Pressure Dependence of the Crystal Structures and EPR Spectra of Potassium Hexaaquacopper(II) Sulfate and Deuterated Ammonium Hexaaquacopper(II) Sulfate

Abstract

The crystal structures of (ND4)2[Cu(D2O)6](SO4)2 at 295 K, determined by X-ray diffraction at pressures of 1 bar, ∼1.5 kbar and ∼3.0 kbar, are reported. Between 1 bar and 1.5 kbar, the crystal structure changes to one almost identical to that of the corresponding hydrogenous compound at 1 bar. The structural change involves a 90° switch in the direction of the long bonds of the Cu(D2O)6 2+ ion, accompanied by a change in the hydrogen-bonding interactions of the ammonium cations. Comparison of these two structures with those at ∼15 K shows that at room temperature, for both the high (H) and low (L) pressure phases, the Cu complexes are in thermal equilibrium with the other structural isomer. Though one of these is energetically preferred in the H and the other in the L phase, the variation of the powder EPR spectrum with pressure indicates a continuous transition between both within ∼1 kbar and suggests that at ∼0.5 kbar the two orientations of the copper complex are similar in energy. However, the underlying thermally induced equilibrium is little affected by pressure, and this is also the case for the Cu(H2O)6 2+ ion in Cu2+-doped K2[Zn(H2O)6](SO4)2. The crystal structure of K2[Cu(H2O)6](SO4)2 at 15 K and 1.4 kbar, determined by time-of-flight neutron diffraction, is similar to that at 295 K and 1 bar, except for indications that here also a thermal population of the higher energy form in which the directions of the long and intermediate Cu−O bonds interchange occurs at 295 K. The powder EPR spectrum of the potassium salt at 295 K shows little change between 1 bar and 11 kbar.

Bonding effects and the crystal structures of (NH4)2[Cu(H2O)6](SO4)2 and its H2 18O substituted form at 9.5 K

“…18 O implies that the B 3 A structural change which occurs on deuteration is not caused by the increase in ligand mass. It was suggested that such an increase should cause the Jahn±Teller radius of the Cu(H 2 O) 6 2+ ion to expand slightly and that this might underlie the form change (Rauw et al, 1996). This proposal is invalid, however, because although the coordinates used to describe the potential energy surface in dimensionless units (as presented by Rauw et al, 1996) are dependent on mass, when these are transformed to dimensioned units they become independent of mass.…”

“…It was suggested that such an increase should cause the Jahn±Teller radius of the Cu(H 2 O) 6 2+ ion to expand slightly and that this might underlie the form change (Rauw et al, 1996). This proposal is invalid, however, because although the coordinates used to describe the potential energy surface in dimensionless units (as presented by Rauw et al, 1996) are dependent on mass, when these are transformed to dimensioned units they become independent of mass. While the isotope mass in¯uences the kinetic energy of the system, it has only a very small effect upon its potential energy and the effect on bond lengths is expected to be many orders of magnitude less than that considered here (Van Vleck, 1936;Cencek et al, 1998).…”

“…An alternative suggestion is that deuteration affects the structure through its effect on the mass of the H 2 O ligand, because the Jahn±Teller distortion depends on coupling with a vibration. The ligand mass might in¯uence the Jahn±Teller coupling constant and thus the geometry of a Cu 2+ complex, and this could underlie the structural difference (Rauw et al, 1996).…”

“…This is a trend which is independent of pressure or whether the metal ion is Cu 2+ or Cr 2+ and includes the copper(II) Tutton salts with alkali metals as the counter ions, where the CuÐO bonds have the arrangement of form A. There, neutron diffraction studies have shown that & is 0.39 (2) A Ê in the potassium salt at 14 K and 140 MPa (1.4 kbar; Rauw et al, 1996) and 0.379 (1) A Ê in the rubidium compound at 77 K and ambient pressure (Smith et al, 1975). It thus appears that the lattice interactions in form B favour not only a different O atom for the long bond of the Jahn±Teller distortion, but also a smaller overall distortion from a regular octahedral geometry compared with form A.…”

EPR of two Cu2+ complexes showing dynamic Jahn-Teller effects

“…Agreement with experiment is satisfactory showing that the complex present in the doped zinc(II) compound and the pure copper(II) compound have similar potential surfaces. This contrasts with the Tutton salts, where the temperature dependence of the EPR spectra of the doped systems commences at significantly lower temperatures than for the corresponding pure copper(II) compounds [6,10,17]. The energy separation between the lowest vibronic wavefunctions, -81 cm -', obtained with the RHW method is quite similar to that obtained by Simmons et al [13] with the SG approach, -77 cm -', confirming that this model is a good approximation for the compound.…”