O-Atom Scrambling in the Aqueous Isomerization of Pernitrous Acid

Raman spectroscopy has been used to determine the isotopic distribution of oxygen during the isomerization of pernitrous acid to nitrate in¹⁸OH₂. Decomposition of unlabeled pernitrous acid in carbon-dioxide-free phosphate-buffered solutions of¹⁸OH₂at pH 6.8 results in the incorporation of 13 ± 1%¹⁸O into the nitrate product with no detectable double-isotopic incorporation and 83% of the product having complete retention of its oxygen atoms. The mechanistic implications of this are contrasted for three frequently considered pathways for peroxide bond cleavage and it is concluded that the results are most consistent with the formation of reactive intermediate which can either rearrange or undergo hydrolytic O-atom exchange with water.