EPR evidence for the restricted mobility of NO 2 in gamma irradiated thorium nitrate pentahydrate Th(NO 3) 4·5H 2O

Abstract

Electron paramagnetic resonance (EPR) studies were conducted on gamma irradiated polycrystalline sample of thorium nitrate pentahydrate, Th(NO 3) 4·5H 2O, in the temperature range of 100–300 K. The most prominent species with triplet hyperfine structure in the EPR spectrum was identified as NO 2. The EPR spectrum gave evidence for the stabilization of NO 2 in at least three different sites slightly differing in spin Hamiltonian parameters (Site 1: g x = 2.0042, g y = 1.9911, g z = 2.0020, A x = 54.20 G, A y = 48.50 G and A z = 65.25 G; Site 2: g x = 2.0042, g y = 1.9911, g z = 2.0020, A x = 54.20 G, A y = 48.50 G and A z = 67.85 G; Site 3: g x = 2.0045, g y = 1.9911, g z = 2.0015, A x = 54.20 G, A y = 49.05 G and A z = 72.45 G). The EPR spectra for Site 1 revealed molecular dynamics of NO 2 from a slow motion region to fast motion region as the sample temperature was varied from 100 to 300 K. This led to a change in EPR spectrum from orthorhombic to axial, indicating preferred rotation of NO 2 molecule about the O–O bond direction. However, the NO 2 molecule at Site 2 was found to be rigid throughout the entire temperature range. The differences in the mobility of NO 2 molecules occupying the two sites could be attributed to the fact that in one case NO 2 was bonded to thorium or water and in the other case it was weakly bound. The NO 2 bound to thorium through two oxygen atoms or bound to thorium on one side through one oxygen atom and hydrogen bonded to water on the other side remains rigid throughout the entire temperature range, while NO 2 situated at interstitial sites or adsorbed on the surface exhibits mobility with increase in temperature above 100 K.