Femtosecond study of the effects of ions on the reorientation dynamics of water

We study the effects of ions on the reorientation dynamics of liquid water with polarization-resolved femtosecond mid-infrared spectroscopy. We probe the anisotropy of the excitation of the O-D stretch vibration of HDO molecules in solutions of NaCl. NaI and N(alkyl)(4)Br (tetra-alkylammoniumbromide) salts in 8% HDO: H2O. We find that the reorientation of the O-D groups of HDO molecules hydrating the Cl- and I- anions occurs on two different time scales with time constants of 2 +/- 0.3 ps and 9 +/- 2 ps. The fast component is due to a wobbling motion of the O-D group that keeps the hydrogen bond with the halogenic anion intact. For solutions of N(alkyl)(4)Br salts we observe a very strong slowing down of the reorientation of water that is associated with the hydration of the hydrophobic alkyl groups of the N(alkyl) : ions. (C) 2012 Elsevier B.V. All rights reserved.