Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: An ab initio mixed quantum-classical study J. Chem. Phys. 126, 074304 (2007) To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu + ͑H 3 O͒ 1-6 and Au + ͑H 2 O͒ 1-6 clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag + ͑H 2 O͒ n ͑which have the coordination numbers of 3-4͒ as well as the hydrated alkali metal ions ͑which have the coordination numbers of ϳ6͒. For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.