Infrared photodissociation spectroscopy of [Mg·(H₂O) _1-4]⁺ and [Mg·(H₂O) _1-4·Ar]⁺

Infrared photodissociation spectra of [Mg·(H₂O) _1-4]⁺ and [Mg·(H₂O) _1-4·Ar]⁺ are measured in the 3000-3800 cm ^-1 region. For [Mg·(H₂O)_1-4] ⁺cluster geometries are optimized and vibrational frequencies are evaluated by density functional theory calculation. We determine cluster structures of [Mg·(H₂O)_1-4]⁺ by comparison of the infrared photodissociation spectra with infrared spectra calculated for optimized structures of [Mg·(H₂O) _1-4]⁺. In the [Mg·(H₂O) _1-3]⁺ ions, all the water molecules are directly bonded to the Mg⁺ ion. The infrared photodissociation spectra of [Mg·(H₂O)₄]⁺ and [Mg·(H ₂O)·Ar]⁺ show bands due to hydrogen-bonded OH stretching vibrations in the 3000-3450 cm^-1 region. In the [Mg·(H₂O)₄]⁺ ion, three water molecules are attached to the Mg⁺ ion, forming the first solvation shell; the fourth molecule is bonded to the first solvation shell. As a result, the most stable isomer of [Mg·(H₂O)₄]⁺ has a six-membered ring composed of the Mg⁺ ion, two of the three water molecules in the first solvation shell, and a termination water molecule.