Unravelling the microhydration frameworks of prototype PAH by infrared spectroscopy: naphthalene-(water)_1-3

Hydration of aromatic molecules is a fundamental chemical process. Herein, microhydration framework of the prototypical neutral polycyclic aromatic hydrocarbon (PAH), naphthalene (naphthalene-(water)n≤3), is investigated by infrared spectroscopy inside helium nanodroplets. The measured data are analyzed by quantum chemical calculations at the MP2/6-311++G(d,p) level. This combined experimental and theoretical approach demonstrates that water binds to the naphthalene ring via π hydrogen bond (H-bond) for n = 1 case. Further addition of the solvent molecules occurs via the formation of a H-bonded water network facilitated by the nonadditive cooperative force. No isomers are observed in which the solvent molecules separately bind to the aromatic ring. For n = 3 case, we observe the formation of a cyclic H-bonded water moiety. Comparison with corresponding cationic and anionic naphthalene±-(water)n clusters demonstrates the charge-induced modification of the hydration motif. Our results are further compared with the prototypical benzene-(water)n complexes to comprehend the effect of an additional phenyl ring on the solvation network.