EPR evidence for the restricted mobility of NO_2 in gamma irradiated thorium nitrate pentahydrate Th(NO_3)_4· 5H_2O

摘要

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 Site2 was found to be rigid throughout the entire temperature range. The differences in the mobility of NO2 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.