The structure of a methyl iodide monolayer adsorbed on graphite

Abstract

The structure of an adsorbed layer of methyl iodide on graphite has been determined by a combination of X-ray and neutron diffraction. The iodine atom positions were determined from the X-ray pattern and the methyl group positions from the neutron pattern. The experimentally determined unit cell has lattice parameters a = 1·540 nm, b = 0·578 nm and β = 90°. There are four molecules in the unit cell arranged to give two zig-zag chains of iodine atoms. Within the chain the iodine atoms are separated by 0·383 nm, which is similar to the value of 0·389 nm for bulk methyl iodide. The methyl groups are arranged so that the orientations of the molecular dipole moments in the two chains are opposed. All four molecules lie approximately flat on the surface and the layer resembles the (010) plane of the bulk crystal.