The orientation of the electric field gradient tensor in CO-liganded myoglobin.

The EFG-tensor at the position of the Fe-atom of CO-liganded sperm whale myoglobin has been investigated by nuclear gamma-resonance absorption experiments on single crystals. In addition the temperature dependence of the quadrupole splitting of the 14.4 keV level of the iron nucleus was measured. An unambiguous solution for the magnitude and the orientation of the field gradient tensor could only be obtained with the assumption that a C2-axis perpendicular to the haem plane is one principal axis of the electric field gradient tensor. Within this solution the electronic structure of the iron is described by a singlet ground state with Nz = 0.75 and the largest EFG component perpendicular to the haem plane.