Lattice, rotation and intramolecular vibrations of ferrocene, Fe(C 5 H 5 ) 2 , crystallites of the factor group in the disordered phase are calculated using the correlation theorem based on group theory. The correlation between the species of the C 1 site symmetry occupied by cyclopentadienyl molecules and those of the factor group C 2 h of the crystal are calculated. The number of lattice vibrations of the cyclopentadienyl molecules is found to be 12, with active modes in Raman and infrared (IR) spectra. The same number of rotations for the cyclopentadienyl molecules is expected to be allowed in both spectra. The active number of intramolecular vibrations for the cyclo-pentadienyl molecules having D 5 molecular symmetry is expected to be 80 vibrations in both the Raman and the IR spectra. The effect of -irradiation with different doses and heat treatment at different temperatures on the IR spectra of ferrocene in the energy range 4000-200 cm -1 is discussed. A number of bands continuously shifted their position, and a decrease in intensity with increasing -dose is observed. New bands appeared in this spectral region for different annealing temperatures and different -doses. These changes are discussed in terms of intermolecular interactions between molecules within the unit cell.