POLAR PYRAMIDAL MOLECULES ON A GRAPHITE SUBSTRATE

We study the energetics of single CH4, CH3F, CH3Cl, CF3H, CF3Cl, and CF4 molecules physisorbed on a graphite substrate using the Lennard-Jones 6-12 and Coulomb pair potentials. The ground state orientations and positions of these molecules have been obtained. In contrast to CH4 and CF4, for the dipolar molecules we found a shift in the preferred adsorption site slightly off of the atop site. This shift removes the existing orientational degeneracy of the adsorption potential associated with the three-fold rotational symmetry of the graphite lattice, but introduces a new degeneracy due to the three possible directions of the shift. The softening of the potential well due to this off-center displacement should show up in the low-temperature thermodynamic properties. © 1990.