B. Smaller

Paramagnetic Species Produced by γ Irradiation of Organic Compounds

Some alkanes, olefins, alcohols, ethers, and a few other organic compounds have been irradiated with Co60 γ rays at low temperatures. The frozen irradiated samples have been examined for paramagnetic resonance, and the observed spectra in several cases have been identified with specific radicals. The methyl, ethyl, and allyl radicals are among those identified. The results in each case have been considered in relation to the present knowledge of the radiation chemistry of the compound. The mode of radical formation in many cases appears to be quite specific. Thus, in normal alkanes the major process for radical formation is by loss of a specific H atom. Hyperconjugation is used to interpret the resonance spectra.

Electron Paramagnetic Resonance Studies of Transient Free Radicals Produced by Pulse Radiolysis

Electron paramagnetic studies have been conducted on the transient free radicals produced by pulse radiolysis in liquids. The instrument design permitted observation of radical lifetimes ≥2 − 3 μsec and corresponding rate constants ≤1010M − 1·sec − 1. The effect of radical scavengers such as sulfur‐containing compounds or oxygen on the transient behavior of the cyclopentyl and cyclohexyl radical were studied and the pertinent rate constants determined for the reaction. Recombination rate constants for cycloalkyl as well as alkyl‐ and cycloalkyl‐peroxy radicals were compared along with the activation energy for representative species.

EPR Pulse Radiolysis Studies of the Hydrogen Atom in Aqueous Solution. I. Reactivity of the Hydrogen Atom

The transient properties of hydrogen and deuterium atoms produced by pulse radiolysis in water have been studied by EPR. Details of the EPR instrumentation are described. From decay characteristics of the EPR signal on addition of various solutes, rate constants for the reaction of atomic hydrogen with various classes of molecules have been determined. Agreement of our results with prior indirect determinations is, in general, good. The isotope effect is also in agreement with expectation.

Paramagnetic Species in Gamma‐Irradiated Ice

The paramagnetic resonance spectra of H2O and D2O ice irradiated at 77°K have been examined. The absorbing species have been identified as H and OH (or D and OD), a significant result for the radiation chemistry of aqueous solutions. The yield of radical pairs/100 ev is about 0.14. The hyperfine splittings of the H doublet and D triplet are a factor of 16 less than are observed in the gas phase. This result is attributed to the effect of the solid on the electronic state of the H or D atoms. The spectra of H2O2 in H2O and D2O2 in D2O irradiated at 77°K support the identification of OH (or OD) absorption. In annealing experiments the H and OH disappear near 115°K. Results on solid ammonia and solid formic acid irradiated at 77°K are also described.

Paramagnetic Resonance of Some Silver (II) Compounds

The electron paramagnetic resonance of acidic solutions of silver (II) and several silver (II) solid complex compounds has been investigated. In all cases a strong g anistropy permitted the evaluation of the principal g values. In two cases, the hyperfine splitting could be measured without ambiguity. Parallel measurements of the optical and ultraviolet absorption in the case of silver (II) solutions indicated that the hole is not localized on the paramagnetic ion.

Paramagnetic Resonance of Gamma‐Irradiated Single Crystals of Ice at 77°K

The anisotropy of the paramagnetic resonance spectra of ice crystals irradiated and measured at 77 deg K was studied in- an effort to identify the paramagnetic species indicated by previous experiments. The doses were approximately 1021 ev/cc each, and the spectra were obtained with the c axis parallel and perpendicular to the axis of rotation, which was kept perpendicular and parallel to the static and microwave magnetic fields, respectively. The parallel run showed sixfold symmetry while the perpendicular run showed one canonical orientation with a minimum doublet separation, a maximum g value, and two mirror planes. The spectra show twelve-fold periodicity at 15 deg orientations to each mirror; the results are interpreted in terms of a cylindrically symmetric radical. Polycrystalline ice was also examined, giving two lines with sixfold symmetry in the parallel run; this result indicates the presence of H 2O+.

Triplet‐State Zero‐Field‐Splitting Correlations in Substituted Molecules

The triplet‐state properties of a large number of unsaturated ring compounds have been investigated by optical and electron paramagnetic techniques. Correlative effects have been noted between optical zero‐field‐splitting (ZFS) shifts and chemical data. Redistribution of spin density due to substituents as evidenced by ZFS shifts appear to be directly related to measures of π charge redistribution using other techniques. For the naphthalene derivatives the dependence of ZFS shift on spin density at substitutional carbon site has been found also to correlate with triplet lifetimes.

Energy Transfer between Triplet States in a Viscous Medium

The mechanism of energy transfer between triplet states of aromatic molecules has been studied using deuterated phenanthrene as donor and deuterated naphthalene as acceptor. The investigation reveals that at high viscosities the transfer rate constant kt can be accounted for in terms of an electron exchange interaction and becomes diffusion controlled as the viscosity η is decreased and molecular motion becomes appreciable. The results covering viscosities above 104 P are in good agreement with a Debye diffusion‐controlled collision reaction, ktη=const. The interaction is probably electron exchange in nature with range Rc≈20 A. The relative magnitudes of Rc and r = 〈r2〉½ determine the characteristics of the energy‐transfer phenomenon.

Paramagnetic Resonance of Some Copper Complex Compounds

Electron paramagnetic resonance spectra of Cu alpha picolinate, Cu dipyridyl/sub 2/ SO/sub 4/, Cu pyridine/sub 4/ SO/sub 4/, and Cu pyridine/sub 4/ S/sub 2/O/sub 8/ were observed. A dual modulation spectrometer operating at 9 k Mc was used at 77 deg K. Derived spectra and sorption curves are plotted and results are summarized in tabular form. (L.N.N.)