Polycyclic aromatic hydrocarbons (PAHs) and their oxygenated products (oxi-PAH) are considered as important pollutants of the Earth’s atmosphere since they are emitted by the combustion of fuels. ^[1] The study of their intermolecular interactions is essential to understand the formation of their aerosols. In this work, we have studied at molecular level the interactions present in the hydration of the oxi-PAH, α- and β-naphthaldehyde. This study has been performed using a supersonic jet Fourier transform microwave (FTMW) spectrometer in the 4-15 GHz range, with the support of theoretical calculations. Both isolated α- and β-naphthaldehyde species could present two possible structures: cis, the most stable one for α, and trans for β. ^[2] Our calculations show that there are three low energy monohydrates predicted for each conformer, cis/trans, in an energy range of 1500 cm^-1. Experimentally, one conformer has been observed in gas phase for α and two for β, corresponding to the calculated most stable structures. All species are stabilized by intermolecular hydrogen bonds between the water molecule and the aldehyde group of naphthaldehyde: for the α isomer, the oxygen of the aldehyde acts as proton acceptor and the aldehyde hydrogen as proton donor; for the β isomer, the oxygen of the aldehyde acts as proton acceptor and one of the ring hydrogens as a proton donor.^[3]

[1] Karavalakis G. et al. Sci. Tot. Environ., 409, 4, 738, 2011. [2] Goubet M., et al. J. Phys. Chem. A, 124, 4484, 2020. [3] This work is supported by the CaPPA project and by the CPER ClimiBio funded by the French National Research Agency (ANR) through the PIA 11-LABX-0005-01, the I-SITE ULNE/ANR-16-IDEX-0004 ULNE, the Regional Council Hauts-de-France and the European Funds for Regional Economic Development (FEDER).