Hyperfine interaction of the CuCl4(NH3)22- centre: dependence on metal-ligand distance

Abstract

EPR measurements on NH4Cl1-xBrx and CsCl crystals containing the CuCl4(NH3)22- centre together with MS-X alpha calculations as a function of the Cu2+-N(Rax) and Cu2+-Cl-(Req) distances have been carried out. The calculations indicate that the isotropic contributions to the hyperfine tensor, A4s, reflecting the probability of finding the a1g*( approximately 3z2-r2) unpaired electron on the 4s orbital of Cu2+, is very sensitive to Rax and Req changes. The change experienced by the experimental hyperfine value A/sub /// on passing from NH4Cl to NH4Cl0.92Br0.08 supports this conclusion and leads to ( delta A4s/ delta Rax) approximately=-2( delta A4s/ delta Req) equivalent to -30 G pm-1. These figures support the attribution of the significant increase, delta T(A/sub ///)=60 G, experienced by A/sub /// of NH4Cl:Cu2+(II) below the phase transition at Tc=242.5 K principally to an anomalous relaxation of the Cu2+(II) centre involving a decrement of Rax but an increase of Req in accord to the conclusion from previous optical and Raman studies. This work stresses the possibility of detecting from CuCl4(NH3)22- delta Rax changes of the order 10-5 AA through ENDOR measurements of the hyperfine tensor. To the authors' knowledge this is the highest sensitivity reached so far for measuring variations of metal-ligand distances through spin-Hamiltonian parameters.