Van der Waals modes of solute/solvent clusters: benzene-methane, -deuteriomethane, and -carbon tetrafluoride

Clusters of benzene(CD/sub 4/)/sub 1/ and -(CF/sub 4/)/sub 1/ are created in a supersonic molecular jet and are studied by two-color time-of-flight mass spectroscopy. The clusters' S/sub 1/ reverse arrow S/sub 0/ intermolecular vibronic structures are characterized by calculational modeling of the clusters' intermolecular motion. The calculations include (1) an intermolecular normal-coordinate analysis (NCA) which treats all six van der Waals modes under a harmonic oscillator assumption and (2) a three-dimensional hindered rigid-rotor analysis (3D-HRRA) which treats the intermolecular torsional motion. Agreement between calculation and experiment is excellent for binding energy, symmetries, and van der Waals mode energies. The cluster spectra and calculated intermolecular modes are compared to those of benzene(CH/sub 4/)/sub 1/ reported previously. A major conclusion of this work is that the clusters behave rigidly with regard to internal rotation of the cluster subunits and that the clusters possess unique equilibrium geometries. The internal torsional motion is oscillatory and is constrained by an orientationally dependent intermolecular potential whose barrier height is of the order of the cluster binding energy.