Structural and EPR study of the dependence on deuteration of the Jahn-Teller distortion in ammonium hexaaquacopper(II) sulfate, (NH4)(2)[Cu(H2O)(6)](SO4)(2)

The variation of the EPR spectra with degree of deuteration of the partiall y deuterated Tutton salt ammonium hexaaquacopper(II) sulfate, (NH4)(2)[Cu(H 2O)(6)](SO4)(2), has been measured at 293 K. The measurements indicate that the structure changes quite abruptly from that of the purr: hydrogenous sa lt to that of the fully deuterated salt at similar to 50% deuteration. The structure of a crystal in which similar to 42% of the hydrogen atoms were r eplaced by deuterium was elucidated at 15 K by single-crystal time-of-fligh t neutron diffraction. The hexaaquacopper(II) complex exhibits an orthorhom bically distorted, tetragonally elongated octahedral coordination geometry (Cu-O bond distances of 2.281(1), 2.007(1), and 1.975(1) Angstrom). The str ucture is very similar to that reported for the undeuterated salt at 9.6 K, and markedly different from that of the fully deuterated compound at 15 K, which has similar Cu-O bond lengths but with the directions of the long an d intermediate bonds interchanged. There is no evidence for disorder or par tial switching of the Cu-O bond directions. This is consistent with the tem perature dependence of the EPR spectrum of the similar to 42% deuterated co mpound, which indicates a thermal equilibrium between the two structural fo rms close to room temperature similar to that reported for the undeuterated compound, but complete reversion to the low-temperature phase on cooling t o 5 K. The possible influence of deuteration upon the hydrogen-bonding dist ances and the bearing of this upon the structural modifications of the comp ound are discussed.