The radiation-induced formation of excited states of aromatic hydrocarbons in cyclohexane IV. Effect of electron scavengers

Abstract

The effects of the electron scavengers nitrous oxide, nitromethane, carbon dioxide, sulphur hexafluoride and chloracetic acid on the yields of naphthalene excited states observed in pulse radiolysis of deaerated naphthalene-cyclohexane solutions have been investigated. The excited-state yields were reduced by all the solutes examined although they showed marked differences in their effects on the formation of singlet and triplet excited naphthalene. It is proposed that in the absence of a second solute naphthalene excited states are produced as a result of electron and positive charge scavenging by naphthalene followed by the ion-recombination reaction Naph + + Naph - -> b s (or b ' s ) 1 Naph* + b t (or b ' s ) 3 Naph*, where b s and b t are the number of singlet and triplet excited states of naphthalene formed initially per geminate ion-pair recombination and b ' s and b t are the corresponding efficiencies per free ion-pair recombination. The concentration dependences of the singlet yield G( 1 Naph)*, the triplet yield formed directly on ion recombination, G( 3 Naph*) r , and the total triplet yield, G( 3 Naph*) = G( 3 Naph*) r + ^ t G( 1 Naph*) reported in parts II and III of this series can be fitted to the expressions G( 1 Naph*) = b ' s G gi {α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}, G( 3 Naph*) = B ' t G fi + B t G gi >{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}, using the following parameters: G fi = 0.15, G gi = 3.9, a_ = 3.291/mol, a + = 50001/mol, b 8 = 0.863, b ' s = 1.22, B t = b t + (fi t b s = 1.34 and B ' t = b ' t + (j) t b ' s = 1.89. Of the various competing electron scavengers investigated only the results of the nitromethane system at [CH 3 NO 2 ] > 10 -2 mol/l could be explained solely in terms of a simple competition for free and geminate ions by the two solutes. However, it was found that the nitrous oxide and carbon dioxide systems could be rationalized on a similar model by taking account of possible secondary ion reactions and/or excited state formation as a result of ion recombination between Naph + and the negative ion of the competing scavenger. The secondary process occurring in the sulphur hexafluoride and chloracetic systems are more complex and an empirical fit of the data was not possible.