Vibrational and orientational dynamics of water in aqueous hydroxide solutions
Abstract
We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH- from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H2O. For the O-H vibration of HDO molecules outside the first hydration shell of OD-, we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D2O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ∼7 surrounding water molecules, considerably less than previously reported for the proton.
- Publication:
-
Journal of Chemical Physics
- Pub Date:
- September 2011
- DOI:
- 10.1063/1.3643763
- Bibcode:
- 2011JChPh.135l4517H
- Keywords:
-
- dielectric relaxation;
- molecular configurations;
- molecular moments;
- sodium compounds;
- terahertz wave spectra;
- time resolved spectroscopy;
- vibrational states;
- water;
- 33.20.Tp;
- 33.15.Bh;
- 33.15.Kr;
- 33.15.Mt;
- 33.20.Bx;
- Vibrational analysis;
- General molecular conformation and symmetry;
- stereochemistry;
- Electric and magnetic moments polarizability and magnetic susceptibility;
- Rotation vibration and vibration-rotation constants;
- Radio-frequency and microwave spectra