A neutron diffraction study of xylitol: derivation of mean square internal vibrations for H atoms from a rigid-body description.

A neutron diffraction study of xylitol (C(5)O(5)H(12)) is presented. The nuclear anisotropic displacement parameters have been analysed showing that the carbon-oxygen skeleton conforms to a rigid-body (TLS) description. Applying this TLS model to the xylitol H atoms allows characterization of the internal molecular displacements of the H nuclei, assuming that the observed H nuclear mean-square displacements are a sum of the internal displacements and rigid-body displacements. These internal molecular displacements are very similar for chemically equivalent H atoms and in good agreement with the values obtained by other methods. In all cases the smallest eigenvector of the residual mean-square displacement tensor is almost parallel to the X-H bond. The use of ab initio calculations to obtain the internal vibrations in xylitol is discouraging. Another 12 structures extracted from the literature which have been investigated by neutron diffraction were subjected to a similar analysis. The results for the nine compounds investigated at low temperature conform to the results from xylitol and provide estimates of the internal vibrations of H atoms in a range of chemical environments.