An X-Ray Diffraction Study

Abstract

X-ray oscillation photographs reveal both diffuse and discrete scattering for alkanone guest molecules within inclusion compounds of 5-undecanonehrea grown from methanol. Guest molecules that give rise to diffuse scattering are ordered only along the channel axis, whereas those guest substructures that give rise to discrete scattering are ordered in three dimensions. Precession photographs and diffractometry show that within domains containing three dimensionally ordered guests, the offset (denoted δ_g) between the heights of guest molecules in adjacent channels is exactly zero. These guests therefore share overall hexagonal symmetry with the host, as do a large number of n-alkanones of varying chain length and symmetry properties. Although side by side ordering optimizes dipole-dipole interactions between alkanones in adjacent channels, the large inter channel distance (8.22Å) and the lack of symmetry in 5-undecanone suggest that other factors could influence guest molecule ordering. A careful diffractomemc study from -24°C to +20°C reveals that 5-undecanone/urea is a commensurate system in which the periodic repeats of the host and guest structures follow the relationship 3c _h = 2c _g. Both dipole-dipole interactions and commensurability of host and guest are thought to contribute to the ordering of guests in this system.

Key Words:

• Urea Inclusion Compound
• Commensurate structure
• Dipole Organization
• Long-range Interactions
• X-ray diffraction
• Alkanone
• Dipole-dipole interactions