WATER Ira Waluyo Nilsson Group Stanford Synchrotron Radiation Lightsource SASS Talk 10/14/09

Ban DHMO! “DHMO is a colorless and odorless chemical compound…Its 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

What is DHMO? “Dihydrogen monoxide” H 2 O a.k.a water It is everywhere Covers 2/3 of Earth’s surface Comprises 50-65% of human body No water = no life But it exhibits strange properties and it’s liquid structure is still a mystery….

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 At the bottom of the glass is 4 °C water Sssssssssssssssssssssss s Temperature/ °C ddd ddd density Normal liquid

Some Anomalies of Water Temperature °C Molecular mass Room Temp H2OH2O SnH 4 GeH 4 SiH 4 CH 4 H2SH2S H 2 Po H 2 Te H 2 Se Water should be a gas at room temperature Why not? High Boiling Point

Water and the Hydrogen Bond H2OH2O 3 Å O-H chemical bonds Lone pairs   electrostatic interaction -- ++ 2 Å1 Å tetrahedral coordination Seems simple so far…what’s the fuss about?

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 Ice Tetrahedral structure Two extreme models for water Röntgen 1892 Mostly accepted picture Old debate prior to 1980 The Controversy: Mixture vs. Continuum Model

X-ray Absorption Spectroscopy XAS: probes unoccupied states Dipole selection rule O1s  O2p Franck-Condon Principle Electronic excitation time scale much faster than nuclear motion Atoms can be considered frozen during excitation XAS represents a snapshot structure

Water Structure from XAS 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 ambient 60-75%25-40% Single donor (asymmetrically distorted H-bonds) Double donor (tetrahedral-like H-bonds) Some people were not very happy about this….

X-ray Emission Spectroscopy XES: probes occupied states Hypothetical Water Homogeneous Gas Ice 1b 1 Increasing hydrogen bonding Tokushima et al. Chem. Phys. Lett. 2008, 460, 387

X-ray Emission Spectroscopy XES: probes occupied states Tetrahedral: 20-30% Distorted: 70-80% Experimental Result Real Water Two peaks = two components Gas Ice 1b 1 Increasing hydrogen bonding Tokushima et al. Chem. Phys. Lett. 2008, 460, 387

Small Angle X-ray Scattering Probe for density variations in liquids on the nanometer scale

Small Angle X-ray Scattering of Liquid Water Enhancement showing heterogeneity Ångstrom in Size Hypothetical Water Homogeneous Experimental Water Minimum gives us size Huang et al., PNAS 2009, 106, 36

Mixture Model The two components in liquid water: High density liquid water (HDL) – disordered Low density liquid water (LDL) – ordered HDL LDL In pure liquid water, LDL and HDL structures interconvert continuously Huang et al., PNAS 2009, 106, 36

What happens when the H-bond network is disrupted? e.g. temperature increase, addition of salt

Temperature Dependence XASXES 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 1b 1 ” to 1b 1 ’ peak increase Consistent with XAS (LDL converted to HDL) 1b 1 ” peak shifts closer to gas phase Also consistent with XAS (HDL thermally excited, more gas-like) Huang et al., PNAS 2009, 106, 36

Salt Addition Näslund et al., J. Chem. Phys. A 2005, 109, 5995 KCl and AlCl 3 have opposite effects KCl: similar to temperature increase More distorted H-bonded species K + : “structure-breaker” AlCl 3 shifts spectrum to higher energy More strongly H-bonded species Al 3+ : “structure-maker”

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 Mixture models “Small number of different species with well defined bond angles/lengths.” WRONG! Controversial but supported by experiments and simulations