Isotope effects in liquid water probed by x-ray Raman spectroscopy

U. Bergmann, D. Nordlund, Ph. Wernet, M. Odelius, L. G. M. Pettersson, and A. Nilsson

Phys. Rev. B 76, 024202 – Published 5 July 2007

Abstract

The isotope effect on the local structure of liquid water at room temperature is studied by x-ray Raman spectroscopy. The difference between the room-temperature spectra of liquid $H_{2} O$ and $D_{2} O$ is compared to the difference spectrum between liquid $H_{2} O$ at 22 and $2 ° C$ . The spectral changes between $H_{2} O$ and $D_{2} O$ can partly be attributed to structural changes similar to a temperature change, in agreement with diffraction data. Additionally, we find that isotope substitution affects the local asymmetry in the hydrogen-bonded network: hydrogen-bonding configurations are more asymmetric on the donor side for $H_{2} O$ than for $D_{2} O$ . A cluster model is used to computationally illustrate the spectral changes that arise due to the increased asymmetry, capturing all essential features in the difference spectrum. We infer from our study that quantum effects contribute to the formation of asymmetrical species in the liquid.

Received 1 May 2007

DOI:https://doi.org/10.1103/PhysRevB.76.024202

Authors & Affiliations

U. Bergmann¹, D. Nordlund¹, Ph. Wernet^1,2, M. Odelius³, L. G. M. Pettersson³, and A. Nilsson^1,3

¹Stanford Synchrotron Radiation Laboratory, P.O. Box 20450, Stanford, California 94309, USA
²BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany
³FYSIKUM, Stockholm University, AlbaNova, S-106 91 Stockholm, Sweden

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Vol. 76, Iss. 2 — 1 July 2007

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Images

Figure 1
(Color online) XRS spectra recorded from two independent experimental runs, (a) $H_{2} O$ at $22 ° C$ and (b) $D_{2} O$ at $22 ° C$ , and (c) the isotope difference spectra, $H_{2} O - D_{2} O$ at $22 ° C$ . Also, the isotope difference spectrum from the averaged spectra $(run 1 + run 2)$ is plotted for comparison. The error bars in the difference spectra, based on the Poisson distribution of total counts from the detector, are plotted with a confidence interval of 68%. Spectra from run 1 and run 2 are nearly identical, and the isotope difference spectra from the two runs are very well reproduced (Ref. 28).Reuse & Permissions
Figure 2
(Color online) XRS spectra of liquid $D_{2} O$ at $22 ° C$ [(a), solid line] and liquid $H_{2} O$ at $2 ° C$ [(b), solid line] compared to $H_{2} O$ at $22 ° C$ [(a) and (b), dashed line]. Top panels: Difference of the spectra in the bottom panels [(a) $D_{2} O - H_{2} O$ at $22 ° C$ , (b) $H_{2} O 2 ° C - H_{2} O$ $22 ° C$ ]. The error bars in the difference spectra, based on the Poisson distribution of total counts from the detector, are plotted with a confidence interval of 68% (Ref. 28).Reuse & Permissions
Figure 3
(Color online) O $K$ -edge XRS isotope and temperature difference spectra. The spectra are plotted without the error bars shown in Fig. 1. Vertical lines (gray) are drawn as a guide for the eyes for the preedge, the main edge, and the postedge. From top to bottom: (a) $H_{2} O 2 ° C - H_{2} O 22 ° C$ , (b) $D_{2} O - H_{2} O$ at $22 ° C$ , and (c) the $D_{2} O - H_{2} O$ difference at $22 ° C$ taken after a Gaussian broadening with full width at half maximum of $0.5 eV$ of the $D_{2} O$ spectrum (Ref. 28).Reuse & Permissions
Figure 4
(Color online) Comparison of model calculations with measured spectra. (Top) Calculated spectra illustrate the effect of changes to the H-bond lengths starting from two different geometries: an icelike (DD) cluster (dotted lines) with O-O distances of $2.80 Å$ and an asymmetric (SD) cluster (solid lines) generated by increasing the H-bond distance of one of the donors by $0.6 Å$ (O-O D1). Difference spectra of the distorted minus original cluster are shown for (a) elongation of both donating H bonds by $0.1 Å$ and (b) elongation of the weak donor H bond (O-O D1) by $0.1 Å$ and shortening of the strong donor H bond (O-O D2) by $0.1 Å$ (for DD species, the original D1 and D2 bonds are equivalent). (c) Similar to (b) but with a $0.2 Å$ elongation and $0.05 Å$ shortening. (Bottom) Experimental difference spectra of (d) $D_{2} O$ at $22 ° C$ minus $H_{2} O$ at $12 ° C$ (linearly interpolated from the spectra at 2 and $22 ° C)$ and (e) $D_{2} O$ at $22 ° C$ minus $H_{2} O$ at $2 ° C$ , representing the isotope effect with a temperature effect subtracted (Ref. 28).Reuse & Permissions

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