Probing Axial Water Bound to Copper in Tutton Salt Using Single Crystal 17O-ESEEM Spectroscopy

Electron spin echo envelope modulation (ESEEM) signals attributed to axial water bound to Cu2+ have been detected and analyzed in Cu(II)-doped O-17-water enriched potassium zinc sulfate hexahydrate (Tutton salt) crystals. The magnetic field orientation dependences of low frequency modulations were measured to fit hyperfine and quadrupole coupling tensors of a O-17 (I = 5/2) nucleus. The hyperfine tensor (A, A(yy), A(zz): 0.13, 0.23, -3.81 MHz) exhibits almost axial symmetry with the largest value directed normal to the metal equatorial plane in the host structure. Comparisons with quantum chemical calculations position this nucleus about 2.3 A from the copper. The isotropic coupling (-1.15 MHz) is small and reflects the weak axial water interaction with a d(x2-y2) unshared orbital of copper. The O-17-water quadrupole interaction parameters (e(2)qQ/h = 6.4 MHz and eta = 0.93) are close to the average of those found in a variety of solid hydrates. In addition, the coupling tensor directions correlate very closely with the O8 water geometry, with the maximum quadrupole direction 3 from the water plane normal, and its minimum coupling about 2 from the H-H direction. In almost all previous magnetic resonance O-17-water studies, the quadrupole tensor orientation was based on theoretical considerations. This work represents one of the few experimental confirmations of its principal axis frame. When Cu2+ dopes into the Tutton salt, a Jahn Teller distortion interchanges the relative long and intermediate metal O7 and O8 bond lengths of the zinc host. Therefore, only those unit cells containing the impurity conform to the pure copper Tutton structure. This study provides further support for this model. Moreover, coupling interactions from distant (H2O)-O-17 such as in the present case have important implications in studies of copper enzymes and proteins where substrates have been proposed to displace weakly bound water in the active site.