ORIENTATION OF ELECTRIC-FIELD-GRADIENT TENSOR IN CRYSTALLINE DIRHENIUM DECACARBONYL

The orientation of the principal-axis system of the field-gradient tensor in a single crystal of Re2(CO)10 was studied at room temperature. The z and y axes were determined independently from curves of constant frequency for the +3/2↔+5/2 and -3/2↔-1/2 transitions at a magnetic field of 2 kG. Two sets of principal axes related by the twofold axis and mirror plane of the monoclinic crystal were found. The rheniumrhenium bond coincides with the intersection of the two y-z planes and forms an angle of 25° with the z axes. The deviation of the z axes from the Re-Re bond is attributed to distortions of the molecule in the solid due to nearest-neighbor interactions of carbonyl groups. The total field-gradient tensor is described as the sum of an axially symmetric tensor with z axis along the Re-Re bond-dominated by intramolecular contributions-and a second axially symmetric tensor arising from the distortion of the molecule in the lattice with z axis in the y-z plane of the total field-gradient system at an angle of 24° from the Re-Re bond. The two tensors have z components of opposite sign and the same order, giving rise to the large η value observed. The descrease of η with increasing temperature is attributed to a reduction of the distortions of the molecule by the vibration of the CO groups.