Knowledge Bank

ESR spectra have been obtained for $LiO2$ trapped in krypton and nitrogen matrices, and are consistent with an ionic model, $Li+O2-$, of $C2V$ symmetry. Each spectrum has a triplet structure arising from the three principle values of the g-tensor. In Kr, the values are $gzz=2.0570,gxx=2.0095$ and $gyy=2.0024$. For the $N2$ matrix, $gzz=2.0667$ and $gxx=2.0080(gyy$ was not determined because of overlap with intense N atom transistions centered at $g=2.0014$). The $gzz$ transistion in $N2$ has a well resolved hyperfine quartet structure arising from $\mathrm{<mrow\; data-mjx-texclass="ORD">7</mrow>}Li(I=3/2)$, with $azz=2.33G$, and the $gyy$ transistion in Kr has $ayy=2.65G$. Hyperfine structure was not resolved for $gxx$ in either matrix. An apparent but poorly resolved quadrupole interaction is observed for $gzz(N2)$ and $gyy(Kr)$. The alkali cation lifts the degeneracy of the $O2\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ HOMO and splits it into an in plane orbital, $\Pi X\ast (b1)$, and an out of plane orbital, $\Pi y\ast (a2)$. The splitting parameter between the two orbital, $|\delta |$, decreases down the series $NaO2,KO2,RbO2$, then increases for $CsO2(1)$. $|\delta |$ is inversely proportional to $△gzz$, the shift from the free electron g value. The $gzz$ values for $LiO2$ support the ``inversion model'' for $CsO2$ where the ground state configuration changes from $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A2$ to $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$.