W. F. Libby

Chemistry of Positive Ions. III. The Radiation Chemistry of Solid n‐Hexane at 77°K

The mechanism of the chemical effects of ionizing radiation on solid straight‐chain saturated aliphatic hydrocarbons such as polyethylene was studied by use of n‐hexane. This hydrocarbon was short enough to allow analysis of the cross‐link C12 products and of the hexenes formed. It was thought to be long enough to serve as a legitimate substitute for polyethylene. In any case, the yields of H2, olefines, and cross linkages were very similar in the two systems and we believe the mechanistic implications found here for solid n‐hexane at 77°K should apply even to polyethylene.The results indicate: (a) the active species is the original ion radical produced by the ionizing radiation (Co60 gammas); (b) the ion radical reacts before relaxing to redistribute the positive charge or to isomerize in any way, and the products show that there are three primary ions formed corresponding to the first, second, and third carbon atoms in the chain; (c) the ion radical reacts with nearest neighboring groups to bond and rel...

Chemistry of Positive Ions. I. General Theory Particularly for the Radiation Induced Cross Linkage of Polymers and Polymerization of Saturated Hydrocarbons

The chemical properties of positive ions are considered to be analogous to those of the corresponding neutral atom or molecule. However, the charge strengthens bonds and provides the long‐ranged attractive polarization force which causes the reaction cross sections to be very large. For organic ions dehydrogenation to form carbonium ions occurs frequently, so in addition to the parent ion radical with its great reactivity as a type of super fluorine atom, the carbonium ions with their extremely acidic (electrophilic) properties cause a wholly different set of reactions. These are analogous to those of carbene and may lead to the radiation‐induced polymerization of saturated aliphatic hydrocarbons. Both ion radicals and carbonium ions can play important roles in the radiation induced cross linkage of polyethylene and other polymers. It is predicted that the effect of phase will be very important in radiation chemistry, because the cage effect of the surrounding close‐packed molecules in the liquid and soli...

Size Effects Among Isotopic Molecules

It is known that equilibrium constants for isotopic reactions are temperature-dependent owing to energy differences in zero-point energy, but it is less well-known that there is a possible pressure effect owing to volume differences in the zero-point state. For example, for the reactions H/sub 2/O/ sup 18/+ CaCO/sub 3/ (?) H/sub 2/O + CaCO/sup 18//sub 3/ (calcite), the volu me change is calculated from the frequency shift to be 0.14 cc/mole, and sigma ln k/ sigma P =1.9 x 10/sup 6/s atm/sup -1/ at 298 deg K. This means that a change in k should be measurable in the laboratory at high pressures. (D.L.C.)

Chemistry of Positive Ions. VI. Positive‐Ion Chemistry in Solid Methane

The polymerization of solid methane at 77°K by γ rays was studied at two dose rates in the dose range 4 to 150 Mrad, corresponding to 0.04% to 1.4% conversion. The polymer contains an average of 20 carbon atoms per molecule and its empirical formula is C20H40, independent of the dose. The yield is 0.32 methane molecules converted to polymer per 100 eV absorbed and any induction dose must be less than 1 Mrad (0.01% conversion). We report a detailed characterization of the polymer based on physical and spectroscopic studies. Products up to heptane are identified. Preliminary studies of solid ethane and propane afforded polymer with lower yields and fewer numbers of monomer units than the methane polymer.From an analysis of the dose and dose‐rate dependence of the polymerization of solid methane, we conclude that only two types of mechanism are possible. The most useful mechanism is one in which a carbonium ion is transformed directly into a heavy hydrocarbon ion. This process would require about 6 eV of exc...

Nature of the Chemical Reactivity of Radiobromine Atoms Produced by Isomeric Transition

The hot‐atom chemistry of n‐propyl bromide has been examined further. By comparing the differences and similarities of the (n, γ) and isomeric transition processes, an over‐all theory is evolved which accounts for the identical results produced by both nuclear processes in the solid and liquid alkyl halides. The charge developed by the Auger process following internal conversion causes the molecule to explode and gives a neutral recoiling atom which then reacts as the (n, γ) atoms. The neutralization after rupture is rapid in the solid and liquid. In the gas this is not so and it has been possible to demonstrate experimentally a difference between the retentions in the gas phase.

Chemistry of Positive Ions. II. Ion—Molecule Reactions in Radiolysis of n‐Hexane at Low Temperatures

n-Hexane was gamma irradiated at various temperatures. It was found that the yield of n-dodecane at room temperature was not affected by 0.03 M added iodine, or by 1% added pentene. Also, 8 mole% propylamine did not reduce the yield. The scavenger results at room temperature indicate that n-dodecane is formed by an ionic process involving the original ion radicals. An increase in yield showed that the process becomes more efficient at low temperatures. It is proposed that the product is formed by a hydrogen-atom abstraction reaction. (P. C.H.)

Electron Transfer Among the Transition Elements; the Controlling Role of the Franck—Condon Principle on Rates

The principle [W. F. Libby, J. Phys. Chem. 56, 863 (1952)] that electrons cannot be readily exchanged between aqueous ions of different valence, unless they possess sufficient geometrical similarity to reduce to a minimum the energy transfer required by the simultaneous and instantaneous conversion of an ion of one valence to another while a second is changed in the exact opposite way, has now been firmly established. The Franck—Condon principle as applied to electron movement as well as to optical transitions thus has been found to be completely commanding in the case of electron exchange reactions between transition element ions in aqueous solution; e.g., Fe2+ and Fe3+ exchange electrons slowly while Fe(CN)64+ and Fe(CN)63— exchange very rapidly. Rapid electron exchange occurs also between MnO4—— and MnO4— even though the oxygen atoms do not exchange. Thus a type of tunneling of electrons is firmly proved to occur when the source and sink (reducing and oxidizing) ions (or molecules) satisfy the requirem...

Ionizing Photolysis of Methane

The photolysis of methane using a filtered 584‐A light source yields primarily ethylene and a polymer with small amounts of other products. An interpretation of the meaning of these results is offered. Information on the use of thin aluminum films to obtain pure 584‐A light is given.