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. 2020 Sep 24;124(38):8309-8316.
doi: 10.1021/acs.jpcb.0c03614. Epub 2020 Sep 9.

Slowing Down of the Molecular Reorientation of Water in Concentrated Alkaline Solutions

Roberto Cota  1   2 , Eliane P van Dam  2 , Sander Woutersen  1 , Huib J Bakker  2
Affiliations

Slowing Down of the Molecular Reorientation of Water in Concentrated Alkaline Solutions

Roberto Cota et al. J Phys Chem B. .

Abstract

It is generally accepted that the hydroxide ion (OH-) is a strong hydrogen bond acceptor and that its anomalously high diffusion constant in water results from a Grotthuss-like structural diffusion mechanism. However, the spatial extent over which OH- ions influence the dynamics of the hydrogen-bond network of water remained largely unclear. Here, we measure the ultrafast dynamics of OH groups of HDO molecules interacting with the deuterated hydroxide ion OD-. For solutions with OD- concentrations up to 4 M, we find that HDO molecules that are not directly interacting with the ions have a reorientation time constant of ∼2.7 ps, similar to that of pure liquid water. When the concentration of OD- ions is increased, the reorientation time constant increases, indicating a strong slowing down of the structural dynamics of the solution.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Linear IR spectra of 8% HDO isotopically diluted in NaOD/D2O solutions at four different concentrations. The spectra are normalized to the maximum of the OH stretch peak at 3400 cm–1 in neat D2O. The low frequency shoulder is assigned to OH groups of HDO molecules that form strong hydrogen bonds with OD– ions, while the high frequency shoulder OH– ions are weak hydrogen-bond donors. The inset shows the deuteroxide continuum far from the OH spectral region. The purple-shaded bar indicates the region where the transient absorption spectra were measured.
Figure 2
Figure 2
Transient spectra of aqueous solutions of NaOD in D2O at 0, 3, and 8 M concentrations. All samples contain 8% HDO. The dots indicate the experimental data, while solid lines are fits to the relaxation model described in the main text.
Figure 3
Figure 3
Parallel (blue), isotropic (yellow), and perpendicular (red) absorption change Δα as a function of delay time following the excitation of the OH vibration of HDO in a 3 M NaOD/D2O solution. The inset zooms in the isotropic signal at long delay times. The solid lines are fits to the models presented in the text.
Figure 4
Figure 4
Schematic energy level diagram of the model that describes the vibrational dynamics of hydroxyl groups around 3400 cm–1. Solid arrows indicate the channels for vibrational relaxation. The intermediate step represents the delayed adaptation of the hydrogen-bond network to the resulting vibrations from the fast relaxation process. The details of the model are described in the text.
Figure 5
Figure 5
Spectral decomposition of the transient absorption spectrum measured at 0.45 ps delay time for a solution of HDO and 3 M NaOD in D2O solution. The black circles represent the experimental data, and the gray curve results from a least-square optimization of the model given by eqs 3–5 to the experimental data.
Figure 6
Figure 6
Anisotropy as a function of delay time for NaOH/D2O solutions. The solid lines are fits to the experimental data via the model described in the main text.
Figure 7
Figure 7
Reorientation time constant of bulk-like HDO molecules in NaOD/D2O solutions as a function of the OD– concentration (blue squares). Bulk-like HDO molecules imply HDO molecules that are not directly hydrogen bonded to OD– ions, that is, HDO molecules outside the first hydration shell of the OD– ions. The dashed black curve shows the molecular reorientation time constant of bulk neat water. The dashed blue curve is a guide to the eye.
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