ESR SPECTRA OF MATRIX ISOLATED ALKALI TETROXIDE MOLECULES

$^{1}$ D. M, Lindsay, D. R. Herschbach, and A. L, Kwiram, J, Chem. Phys. 60, 315 (1974). $^{2}$ D. H. Lindsay, D, R. Herschbach, and A. L. Kwiram, Chem. Phys, Lett. 25, 175 (1974), $^{3}$ F. J. Adrian, V, A, Bowers, and E, L, Cochran, J. Chem. Phys. 61, 5463 (1975). $^{4}$ R. R, Smardzewski and L. Andrews, J. Chem. Phys. 57, 1327 (1972), $^{5}$ M. E. Jacox and D, E, Milligan, Chem. Phys. Lett. 14, 518 (1972). $^{6}$ R. D. Spratley and G. C. Pimentel, J. Am. Chem. Soc. 88, 2394 (1966).""Author Institution: Department of Chemistry, Harvard University; Department of Chemistry, University of WashingtonOn 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