The infrared absorption spectrum of C2F6 physisorbed on graphite in the commensurate 2x2 phase has two strong, narrow peaks associated with the nu(5) molecular vibration. They are interpreted as a Fermi resonance between nu(5) and the nu(8)+nu(11) combination band, which are shifted into near-coincidence by dynamic dipole coupling between the adsorbate molecules. The splitting and relative strengths have been measured as the coupling is reduced in a lower-density, tilted, incommensurate phase and are consistent with theory. It is shown that two alternative descriptions, as Fermi resonance between exciton modes of the layer or as dynamic dipole coupling between molecules with two vibration modes, are essentially equivalent. To fit the experimental frequency shifts and absolute absorption strengths, it is necessary to use a value for the nu(5) vibrational polarizability of the adsorbed molecule that is somewhat smaller than the value for the free molecule.