UNEXPECTED DIHYDROXYLATION OF METHACRYLAMIDE PHOTOINDUCED BY CHROMIUM-(VI) IN AQUEOUS-SOLUTION

The excitation at 365 nm of chromium (VI) in aqueous solution in the p resence of methacrylamide (MAM) leads to the formation of only one pho toproduct, The quantum yield of the reduction of chromium (Vt) to chro mium (III) is very high in acidic media (Phi = 0.2 at pH = 2-4) and de creases to 2 x 10(-3) at pH > 7. The titration profile of the curve gi ving Phi as a function of pH is assigned to the protolytic equilibrium between HCrO4- and CrO42-. Phi is also directly proportional to the M AM concentration, reflecting a quenching reaction between chromium (VI ) species in the excited state and MAM. The narrow signal of chromium (V) at g = 1.9788 observed by ESR spectroscopy is due to the thermal r eaction between chromium (VI) and radicals arising from MAM. This obse rvation accounts for the total inhibition of methacrylamide polymeriza tion by chromium (VI) when the solution is allowed to stand in the dar k. The photoreaction is assumed to go through the formation of a cycli c chromium (IV) ester by addition of the excited HCrO4- anion on the double bond. The decomposition of this intermediate yields the dihydro xy derivative 2,3-dihydroxy-2-methylpropionamide, not yet described.