Formation and crystal structures of the hydrates of 18-crown-6

The melting diagram of the system 18-crown-6/water has been set up and the existence of four binary hydrates confirmed. Crystal structures have been determined for three of these, in addition to that for the hexahydrate already published. The hydrates are monoclinic, a tetrahydrate (previously reported as a 4.5-hydrate) with space group P2(1)/c and Z = 2 formula units per unit cell, an octahydrate with I2/a and Z = 4, and a dodecahydrate with P2(1)/n and Z = 2. In each structure the crown ether molecule has the (pseudo) D-3d conformation common for many of its complexes, and is immediately hydrogen-bonded by a group of four water molecules in a similar, characteristic pattern. In the tetrahydrate, with one of the waters disordered, the total connectivity is that of a 1D chain, while the overall structures of the octa- and dodecahydrate are 3D networks. Four of the six independent water molecules in the dodecahydrate are linked with each other into a rare 2D ice-like layer.