S2–S0Spectra of Jet-Cooled Xanthione and Benzopyranthione: Microscopic Solvation by Rare Gases

Abstract

The laser-inducedS2–S0fluorescence excitation and dispersed emission spectra of two aromatic thiones, xanthione (XT) and benzopyranthione (BPT), have been measured under jet-cooled conditions. These emission spectra, together with the corresponding FT-IR and FT-Raman spectra andab initioCEP-31G calculations, have been used to achieve a complete vibrational analysis of BPT in its ground state, complementing that of XT, previously published. The 1:1 and 1:2 van der Waals complexes of XT and BPT with He, Ne, Ar, Kr, and Xe have been synthesized and characterized using their microscopic solvation shifts and the frequencies of their intermolecular (van der Waals) vibrations. In all cases the adatom preferentially occupies a position over the pyranthione ring in the 1:1 complex, and the 1:2 complex has a symmetrical sandwich (1|1) structure. No (2|0) isomer is observed. An analysis of the microscopic solvent shifts of the origin bands of the complexes indicates that dipole-induced dipole interactions contribute significantly to the binding energies. The binding energies and geometries of the 1:1 and 1:2 van der Waals complexes of XT, BPT, and pyranthione (PT) have been calculated using pairwise atom-atom Lennard–Jones 6–12 potentials and are consistent with the structures derived from experiment. The out-of-plane intermolecular stretching vibrations can be clearly observed in the excitation spectra of most complexes, and their assignments have been confirmed using model vibrational potentials.Ab initioSCF calculations including second order Møller–Plesset correlation are also reported for the PT·rare gas systems, and show nice agreement with the Lennard–Jones calculations and experiment.