INFRARED SPECTRA OF $H_{2}O^{+} -Ar_{n}$ COMPLEXES (n=1-14)

Abstract

IR spectra of the open-shell ionic $H_{2}O^{+} -Ar_{n}$ complexes (n=1-14) are recorded in the vicinity of the O-H stretch vibrations $(\nu_{1}, \nu_{3})$ by means of photodissociation spectroscopy in a tandem mass spectrometer. Rotational and vibrational anal ysis of the dimer spectrum clearly shows that the first Ar ligand forms a linear ionic hydrogen bond to the $H_{2}O^{+}$ cation. Complexation transforms the symmetric (antisymmetric) O-H stretch of $H_{2}O^{+}$ into bound and free O-H oscillators in the $H_{2}O^{+} -Ar$ dimmer. Strong coupling between the bound O-H stretch ($\nu_{1}$) and the intermolecular bond leads to a large red shift in the $\nu_{1}$ frequency and a short lifetime. In contrast, the free O-H stretch ($\nu_{3}$) is only little affected upon Ar complexation, and the resulting long lifetime allows for the resolution of the rotational structure (including spin-rotation). The second Ar ligand binds to the second proton of the water cation ($C_{2u}$). Complexation with further Ar ligands cause smaller frequency shifts owing to weaker intermolecular bonds. Ab initio calculations show good agreement with the experimental data.