Discovery and ESR spectra of matrix stabilized hydronium radicals H 3 O and D 3 O

The photolysis with ultra violet light from 2800 to 3500 Å of aqueous ceric perchlorate‐perchloric acid solutions frozen to —180° in the cavity of an ESR apparatus produces the doublet spectrum of H 2 O + . On annealling the glassy matrix to —120° and recooling to —180°, the H 2 O + spectrum disappears and is replaced by a 1:3:3:1 quartet characteristic of a species having axial symmetry and three equivalent hydrogens. The quartet is assigned to the neutral H 3 O species rather than (H 3 O) 2+ because it is a chemical reducing agent, stable toward isotopic exchange and basic media, and has a proton splitting factor, a H, which is too low for (H 3 O) 2+ when compared to other known isoelectronic species. The similarity in ESR and isotopic exchange properties of H 3 O and methyl radical raises the question whether H 3 O is planar or tetrahedral or partially complexed to some other species such as Ce(III). In heavy water solvent H 3 O, D 3 O, H and Datom spectra are all obtained together upon direct irradiation at —180° without annealling. The atoms probably arise from reactions of the type, H 3 O ⇌ H + H 2 O. It appears that H 3 O may be stable with respect to H + H 2 O and we estimate the bond dissociation energy of H 3 O < 7 kcal/mole. H 3 O and D 3 O have the same ḡ = 2.0035 ± 0.0003 but a H(H 3 O) = 22.84 G and αD(D 3 O) = 3.65 G.