The Electronic Structure of Copper Dichloride (CuCl2)

The transition elements in the periodic table are characterised by the presence of partially filled d-shells. Molecules that contain such a transition element have a complicated electronic structure because it takes very little energy to move an electron from one d-orbital to another. In consequence, the electronic properties of such molecules are not straightforward to study, either theoretically or experimentally.Crystalline copper dichloride, CuCl2.4H20, is a stable, yellow solid. However, on heating, the water of crystallisation is boiled off to leave the triatomic molecule CuCl2 which is a chemically unstable molecule in an open-shell state (a free radical ). Despite its small size, it possesses a complicated electronic structure. Some progress has been made in the last ten years on experimental measurements of its properties by spectroscopic methods. Theoretical calculations of its structure have not given reliable results so far. The electronic structure of CuCl2 is expected to be the simplest of the transition metal dihalides because there is effectively only one unpaired electron (d9 configuration). The spin of this electron has 2 possible orientations relative to the orbital angular momentum in the ground electronic state; as a result, there are two states of different energy, the so-called spin components. All experimental observations of CuCl2 so far have involved the molecule in the lower of these two states; consequently, the energy separation between them is not known. The objective of the present proposal is to measure this splitting. It is important to determine this quantity because:(i) the separation is of interest in its own right because it gives information on the electronic structure of CuCl2,(ii) knowledge of the separation would allow perturbations in higher vibrational levels to be analysed and(ii) an experimental measurement of the splitting gives a benchmark value to guide theoretical calculations of the electronic properties of CuCl2.