Elucidating the structure and binding nature of thianaphthene dimers using gas-phase infrared spectroscopy

Crucial for many biological systems, in astrochemistry, and for fundamental chemistry in general, the conformations adopted by weakly bound complexes of polycyclic aromatic hydrocarbons (PAHs) have been the focus of debate for decades. Still, there are great challenges in forming such complexes in the laboratory, measuring accurate spectroscopic information sensitive to structure, and computing molecular properties for systems subtly bound by dispersive interactions. Here, we employ a combination of gas-phase infrared spectroscopy with extensive density functional theory (DFT) calculations to unambiguously determine the preferred conformation adopted by dimers of neutral thianaphthene, a PAH composed of one six- and one five-membered ring with an incorporated sulfur atom. A very wide spectral range from 350 to 3150 cm^-1 is covered, allowing a determination of the effect of complexation on fingerprint vibrations as well as C-H stretches. A comparison of the recorded infrared spectra of monomers and dimers in combination with detailed vibrational calculations assigns a π-stacked configuration for the complex. This agrees with energetic arguments, where DFT predicts the isomeric T-shaped configuration to be 0.13 eV higher in energy. The potential energy surface of the complex is explored using the nudged elastic band (NEB) method and the nature of the interaction between neutral monomers is investigated based on the local energy decomposition (LED) analysis. The π-stacked dimer is overwhelmingly stabilized by π⋯π dispersion, an interaction that is much weaker in the T-shaped configuration, despite the effect of C-H⋯π forces. The methodology applied here to thianaphthene is extendible to dimers without a permanent dipole moment, hence invisible to microwave spectroscopy, as well as larger clusters of PAHs.