ESR Spectrum of HO2 in Solutions of Hydrogen Peroxide in Water at 77°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: $g_1\text{\hspace{0.17em}=\hspace{0.17em}2.0353}$ , $g_2\text{\hspace{0.17em}=\hspace{0.17em}2.0086}$ , and $g_3\text{\hspace{0.17em}=\hspace{0.17em}2.0042}$ ; ${\mathrm{|A}}_1\text{|\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}39.2}$ , ${\mathrm{|A}}_2\text{|\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}10.0}$ , and ${\mathrm{|A}}_3\text{|\hspace{0.17em}/\hspace{0.17em}h\hspace{0.17em}=\hspace{0.17em}43.9}$ 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 HO₂ 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 H₂O₂–H₂O matrix. This is attributed to hydrogen bonding in the H₂O₂–H₂O matrix.