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)₄Br (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)₄Br 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)₄⁺ ions.