title: 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:H(2)O. 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:D(2)O. 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.
authors.affiliation: FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.
authors.forename: Johannes
authors.initials: J
authors.lastname: Hunger
authors.forename: Liyuan
authors.initials: L
authors.lastname: Liu
authors.forename: Klaas-Jan
authors.initials: KJ
authors.lastname: Tielrooij
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.forename: Huib
authors.initials: H
authors.lastname: Bakker
mesh_terms: D006878:Hydroxides / Q000737:chemistry*; D012996:Solutions; D013816:Thermodynamics; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D006878:Hydroxides; D012996:Solutions; D014867:Water; C031356:hydroxide ion