The reactivity of biogenic monoterpenes towards OH. and SO4-. radicals in de-oxygenated acidic solution

The reactivity of some selected biogenic monoterpenecompounds towards important aqueous phase free-radicaloxidants, namely OH· and SO^sub 4^^sup -^·, have beeninvestigated using the complementary experimentaltechniques of pulse radiolysis and laser flashphotolysis (λ = 248 nm). Rate constants forthe reactions of the OH· radical with cis-verbenol andmethacrolein have been determined to be (6.8 ± 0.5) ×10^sup 9^ dm^sup 3^ mol^sup -1^ s^sup -1^ and (8.0± 0.7) × 10^sup 9^ dm^sup 3^ mol^sup -1^s^sup -1^,respectively (T = 20 °C, pH 4.0, Ionic strength 0 mol dm^sup -3^). Rate constants and activationenergies for the reactions of the SO^sub 4^^sup -^·radical have been measured for the following compounds(T = 20 °C, pH 4.0, Ionic strength = 0.03 moldm^sup -3^): α-pinene (k = (3.1 ± 0.1) ×10^sup 9^ dm^sup 3^ mol^sup -1^ s^sup -1^;E^sub act.^ =(8.9 ± 1.3) kJ mol^sup -1^), α-terpineol(k = (4.1 ± 0.1) × 10^sup 9^ dm^sup 3^mol^sup -1^s^sup -1^; E^sub act.^ = (13.4 ± 0.6) kJmol^sup -1^), cis-verbenol (k = (3.2 ± 0.2) ×10^sup 9^ dm^sup 3^ mol^sup -1^ s^sup -1^;E^sub act.^ =(10.0 ± 0.7) kJ mol^sup -1^), verbenone (k = (1.6± 0.1) × 10^sup 9^ dm^sup 3^ mol^sup -1^s^sup -1^;E^sub act.^ = (6.1 ± 0.7) kJ mol^sup -1^), myrtenal(k = (1.85 ± 0.1) × 10^sup 9^ dm^sup 3^mol^sup -1^s^sup -1^; E^sub act.^ = (7.5 ± 0.7) kJmol^sup -1^)and methacrolein (k = (1.18 ± 0.1) × 10^sup 9^dm^sup 3^ mol^sup -1^ s^sup -1^). In most instances theabsorption spectra of the intermediate products formedby these reactions have been measured which, inconjunction with strategic conductiometric studies,have been used to suggest plausible mechanisms for theoxidation in acidic de-oxygenated solution.[PUBLICATION ABSTRACT]