The influence of the motion of water molecules on proton dipolar coupling tensors in Sr[Fe(CN)5NO]·4H2O

A 1 H single crystal NMR study on strontium nitroprusside Sr[Fe(CN) 5 NO]·4H 2 O has demonstrated that water molecules are not static in the unit cell but undergo librations. The dipolar coupling tensors have been measured at 287 K and 120 K for three types of water molecule, W1, W2 and W3, existing in the unit cell of SrNP. The tensors are motionally averaged and the results are interpreted in terms of librational motion of the water molecules. The models for such motions are proposed and the angles of libration are calculated for each of the water molecules. At 120 K the asymmetry parameters of the tensors are still different from zero indicating that even at this temperature the water molecules in SrNP are characterized by great mobility. Despite the dipolar-dipolar (DD) tensors not being axially symmetric the unique eigenvectors still give reliable information about the average proton-proton directions within the unit cell of SrNP. It was found that the directions of the proton-proton vector given by the largest eigenvector of the DD tensor for W1 are quite different at 286 K and at 120 K. This result is interpreted as being due to a rearrangement of the W1 water molecule, which takes place at the phase transition at 198 K.