Intermolecular vibrations of a crystalline molecular complex

The nature of intermolecular vibrations in a crystalline molecular complex is examined using a simple model of a crystal with one molecule of a 1:1 complex per unit cell. It is shown that two limiting descriptions can result depending on the coupling strength between molecular components in the complex: (i) The giant molecule limit describes the case of a strong coupling where the intermolecular motions are the motions of the complex as one entity, (ii) The sublattice limit is applicable in the weak coupling case. Here, the motion can be characterized as an intermolecular motion belonging to the sublattice of the individual components. The phonon bands of naphthalene:octafluoronaphthalene crystals are examined within the framework of the simple model. The results predict phonons to be close to the sublattice limit. Our experimental study of Raman phonon spectra using isotopic mixed crystals of the complex is in agreement with the theoretical predictions and show that the low frequency phonons belong predominantly to the C₁₀F₈ sublattice, while the high frequency phonons characterize the C₁₀H₈ sublattice motions.