Phase change and energy transfer effects in the radiation chemistry of organic substances

Abstract

G(-S)values, where S = FeC13 or I2 or DPPH for solutions of S in various organic solvents, have been measured at different temperatures in the range -196-30°C. For all systems in the liquid or glassy solid phases, [S] diminishes linearly with dose, and G(-S) is largely independent of IS] and temperature (provided the phase is not changed), being larger in the liquid than the glass. Crystalline solid solutions fall into two categories. Type-A systems, comprising solutions of FeC13 in diethyl ether or 1,4-dioxan, or DPPH in benzene or diphenyl ether, are similar to glasses.Type-B systems, comprising solutions of FeC13 in benzene, diphenyl ether, diphenyl methane and phenetole (crystalline state only), and I 2 in diphenyl ether, give steeply curved [S]-dose graphs and G(-S)o increases proportionately with increase of [S], the slope of this line being larger the lower the temperature. Thus values of G( -S)O for the solid phase may be much larger than G( -S) for the liquid or glassy phases (in benzene at -196" this factor can be as large as 30), and there is a sudden increase of G( -S)o on freezing.Reasons are given for believing that the reaction occurs whilst the system is solid and that in class-B systems the radiation induces formation of excitons corresponding to the molecular state 1B2u (for benzene), which are propagated over distances of 50-100A. Interaction of these with solutes whose absorption spectrum overlaps the exciton spectrum causes chemical change of the solutes. Class-A crystals cannot form excitons in the appropriate spectral range. That crystallinity is essential for class-B behaviour is shown by the grossly disparate results for those systems which show class-B behaviour when crystalline and class-A behaviour, at the same temperature and solute concentration, when they are in either the supercooled liquid or glassy states. Other observations are recorded and explained on the same hypothesis.

Radiation Chemistry

“…Collinson, Conlay & Dainton (144) have studied the effect of phase on ferric chloride reduction. Certain solvents such as benzene, diphenyl ether, diphenyl methane, and phenatole give strongly concentration-dependent ferrous chloride yields.…”

Radiolysis of ferric hexafluoroacetylacetonate–toluene solutions

“…Some reduction of Co(acac)3 to C~( a c a c )~ occurs durlng the " C O -~ rad~olysis of toluene solutions containing Co(acac)3 (11) and Bamford and Ferrar (12) have shown that manganic trifluoroacetylacetonate, Mn(TFA)3 (TFA = CF3COCHCOCH3), reacts with polymethylmethacrylate and polystyryl radicals to retard the polymerizations of methyl methacrylate and styrene monomers, respectively. These authors presume that these reactions Yesult in reduction of the oxidation state of the metal '. In view of the large yields of excited states produced in the radiolysis of liquid toluene, the known similarities in the radiation physics (1) and chemistry (4) of toluene and benzene, the importance of pararnagneric and heavy atom que~lching of electronical!y excited states (13u) and the fact that at very low doses the paramagnetic radical scavenger FeC13 is observed to chemically quench excited benzene in the liquid state (14) and in low temperature crystalline bcnzene (15) 11 was alliicipdted that similar indirect evidence for the chemical quenching of +CH3* by Fe(HFA)3 might be obtained in this bark. Direct photochemical evidence for this quenching has also been sought. Lintvedt and Kernitsky (16) have assigned the 284 nm (log E = 4.4), 383 nm (log E = 3.6) and 446 nm (log t = 3.6) absorption bands of Fe(HFA)3 in CHC13 to intraligand (a* + a), metal-to-ligand (T + t2,) and ligand-to-metal (e, + a ) transitio~is, respectively.…”