DYNAMICS IN THE HYDROGEN-CYANIDE WATER-SYSTEM - A C-13 NMR-STUDY

Rate constants for proton exchange in aqueous solutions containing CN- and HCN were determined by the C-13 NMR line-broadening technique at two ionic strengths at 25-degrees-C. The rate law is d[HCN]/dt = k0[HCN)] + k1[HCN][OH-] + k2[HCN][CN-]. The rate constants are k0 = 38.7 (32.9) s-1, k1 = 3.5 x 10(8), (3.3 x 10(8), k2 = 3.1 x 10(6) (2.0 x 10(6)) M-1 s-1, in 1 M NaClO4 (in 3 M LiClO4). In the presence of phosphate buffer, three additional terms have to be added to the rate equation, k(p0)[HCN][PO(4)3-] + k(p1)[HCN][HPO(4)2-] + k(P2)[HCN][H2PO4-], with k(p0) 1.3 x 10(9), k(p1) = 3.7 x 10(5) and k(p2) = 1.7 x 10(3) M-1 s-1 in 1 MNaClO4. The activation parameters for the reaction HCN + H2O reversible K-0 k-0 CN- + H3O+ have been determined: DELTA-H double-ended-dagger = 45 +/- 4 kJ mol-1 and DELTA-S double-ended-dagger = -64 +/- 12 J mol-1 K-1.