Dynamical properties of F--water and Cl--water hydrogen bonds in aqueous solution have been studied by ab initio QM/MM molecular dynamics simulations, in which the whole first hydration sphere of the anion was treated at Hartree-Fock level using D95V+, 6-31+G and D95V++ basis sets for F-, Cl- and water, respectively. According to a detailed analysis of the bond distortions and shifts in the corresponding bending and stretching frequencies as well as the mean residence times of water molecules surrounding the ions, F- clearly acts as a 'structure-maker', while Cl- solvation leads to a more flexible structure with frequent re-arrangements of the hydrogen bonds. (C) 2005 Elsevier B.V. All rights reserved.