Pulse radiolysis of aqueous cyanide solutions. Kinetics of the transient OH and H adducts and subsequent rearrangements

OH radical reactions with aqueous cyanide were studied by pulse radiolysis over the pH range 1.9-15. The reaction is characterized by three pH ranges: In acid solution (up to pK (HCN/CN-) = 9.3), OH adds to HCN with k = (6 +- 2) x 107 M-1 s-1, forming HO--CH=N. This radical disappears by second-order kinetics with 2k = (2.8 +- 0.5) x 109 M-1 s-1. In alkaline solutions (pH 9.3 to pK (OH/O-) = 11.9) the OH radical adds to CN- with k = (7.1 +- 0.5) x 109 M-1 s-1. Subsequently a fast protonation equilibrium occurs HO--C=N- + H+ reversible HO--C=NH (pK = 10.2 +- 0.5), the product decaying by an H-atom transfer to the radical .CONH2 with k = (4.0 +- 1.0) x 107 s-1. This latter radical disappears with 2k = (6.2 +- 1.0) x 109 M-1 s-1. In very alkaline solutions (pH > pK (OH/O-) = 11.9) the O- addition to CN- (k = 2.6 x 108 M-1 s-2) and the consecutive protonation and rearrangement to form .CONH- are identical with the results published by Behar. Computer simulation techniques were used to determine the resolved spectra, extinction coefficients, and rate constants from the experimentally observed absorptions. The amount of contribution from remaining H-atom reactions has been analyzed separately. The H addition to HCN (k = 3.6 x 107 M-1 s-1) leads to the CH2=N. radical showing an absorption band with lambda/sub max/ 275 nm. This radical disappears by a second-order process with 2k = (2.6 +- 0.8) x 109 M-1 s-1. The results for the OH reactions have been corrected for the contribution from the small amount of H-atom reactions throughout the mechanism