Temperature independent contribution to the paramagnetic susceptibility of copper(II) acetate

The paramagnetic susceptibility data for copper(II) acetate monohydrate are reanalysed to show the accuracy with which molecular parameters may be obtained and the possibility of discriminating between alternative theoretical models describing the anomalous behaviour. If the usual assumptions of a singlet ground state and excited triplet only are made it is found that the singlet-triplet separation J = 293 ± 4 cm^-1 and the temperature independent contribution to the susceptibility Na = (1.41 ± 0.31) × 10^-9 m³ mol^-1. The hypothesis that there is also a low lying, excited singlet state does not have a statistically significant effect on the agreement between the experimental results and theory unless a much higher value of Na is assigned a priori. A theoretical calculation of Na is attempted yielding Na ˜ 1.09 × 10 ^-9 m³ mol^-1. This result argues against the suggestion of additional low lying states, and also raises the possibility that J may be temperature dependent.