Density functional theory was used to study the hydrogen bonding between the water and the ClO radical. The two energetically low-lying minima are 5 ((2)A') and 6 ((2)A"), with hydrogen bonding occurring between the chlorine atom in the ClO radical and the hydrogen atom in the water; the (2)A' state has a slightly (similar to0.3 kcal/mol) lower energy. Another hydrogen bond occurs between one of the hydrogen atom in the water and oxygen atom in the ClO radical. The optimized geometry parameters, rotational constants and interaction energies for various isomers were calculated at the UB3LYP and UMP2 levels. The infrared spectra and the vibrational frequency shifts are also reported. (C) 2003 Published by Elsevier B.V.