The reaction of the malonic acid derived radical with oxygen. A reinvestigation by pulse radiolysis

Abstract

The dicarboxymethyl radicals (derived from malonic acid) and their peroxyl radicals play an important role in the Belousov–Zhabotinsky reaction, and detailed knowledge of the rate constants are required for modelling this oscillating reaction. Dicarboxymethyl radicals were produced pulse radiolytically in aqueous solutions. They are characterized by absorption maxima at 310 nm at pH 0 (fully protonated, ε = 1210 dm³ mol⁻¹ cm⁻¹) and at 340 nm at pH 8 (dianion, ε = 1070 dm³ mol⁻¹ cm⁻¹). The radicals decay bimolecularly with rate constants of 4 × 10⁸ dm³ mol⁻¹ s⁻¹ at pH 0 and 1.4 × 10⁷ dm³ mol⁻¹ s⁻¹ at pH 8. In the presence of O₂, the dicarboxymethyl radicals add O₂, and their absorption decays with rate constants of 1.6 × 10⁹ dm³ mol⁻¹ s⁻¹ at pH 0 and 1.3 × 10⁹ dm³ mol⁻¹ s⁻¹ at pH 8. Whereas the second pK_a value of the radical is identical to that of the parent compound (5.7), the value of the corresponding peroxyl radical is as low as 3.