X-RAY AND NEUTRON DIFFRACTION STUDY OF A CROWN ETHER - A CAUTIONARY TALE. R.H. Fenn, University of Portsmouth, S.A. Mason, ILL Grenoble, R.A. Palmer and B. Potter, Birkbeck College, London, O.S. Mills, P.M. Robinson and C.I.F. Watt, University of Manchester, UK

Beware all Ye who regard a diffractometer as a black box and do not appreciate the beauty of crystallography.

The compound C₂₄H₃₀O₆.H₂O is a water-bound adduct of the tetragol crown ether 9,l0-dihydro,-l0,l0-dimethyl,-9-hydroxyanthracene, which binds strongly with neutral molecules and can react to form covalent adducts bonded to the 9-carbon.

Diffractometry offered cell geometry which conformed extremely closely with an orthorhombic one-face-centred lattice. However the Laue symmetry was 2/m and spacegroup P2₁ with Z = 4 and two molecules, i.e. 126 atoms, per asymmetricc unit. The primitive cell was related to the centred cell by choice of a diagonal along the ac face.

Meanwhile neutron diffraction data were being collected, at l5K, on Dl9 at ILL, Grenoble. Initially, difficulties were encountered in reconciling the calculated neutron structure factors, based on the atomic parameters from the X-ray monoclinic data, with the observed neutron structure amplitudes. However, there are two possible choices of diagonal which lead to very similar unit cell parameters as the original centred cell has essentially equal diagonals. Once the appropriate transformation of the reflexion indices from the neutron data to the X-ray cell had been applied, difference Fourier syntheses and isotropic least-squares refinement of the 504 parameters led to a R-factor of 5% for the neutron structure.

The structure of the crown adduct shows that the bound water molecule is held not only by a strong hydrogen bond to the 9-hydroxyl but also by two weaker bonds between the water hydroxyls and two oxygens of the crown which cause the water to point into the crown cavity.