Non-covalent interactions are rapidly gaining interest as they are often crucial in determining the properties of materials, and key to supramolecular chemistry and to biochemistry. Non-covalent Polycyclic Aromatic Hydrocarbon (PAH) complexes are in particular relevant to astrochemistry and combustion chemistry where they are involved in the initial steps of condensation and soot formation, respectively. Here, we investigated non-covalent π-π stacking and CH-π interactions in naphthalene and

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... naphthene clusters using high resolution IR-UV spectroscopy in combination with quantum chemical calculations. We identified spectral shifts that occur upon complexation and thereby evaluated predicted potential energy surfaces. The results provide strong indications for a prevalent parallel naphthalene dimer, showing that π-π stacking interactions become significant for bicyclic and larger PAHs.