Temperature dependence of the hyperfine structure splittings of XO 3 molecule‐ions in single‐crystal calcite

The hyperfine structure splittings of three paramagnetic molecule‐ions formed in γ‐ir‐radiated single‐crystal calcite have been measured from 4.2 to 320 K. These molecule‐ions are CO   3 3− , PO   3 2− , and AsO   3 2−Over this temperature range, they exhibit a decrease in hyperfine structure splitting with increasing temperature of between 1 to 2%. An analysis of the data suggests that the loss of unpaired electron spin‐density at the central atom is approximately proportional to the fractional increase in spin‐density at the oxygen‐orbitals. Furthermore, the fractional change in p‐character is about three times the fractional change in s‐character for these species. In all cases, calculated equilibrium bond‐angles are found to be insensitive to temperature. Estimates of the unpaired electron molecular orbitals are given using the hybridization ratios determined from the data.