Crystal-water-induced switching of magnetically active orbitals in CuCl2

The dehydration of CuCl2 center dot 2H(2)O to CuCl2 leads to a dramatic change in magnetic behavior and ground state. Combining density-functional electronic structure and model calculations with thermodynamical measurements, we reveal the microscopic origin of this unexpected incident-a crystal-water-driven switching of the magnetically active orbitals. This switching results in a fundamental change in the coupling regime from a three-dimensional antiferromagnet to a quasi-one-dimensional behavior. CuCl2 can be well described as a frustrated J(1)-J(2) Heisenberg chain with ferromagnetic exchange J(1) and J(2)/J(1)similar to-1.5 for which a helical ground state is predicted.