Infrared spectroscopic and ab-initio studies of the benzene dimer

Due to its high symmetry and small size, benzene serves as a model molecule for the symmetry properties of molecular vibrations in polyatomic molecules and research on its vibrational modes dates back 70 years. One would expect the fundamental frequencies of its 20 symmetry unique vibrational modes to be well known. However, for at least one mode, the B 1u C-H stretch mode, the until recently accepted value was determined only indirectly. Performing experiments on the benzene dimer provides the first direct experimental determination of this mode. Clusters of benzene have also attracted experimental and theoretical attention. Of special interest is the dimer, as it can serve as a prototype for pi - pi interaction in aromatic systems and serves since long as a benchmark system to test our understanding of intermolecular interactions. There seems to be no clear consensus from theory if the dimer structure is a parallel or T-shaped structure, although recently, mostly (distorted) T-shaped structures are predicted as being the lowest energy structures. Raman experiments clearly indicate that for the observed species, the two moieties are symmetrically inequivalent and microwave studies show the presence of a dipole moment. Both these results thus support a T-shaped structure. However, it is not clear whether such a T-shaped structure is the exclusive structure or whether a parallel displaced and a T-shaped structure are is some equilibrium and only the T-shaped structure is visible to the experiment. Here, we present a mechanism in which higher energy gas-phase conformers can be converted into lower energy structures with rare gas atoms catalyzing the process