The structures of 1-bromoheptane and 1-bromononane monolayers adsorbed on the surface of graphite

X-ray diffraction and differential scanning calorimetry (DSC) have been used to confirm the formation of the solid monolayers of 1-bromoheptane (C7Br) and 1-bromononane (C9Br) on the surface of graphite. The X-ray diffraction patterns of these two species can be fitted by two possible monolayer structures with symmetries Pgb and P2. For one of these (C7Br) this ambiguity can be lifted using neutron diffraction and the Pgb plane group can be identified as the correct structural solution. Both of the unit cells contain two molecules forming an approximately 90^o zig-zag chain of bromine atoms with an inter-bromine distance of approximately 3.70?Å, which is similar to twice the Van der Waals radii of the bromine, suggesting rather little non-covalent interaction. The dipole moments in two adjacent chains are opposed leading to no net dipole for the layer as a whole.