Kinetic and thermodynamic character of reducing species produced on pulse radiolysis of acetonitrile

Abstract

During nanosecond pulse radiolysis of liquid acetonitrile a transient optical absorption was observed in the spectral range 500 to 1500 nm. The extreme reactivity of this species towards solutes capable of accepting electrons allowed its assignment as a reducing entity. In the purest liquid prepared this species had a half-life of some 900 ns. Variable temperature studies showed that this reducing species, having an intense maximum near 1450 nm at + 62°C, was drastically reduced in absorption intensity at –40°C, where the decay character changed and a weak maximum was observed near 550 nm. It was concluded that the two species are different chemical forms of a reducing entity joined by an equilibrium. Tentative identification of the reversible change according to (CH3CN)–+ CH3CN ⇌(CH3CN)–2 with the monomeric anion as the infrared absorbing entity was based on thermodynamic considerations and the effects of adding polar liquids. Quantitative examination of the data allowed a value of –34.9 kJ mol–1 to be extracted for the binding energy of the anion dimer. Measurements of the yields of pyrene and trans-stilbene radical anion formed after pulse radiolysis of appropriate solutions led to the evaluation of G(reducing entity)= 1.03. In addition it was found that O– radicals, formed by reaction between (CH3CN)– and N2O, reacted with acetonitrile via both addition and abstraction routes. Further, in solutions containing tetracyanobenzene (TCNB) and O2, capture of the negative entity by O2 was followed by electron transfer from O–2 to TCNB.