Anisotropic thermal expansion in 18-crown-6•2 H2O•2 HNO3

Reaction of aqueous HNO3 with 18-crown-6 results in the isolation of the neutral inclusion compound 18-crown-6.2 H2O.2 HNO3. Variable temperature single crystal X-ray and neutron diffraction data reveal that the triclinic (P (1) over bar) unit cell undergoes a highly anisotropic change in shape with temperature, with the crystallographic beta angle changing smoothly from 66.56degrees to 69.03degrees between room temperature and 20 K. This arises from a change in shape of the crown ether in response to increases in hydrogen bond distances in the HNO3...H2O...crown ether chain.