The copper sulfate hydration cycle. Crystal structures of CuSO4 (Chalcocyanite), CuSO4·H2O (Poitevinite), CuSO4·3H2O (Bonattite) and CuSO4·5H2O (Chalcanthite) at low temperature using non-spherical atomic scattering factors

Abstract

New structure determinations of CuSO₄, CuSO₄·H₂O, CuSO₄·3H₂O and CuSO₄·5H₂O are reported from X-ray diffraction experiments at 100 (1) K. Combined density functional theory (ORCA) and non-spherical atomic scattering factor (NoSpherA2) computations enabled a Hirshfeld Atom Refinement (HAR) using custom atom scattering factors, accurately polarized and charge-compensated atom electron densities and accurate placement and shapes of the water hydrogen atoms. The results clearly demonstrate that all H atoms are involved in conventional H-bonding in the structures of the hydrates, contrary to an early X-ray diffraction determination on CuSO₄·3H₂O. Comparison to neutron diffraction results demonstrates the efficacy of this new HAR method. Colour variation in copper sulfates is interpretable by considering the relative number of sulfato and aqua ligands, given the lower crystal field splitting of the former relative to the latter.