Pulse radiolysis study of aqueous hydrogen cyanide and cyanide solutions

The pulse radiolysis of N₂O saturated solutions of HCN and CN^- has been studied in the pH region 3.7-14. Spectra of three different species were identified. In acid and neutral solutions OH adds to the HCN triple bond and forms the HC(OH)=N radical with λ_max 240 nm and ε₂₄₀ 550 M^-1 cm^-1. This radical disappears according to a second-order rate law with 2k = 1.5 × 10⁹ M^-1 sec^-1. At pH 10.6 OH adds to CN^- with a rate of k (OH + CN^-) = 7.1 × 10⁹ M^-1 sec^-1 to form HOĊ=N^-. This adduct protonates and rearranges to the formamide radical ĊONH₂ with λ_max <225 nm and ε₂₂₅ 1400 M^-1 cm^-1. The second-order rate constant for disappearance of the ĊONH₂ radical is 2k = 6.2 × 10⁹ M^-1 sec^-1. At pH 14, O^- reacts with CN^- with a rate constant of k (O^- + CN^-) = 2.6 × 10⁸ M^-1 sec^-1. The adduct rearranges in the same way as at lower pH to produce (ĊONH)^- with λ_max 245 nm and ε₂₄₅ 2200 M^-1 cm^-1. The (ĊONH)^- decays in a second-order rate 2k = 2.2 × 10⁹ M^-1 sec^-1. The formamide radical is in acid-base equilibrium with pK (ĊONH₂ ⇌ ĊONH^- + H⁺) = 12.25.