First observation of ionic π-hydrogen bonds; vibrational spectroscopy of dihydrated naphthalene anion (Nph−(H2O)2)

Hydrated structure of Nph−(H2O)2 was investigated using infrared (IR) spectroscopy. Among OH stretching vibrations, three π-hydrogen bonds to anionic naphthalene ring, namely ionic π-hydrogen (iπH) bonds, and an interwater hydrogen bond were observed. It allowed us to elucidate the geometrical structure of Nph−(H2O)2. The unstable naphthalene bare anion was stabilized through the iπH bonds with the water dimer on its ring, forming the one-sided structure. Besides, the intensity of the aromatic CH stretching modes was found to be enhanced upon the electron attachment without significant lowering of their frequencies.