradiated at 77/sup 0/K, showed the existence of a paramagnetic color center containing two equivalent borom atoms, the dimer radical anion of trimethyl borate. In addition the spectrum shows the characteristic line shapes of parallel and perpendicular components. Orientation of the crystal in a magnetic field enhanced the parallel features. It was concluded that the unpaired electron in the dimer radical anion is largely concentrated in the sigma-bonding orbital between two non planar B(OMe)/sub 3/ units. (JSR)

The isotropic and anisotropic EPR spectra of the tetrafluoroethylene radical anion

Abstract The tetrafluoroethylene radical anion has been generated in solid solutions by electron addition to the parent molecule. Both its isotropic and anisotropic EPR spectra have been observed, including the isotropic 13C satellite lines in natural abundance. The isotropic EPR parameters are aF = 94.3 G, aC = 48.7 G, and g = 2.0027. Two possible geometries, planar and chair, are discussed for this radical anion. The magnitude of the 19F and 13C hyperfine coupling constants are consistent with a planar (D2h) structure provided that the unpaired electron occupies the 5b1u (σ*) rather than the 2b1g (π*) molecular orbital which is predicted by ab initio calculations to be the LUMO of the parent molecule. On the other hand, the EPR parameters do not rule out a chair (C2h) structure if the bending of the CF2 groups introduces only a small distortion from planarity.

Electron spin resonance studies of electron attachment to fluorocarbons and related compounds

Radical anions of three perfluorocycloalkanes and several halogenofluoromethanes have been detected and identified by e.s.r. studies following γ irradiation at 77 K of solid solutions containing up to 5 mol % of the parent compound in neopentane or tetramethylsilane. The isotropic e.s.r. spectra of c-C3F–6, c-C4F–8 and c-C5F–10 are photobleached by visible light and show the second-order structure characteristic of 6, 8 and 10 equivalent fluorines, respectively, the total 19F coupling being approximately the same value (1170 ± 20 G) in each case. Identical e.s.r. spectra were generated in photoionization experiments using tetramethyl-p-phenylenediamine, confirming the radical anion identifications. The equivalence of the fluorines indicates that the unpaired electron is delocalized over the entire molecular framework in an orbital of high symmetry. The e.s.r. spectra of the CF3X– radical anions (X = Cl, Br, I) were anisotropic and showed clear evidence for axially symmetric hyperfine interactions with three equivalent fluorines and the unique halogen. On this basis, a matrix diagonalization program was used to calculate the line positions and the best-fit e.s.r. parameters obtained. Confirmation of the CF3X– identifications was achieved by parallel photoionization experiments and by studies showing that the decay of the CF3X– spectrum in neopentane above 100 K was accompanied by a growth in the spectrum of the CF3 radical. The spin density distributions calculated from the e.s.r. parameters of these congeneric radical anions suggest that the unpaired electron resides in an a1(σ*) antibonding orbital which is composed largely of the p orbitals from carbon and the unique halogen which lie along the C3v symmetry axis of the radical anion. Consistent with this proposal, the spin densities in the s and p oribtials of the unique halogen increase along the series Cl, Br, I, which is the order expected for the effect of decreasing halogen electronegativity.