Structural Characterization of Aromatic−Aromatic Complexes by Rotational Coherence Spectroscopy

Rotational coherence spectroscopy (RCS) has been applied in structural studies of (a) aromatic−aromatic van der Waals complexes of the form M−X, where M = perylene or fluorene and X = benzene or toluene, and (b) aromatic−aliphatic hydrocarbon dimers of the form M−Y, where M is as above and Y = cyclohexane or methylcyclohexane. For all of the perylene complexes the experimentally determined rotational constants are found to be consistent with centrally bound, parallel-stacked structures in which the monomer planes are separated by a distance of 3.5−4.3 A. Analogous geometries also characterize the fluorene−aliphatic species. However, the two fluorene−aromatic complexes have structures that depart from the parallel-stacked form. Both species have slipped geometries in which the monomer planes are not directly over one another. And, in the case of fluorene−benzene, the two aromatic planes are tilted with respect to one another. These differences are attributed to the significant contribution of electrostatic forces in determining the fluorene−aromatic geometries.