ESR SPECTRA OF MATRIX ISOLATED ALKALI TETROXIDE MOLECULES

On warming a matrix of alkali superoxide molecules containing excess oxygen to $15-30^{\\circ}$, a new radical species appears. The ESR spectra of this species indicate the presence of a single alkali cation ($M^{+}$), ionically bound to an anion ($O^{-}_{n}$), where by comparison with previously reported $spectra,^{1,2,3}$ n $\\neq$ 1, 2 or 3. As this synthetic route is similar to that used in studies of the vibrational $spectral^{4,5}$ of the alkali tetroxide molecules ($MO_{4}$), the identification $n = 4$ is established. Symmetry considerations and the g-tensor data indicate that $O_{4}^{-}$ has a trans configuration. This observation is consistent with the predictions of the Wigner-Witmer symmetry rules. These show a correlation between the reactants $O_{2}$ ($^{3}\\Sigma^{-}_{g}$) and $O^-_{2}$ ($^{2}\\Pi_{g}$) and the expected $^{2}B_{g}$ ground state of a trans $O^{-}_{4}$ isomer. By contrast these reactants correlate with an excited state of a cis $O^{-}_{4}$ isomer. For $NaO_{4}$ comparison of the hyperfine data with that predicted by point spin calculations shows that the alkali cation lies above the $O^{-}_{4}$ plane. These same model calculations give agreement only if the anion contains one long oxygen-oxygen bond ($> 1.85$ \\AA). This is consistent with the model of Spratley and $Pimentel,^{6}$ in which the parent species ($O_{2}$ and $O^{-}_{2}$) are bound by a weak [$p - \\Pi^{*}$]$\\sigma$ bond.