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.

This is a preprint of an article published by American Chemical Society in Journal of Physical Chemistry A, 2004, available online: http://pubs.acs.org/doi/abs/10.1021/jp0486752.