WATER

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WATER
Ira Waluyo Nilsson Group Stanford Synchrotron
Radiation Lightsource SASS Talk 10/14/09
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Ban DHMO!
www.dhmo.org
DHMO is a colorless and odorless chemical
compoundIts basis is the highly reactive
hydroxyl radical, a species shown to mutate DNA,
denature proteins, disrupt cell membranes, and
chemically alter critical neurotransmitters.

• Some dangers of DHMO
• Death due to accidental inhalation of DHMO,
  even in small quantities.
• Prolonged exposure to solid DHMO causes severe
  tissue damage.
• Gaseous DHMO can cause severe burns.
• Contributes to soil erosion.
• Often associated with killer cyclones in the
  U.S. Midwest and elsewhere, and in hurricanes
  including deadly storms in Florida, New Orleans
  and other areas of the southeastern U.S

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What is DHMO?
Dihydrogen monoxide H2O a.k.a water It is
everywhere Covers 2/3 of Earths
surface Comprises 50-65 of human body No water
no life
But it exhibits strange properties and its
liquid structure is still a mystery.
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Some Anomalies of Water
Water denser than ice
Density of the liquid higher than the solid
Normal liquid (ethanol, gasoline,etc) Solid more
dense than liquid
Normal liquid
At the bottom of the glass is 4 C water
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Some Anomalies of Water
High Boiling Point
Water should be a gas at room temperature Why not?
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Water and the Hydrogen Bond
H2O
3 Å
electrostatic interaction
tetrahedral coordination
Seems simple so farwhats the fuss about?
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The Controversy Mixture vs. Continuum Model
Ice Tetrahedral structure
Old debate prior to 1980
Two extreme models for water
Mixture models Small number of different
species with well defined bond angles/lengths.

Continuum Models Infinite Network of disordered
tetrahedral water. MD simulations! 3.5
HB/molecule
Röntgen 1892
Mostly accepted picture
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X-ray Absorption Spectroscopy
XAS probes unoccupied states
Franck-Condon Principle Electronic excitation
time scale much faster than nuclear motion Atoms
can be considered frozen during excitation XAS
represents a snapshot structure
Dipole selection rule O1s ? O2p
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Water Structure from XAS
Single donor (asymmetrically distorted H-bonds)
Double donor (tetrahedral-like H-bonds)
Some people were not very happy about this.
Pre-edge sensitive to distorted/broken
H-bond Post-edge sensitive to intact H-bond
Cavalleri et al. Chem. Phys. Lett. 2002, 364,
363 Wernet et al. Science 2004, 304, 995
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X-ray Emission Spectroscopy
XES probes occupied states
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
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X-ray Emission Spectroscopy
XES probes occupied states
Tetrahedral 20-30
Distorted 70-80
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
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Small Angle X-ray Scattering
Probe for density variations in liquids on the
nanometer scale
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Small Angle X-ray Scattering of Liquid Water
Experimental Water
Hypothetical Water Homogeneous
Minimum gives us size
10-20 Ångstrom in Size
Enhancement showing heterogeneity
Huang et al., PNAS 2009, 106, 36
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Mixture Model
LDL
HDL

• The two components in liquid water
• High density liquid water (HDL) disordered
• Low density liquid water (LDL) ordered

In pure liquid water, LDL and HDL structures
interconvert continuously
Huang et al., PNAS 2009, 106, 36
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What happens when the H-bond network is
disrupted? e.g. temperature increase, addition of
salt
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Temperature Dependence
XAS
XES
Increased pre-edge and main edge, decreased
post-edge Double donor (LDL) converted to single
donor (HDL) ? H-bond breaking Existing HDL
thermally excited (becomes more gas-like)
Ratio of 1b1 to 1b1 peak increase Consistent
with XAS (LDL converted to HDL) 1b1 peak shifts
closer to gas phase Also consistent with XAS (HDL
thermally excited, more gas-like)
Huang et al., PNAS 2009, 106, 36
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Salt Addition
Näslund et al., J. Chem. Phys. A 2005, 109, 5995
KCl and AlCl3 have opposite effects
KCl similar to temperature increase More
distorted H-bonded species K
structure-breaker
AlCl3 shifts spectrum to higher energy More
strongly H-bonded species Al3 structure-maker
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Conclusion
Water is more complicated that it seems Current
textbook picture is wrong
Continuum Models Infinite Network of disordered
tetrahedral water. MD simulations! 3.5
HB/molecule
WRONG!
Controversial but supported by experiments and
simulations