Abstract
Single crystals of barium nitroprusside trihydrate (BaNP) were investigated by Fourier transform ‘wide-line’ NMR at 40 K and by proton spin-lattice relaxation time in the temperature range from 320 to 30 K. The analysis of the spectra yielded the dipolar-dipolar (DD) coupling tensors of three different water molecules. Two of the DD tensors show the asymmetry parameters η = 0·08 and 0·09. These results are interpreted in terms of librational motions of the water molecules. A model for such motions is proposed and the angles of librations are calculated. The positions of the proton-proton vectors in the unit cell of the crystal obtained from DD tensors coincide with the neutron diffraction data of BaNP. The proton spin-lattice relaxation times T 1 at low temperatures were interpreted in terms of the dipole-dipole interactions modulated by the oscillation of the proton-proton vector between two minima of an asymmetric double-well potential. The following parameters for this motion have been obtained: E a = 3·4 kJ mol-1, τ0 = 3·9 × 10-13 s and ΔE = 1·50 kJ mol-1.