Very large amplitude intermolecular vibrations and wave function delocalization in 2,3-dimethylnaphthalene · He van der Waals complex

We report a combined experimental and theoretical study of the intermolecular vibrations and van der Waals isomerism of the 2,3-dimethylnaphthalene-He van der Waals complex. Two-color resonant two-photon ionization spectra of the S₀→S₁ electronic transition of 2,3-dimethylnaphthalene-He exhibit five bands within 30 cm ^-1 of the electronic origin. The intermolecular potential energy surface was modeled as a sum of atom-atom Lennard-Jones pair potentials; it exhibits two equivalent global minima on each side of the naphthalene moiety, and a single shallower local minimum adjacent to the two methyl groups. Based on this surface, accurate three-dimensional quantum calculations of the van der Waals vibrational levels using the discrete variable representation method were performed. Careful optimization of the potential parameters lead to a quantitative reproduction of four observed bands as intermolecular vibrational excitations, a vibrationally averaged He atom distance from the aromatic plane 〈z₀〉 = 3.22 Å, and a dissociation energy D₀(S₁)= -60.3 cm ^-1, compatible with experiments. The fifth band is assigned as a van der Waals isomer, corresponding to the local minimum. The quantum calculations were extended up to the dissociation limit, yielding ≈ 173 van der Waals vibrational states. Above 70% of D₀, many vibrational states are completely delocalized over the potential surface, with root-mean-square vibrational amplitudes up to 6 A parallel to and up to 1.5 A perpendicular to the molecular surface. Calculated tunnelling splittings range from < 10 ^-4 cm ^-1 for localized states, to >3 cm ^-1 for highly delocalized ones.