van der Waals vibrational modes of the pentacene⋅R complexes (R=Ne, Ar, Kr)

The vibrational van der Waals modes of the pentacene⋅R van der Waals complexes (R=Ne, Ar, Kr) were theoretically studied. Empirical model calculations according to the method of Ondrechen et al. yielded the pentacene⋅R potential surfaces. One- and two-dimensional vibrational calculations were performed using two different numerical treatments, yielding the vibrational energies and wave functions of the intermolecular pentacene⋅R motions, as well as tunnel splittings and tunneling rates. Estimates for the three-dimensional case were derived from the results for one and two dimensions. Anharmonic coupling between the three van der Waals modes are shown to be relatively unimportant. However, partial counterrotation of the whole complex in space, balancing vibrational angular momentum, turned out to have a sizable effect on one of the modes. The calculated results were compared with available experimental data.