Intermolecular vibrations of the 2,3-dimethylnaphthalene · Ar van der Waals complex: Experiment and quantum three-dimensional calculations

A combined experimental and theoretical study of the intermolecular vibrations of 2,3-dimethylnaphthalene · Ar (2,3-DMN · Ar), for the first excited electronic state (S₁), is reported. Methyl groups at C2 and C3 positions of naphthalene lower the symmetry of the complex, so that transitions involving excitation of the intermolecular long-axis in-plane x mode become allowed in electronic spectra, in addition to the out-of-plane z mode. Two-color resonant two-photon ionization (2C-R2PI) spectrum of the van der Waals (vdW)-mode region (0₀⁰+70 cm^-1) of 2,3-DMN · Ar exhibits six bands to the high-frequency side of the electronic origin 0₀⁰, which arise from excitation of low-frequency intermolecular vibrations of the complex in the S₁ state. Accurate quantum three-dimensional (3D) calculations of vdW vibrational (J=0) levels of S₁ 2,3-DMN · Ar have been performed, using a recently developed quantum method based on the 3D discrete variable representation. Since no approximation is made in the treatment of coupled, very anharmonic vdW vibrations, the calculated eigenstates are essentially exact for the intermolecular potential energy surface (PES) employed, thus enabling direct comparison between theory and experiment. The intermolecular PES was modeled as a sum of atom-atom Leonard-Jones (LJ) pair potentials. Some of the initial LJ parameters were modified until very good agreement was achieved between the calculated and measured vdW frequencies of S₁ 2,3-DMN ·Ar. This allowed assignment of the vdW bands to the blue side of 0₀ ⁰, and resulted in an improved intermolecular PES of the complex. In addition, the quantum 3D calculations provided a quantitative description of the vdW vibrational level structure and floppiness of S₁ 2,3-DMN · Ar up to ∼60-70 cm^-1 above the ground vdW state. The wave functions of all vdW states below ∼49 cm^-1, relative to the ground state, are sufficiently regular to allow assignment of vibrational quantum numbers. At higher excitation energies, mode coupling becomes stronger, and irregular vdW states whose assignment is uncertain, are common.