Hydrogen bond dynamics in aqueous NaBr solutions
- Sungnam Park†,‡ and
- M. D. Fayer†,§
- †Department of Chemistry, Stanford University, Stanford, CA 94305; and
- ‡Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, CA 94025
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Contributed by M. D. Fayer, August 19, 2007 (received for review July 27, 2007)
Abstract
Hydrogen bond dynamics of water in NaBr solutions are studied by using ultrafast 2D IR vibrational echo spectroscopy and polarization-selective IR pump–probe experiments. The hydrogen bond structural dynamics are observed by measuring spectral diffusion of the OD stretching mode of dilute HOD in H2O in a series of high concentration aqueous NaBr solutions with 2D IR vibrational echo spectroscopy. The time evolution of the 2D IR spectra yields frequency–frequency correlation functions, which permit quantitative comparisons of the influence of NaBr concentration on the hydrogen bond dynamics. The results show that the global rearrangement of the hydrogen bond structure, which is represented by the slowest component of the spectral diffusion, slows, and its time constant increases from 1.7 to 4.8 ps as the NaBr concentration increases from pure water to ≈6 M NaBr. Orientational relaxation is analyzed with a wobbling-in-a-cone model describing restricted orientational diffusion that is followed by complete orientational randomization described as jump reorientation. The slowest component of the orientational relaxation increases from 2.6 ps (pure water) to 6.7 ps (≈6 M NaBr). Vibrational population relaxation of the OD stretch also slows significantly as the NaBr concentration increases.
Footnotes
- §To whom correspondence should be addressed. E-mail: fayer{at}stanford.edu
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This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on May 1, 2007.
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Author contributions: S.P. and M.D.F. designed research; S.P. performed research; S.P. and M.D.F. analyzed data; and S.P. and M.D.F. wrote the paper.
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The authors declare no conflict of interest.
- Abbreviations:
- MD,
- molecular dynamics;
- FT,
- Fourier transform;
- AOT,
- sodium bis(2-ethylhexyl)sulfosuccinate;
- FFCF,
- frequency–frequency correlation function;
- CLS,
- center line slope.
- © 2007 by The National Academy of Sciences of the USA
