Electron spin resonance for trapped hydrogen atoms in aqueous media. Comments and calculations on the radicals H3O2+ and H3O

Abstract

Species with e.s.r. spectra similar to those for gas-phase hydrogen atoms are readily obtained by photolysis or radiolysis of aqueous solutions at 77 K. It is suggested that these species are better described as highly distorted H₃O radicals, H₂O—H, with the unpaired electron in a σ* orbital. This distortion may be intrinsic or forced on the system by the fact that the water molecule moiety is strongly hydrogen bonded to its neighbours.

Ab initio Hartree-Fock calculations suggest that if symmetrical H₃O were formed, the proton hyperfine coupling would be ca. 210 G, which means that a 23 G quartet obtained by others in photolysed aqueous solutions of cerium(IV) perchlorate was not due to H₃O. Calculations also showed that H₃O²⁺ could have been responsible for this quartet, but this is chemically improbable. However, we were unable to reproduce the conditions required for the formation of this 23 G quartet, and conclude, with others, that it was probably due to methyl radicals.

The other major product detected by e.s.r. in addition to “hydrogen atoms” in these photolysed solutions was the hydroxyl radical, although in concentrated acid a triplet with A(¹H)= 24 G was obtained which is tentatively assigned to H₂O⁺ cations.