ESR SPECTRUM OF HO2 IN SOLUTIONS OF HYDROGEN PEROXIDE IN WATER AT 77 DEGREES K

The electron spin resonance (ESR) spectrum of the perhydroxyl radical (HO2) has been observed in a hydrogen peroxide-water matrix at 77°K. The radical was produced by either ultraviolet or ionizing radiation, and the initial poor resolution of the spectrum was greatly improved by careful annealing of glassy samples. Accurate measurements of g factors and hyperfine splittings were made at both 9 and 6 GHz. The following parameters, obtained by computer analysis, were found to give the best fit to the spectrum at both frequencies: g1=2.0353, g2=2.0086, and g 3=2.0042; |A1|/h=39.2, |A2|/h=10.0, and |A 3|/h=43.9 MHz. Experiment and theory both show that axis 1 lies along the O-O bond. Theoretical considerations also suggest that axis 2 lies in the molecular plane, that axis 3 is perpendicular to the molecular plane, and that the values of the hyperfine splitting constants are negative. The ESR spectrum of DO2 was observed in a deuterated sample, and computer analysis gave parameters in reasonable agreement with those for HO2, allowing for the difference in nuclear magnetic moment. The parameters for HO2 in H2O2-H2O are similar to those for HO 2 in argon, the small differences being attributed to matrix effects. However, the rotation of the radical about its O-O bond, which was observed for HO2 in argon, does not occur in the H2O 2-H2O matrix. This is attributed to hydrogen bonding in the H2O2-H2O matrix.