Angular intensity distributions of CH4 and C2H6 molecules scattered from a highly corrugated LiF(001) surface have been measured in high resolution. CH4 molecules reveal clear rainbow scattering, while C2H6 molecules do not. This difference is attributed to the structural anisotropy between these molecules; a CH4 molecule is more spherically symmetric than a C2H6 molecule. The microscopical repulsive potential, corrugation of the LiF(001) surface experienced by these molecules is found to decrease as the incident kinetic energy increases. This counterintuitive lessening of the corrugation is a consequence of the potential surface arising from two very different sized substrate ions.