Abstract:
The products formed from HOCH2O2(RO2) radicals during (a) radiolysis of oxygen saturated aqueous solutions of methanol by γ-rays and 1.8 MeV electrons, and (b) photolysis of similar solutions containing H2O2 are HCOOH, HCHO, CH2OHO2H and H2O2. Some of the HCHO is formed initially in the keto form and is subsequently hydrated to CH2(OH)2. The major organic product at pH 2 is HCOOH. At low radiation intensities G(HCOOH)/G(HCHO) and ϕ(HCOOH)/ϕ(HCHO) decrease to zero with increasing pH in the range 3 to 6.5; at high dose rates this range extends from pH 3 to pH 8.5.
These observations and those of an earlier pulse radiolysis study by Stockhausen et al.1 are interpreted quantitatively in terms of reactions of the species HOCH2O4CH2OH(A), HOCH2O4CH2O–(B–), and OCH2O4CH2O=(C=) formed by bimolecular reactions of RO2. At low dose rates HCOOH is formed by unimolecular decomposition of A; at high dose rates it is formed also by bimolecular reactions of B– with B– and its yield is controlled by both the equilibria A ⇌ B–+ H+(pK ∼ 4.6) and B–⇌ C=+ H+(pK ∼ 7.1) or HOCH2O2⇌ O2CH2O–+ H+(pK ∼ 6.8). HCHO results partially from unimolecular decomposition of RO2 into HCHO + HO2 with k≃ 9.9 s–1, and also from reactions of C=, B– and A. Reactions of HOC2H4O2 are similar.
An unusual reaction occurs at high dose rates in the pH region where B– and C= co-exist and causes the product yield to exceed G(RO2); its nature and mechanism are discussed.
