Authors:C. Robert Dennis, I. Potgieter, and S. Basson
The kinetics of the oxidation of hydroxide ions by octacyanomolybdate(V) ions has been studied in the [OH−] range between 0.5 and 4.0 mol dm−3. The reaction displays first-order kinetics in both [Mo(CN)83−] and [OH−] and is consistent with the rate law R = k [Mo(CN)83−] [OH−]. From the Brønsted-Bjerrum equation, it was established that the activated complex is formed by the ions Mo(CN)83− and OH−. A reaction mechanism is proposed for the reaction with a rate constant of 1.13 ± 0.03 × 10−2 dm3 mol−1 s−1, activation enthalpy, ΔH#, of 20.1 ± 09 kJ mol−1 and activation entropy, ΔS*, of −213 ± 3 J K−1 mol−1.
Authors:C. Robert Dennis, Jannie C. Swarts, and Dale W. Margerum
Protonation of copper(II)- and nickel(II) tetrapeptide complexes with bulky α-carbon substituents has been studied. The pKa-values for the second and terminal metal–N(peptide) bond formation have been determined spectrophotometrically. More than one deprotonated peptide nitrogen exists in a complex but the individual protonation steps for the different CuII–N(tetrapeptide) positions could not be accounted for by the variation of the hydrogen ion concentration in the same experiment, as the protonation of the different metal–N(tetrapeptide) positions takes place at different wavelengths. For the NiII-tetra-alanine complex, the proton transfer to the terminal and second deprotonated peptide nitrogens have been detected by varying the hydrogen ion at the same wavelength. The proton transfer to the terminal metal–N(tetrapeptide) of CuII- and NiII-tetrapeptide complexes show first order kinetics with respect to the hydrogen ion and the peptide complex concentration and proceed via an outside protonation pathway. The relatively high pKa values for the terminal deprotonated peptide nitrogen indicate the instability of this metal–N(peptide) bond due to strain in the chelate ring because of atom overcrowding of the bulky α-carbon substituents.
Authors:C. Robert Dennis, I. M. Potgieter, and S. S. Basson
The kinetics of the oxidation of formaldehyde by octacyanomolybdate(V) ions has been studied in aqueous alkaline medium. A formaldehyde-dependent and -independent reaction has been observed. The HCHO-dependent reaction displays first order kinetics in [Mo(CN)83−], [HCHO], [OH−] and alkali metal ion catalysis was observed. The rate law is consistent with: Activation parameters for both the formaldehyde-dependent and -independent reaction have been obtained. The activation parameters for the HCHO-independent reaction provide evidence for the identification of the reaction. A reaction mechanism is proposed.