EFFECT OF CHARGE SCAVENGERS ON RADIATION-INDUCED ISOMERIZATION OF STILBENE IN CYCLOHEXANE

Effects of electron scavengers (benzyl acetate, C6H5NO2, N2O, CH3Cl, and CO2) and cation scavengers (NH3 and piperidine) on radiation-induced isomerization of trans-stilbene in cyclohexane were studied; all but CO2 suppress Gt→c. Effects of the electron scavengers are discussed in relation to empirical equations for quantitative description of the nonhomogeneous kinetics of charge scavenging in γ-irradiated liquid alkanes. Results are consistent with a model in which isomerization is initiated by electron capture by stilbene with subsequent isomerization via stilbene excited states formed in neutralization of stilbene anions by cyclohexane and stilbene cations. The observed absence of any effect of CO2 on Gt→c is attributed to stilbene excitation in reaction of CO2- with a stilbene cation. Study of the radiation-induced isomerization of cis-stilbene is affected by the extreme sensitivity of cis-stilbene isomerization to sample-preparation procedures. Though very large values previously reported for Gc→t in benzene could not be reproduced, present results in both benzene and cyclohexane confirm an appreciable catalytic contribution; in cyclohexane, Gc→t = 7.4 for 0.2 M cis-stilbene, and Gc→t/Gt→c exceeds the corresponding quantum-yield ratio by more than a factor of 4 for 0.1 M stilbene. Participation of ionic processes in the catalytic contribution is indicated by suppression of Gc→t by various charge scavengers. A pronounced catalytic effect is observed with C2H5Br and SF6 (probably via Br and an SFx radical from neutralization of the anions formed by electron capture); e.g., Gc→t ≈ 5500 for 0.01 M C2H5Br and 0.2 M cis-stilbene in cyclohexane.