Extraordinary Properties of...
...An Ordinary Water
Structure of the Water Molecule
Water is a V-shaped molecule with the molecular
formula H2O known since 1781. The chemical
composition: two parts phlogiston (later the gas he
made was proven to be hydrogen) to one part
dephlogisticated air (later this was proven to be
oxygen) was discovered by Henry Cavendish
(1731-1810).
Molecular diameter is about 2.75 Å.
In the liquid state the hydrogen atoms are
constantly exchanging between water molecules
due to protonation/deprotonation processes. It
depends on the other ions present in solution
(Both acids and bases catalyze this exchange).
The rate of this process is characterized by pH
constant. At its slowest rate (at pH 7), the average
time for the atoms in an H2O molecule to stay
together is only about a millisecond.
Structure of the Water Molecule
The water molecule is often described in school and undergraduate textbooks of as
having four, approximately tetrahedrally arranged, sp3-hybridized electron pairs, two
of which are associated with hydrogen atoms leaving the two remaining lone pairs.
In a perfect tetrahedral geometry
the bond-bond, bond-lone pair and
lone pair-lone pair angles would all
be 109.47° and such tetrahedral
bonding patterns are found in
condensed phases such as
hexagonal ice.
Water Molecule: Orbitals
A single atom has a set of electron energy levels and
corresponding electronic orbitals.
In a molecule these orbitals mix to make chemical
bonds.
The five occupied and the lowest three unoccupied
molecular orbitals of the isolated water molecule
s
p
Structure of the Water Molecule
The approximate shape and charge distribution of
an isolated water molecule is different from
tetrahedral: the calculated O-H length is 0.958 Å
and the H-O-H bond-bond angle is 104.500°.
How much electron density is on the oxygen and on
the hydrogen: the iso-electron surfaces are shown.
The average electron density around the oxygen
atom is about 10x that around the hydrogen atoms.
The approximate shape and
charge distribution of an
isolated water molecule.
Water Dimer
The most energetically favorable water
dimer (β is close to as expected if the lone
pair electrons were tetrahedrallly placed =
109.47°/2).
A section through the electron density
distribution (high densities around the
oxygen atoms have been omitted for
clarity).
This shows the tetrahedrality of the bonding
in spite of the lack of clearly seen lone pair
electrons. This tetrahedrality is primarily
caused by electrostatic effects (that is,
repulsion between the positively charged
non-bonded hydrogen atoms) rather than
the presence of tetrahedrally placed lone
pair electrons.
The hydrogen-bonded proton has reduced
electron density relative to the other
protons.
Water Dimer
The most energetically favorable water
dimer (β is close to as expected if the lone
pair electrons were tetrahedrallly placed =
109.47°/2).
The hydrogen-bonded proton has reduced
electron density relative to the other
protons.
Water Hydrogen Bonding
A hydrogen bond is a special type of bond that exists between an electronegative atom and a hydrogen atom (already bonded to another atom).
Typically this occurs where the partially positively charged H+
lies between partially negatively charged O- and N- atoms.
Recently hydrogen bond is also found between fluorine atoms in HF2 and
between water and the smaller halide ions F-, Cl- and Br- (HO-H····Br-)
Carboxylic
acid dimer
In water the hydrogen atom is
covalently attached to the oxygen of a
water molecule (492. kJ/mol) but has
an additional attraction (about 23.3
kJ/mol; almost 5 x the thermal energy
at 25°C) to a neighboring oxygen atom
of another water molecule that is far
greater than any included van der
Waals interaction.
Hydrogen bond cooperativity
When a hydrogen bond forms, the redistribution of electrons changes the ability for
further hydrogen bonding. The water molecule donating the hydrogen atom has
increased electron density in its 'lone pair' region, which encourages hydrogen bond
acceptance (the cooperativity), and the accepting water
molecule has reduced electron density centered on its
hydrogen atoms and its remaining 'lone pair' region, which
encourages further donation but discourages further
acceptance of hydrogen bonds (the anticooperativity).
Water molecules surrounded by four
hydrogen bonds tend to clump together,
forming clusters (for both statistical and
energetic reasons). Hydrogen bonded
chains (that is, O-H····O-H····O) are
cooperative; the breakage of the first bond
is the hardest, then the next one is
weakened, and so on.
Water Hydrogen Bonding
From liquid water simulations at 25 °C, it was estimated that each water molecule
participates in an average of 2.4 - 3.6 hydrogen bonds (depending on the model). At
100 °C, this number decreases by approximately 1/3 of the bond due to the increased
molecular motion and decreased density, while at 0 °C, the average number of
hydrogen bonds increases a little bit.
In our own simulations we saw
that the 3 of HBs varies when the
water is restricted in motion inside
a quantum reef of nanotubes
Hydrogen Bonding: DNA
Hydrogen bonding also plays an important role in determining the 3D structures
adopted by proteins and nucleic bases. Bonding between parts of the same
macromolecule cause it to fold into a specific helical (or other) shape, which strongly
determines the molecule's physiological and biochemical role. The double helical
structure of DNA, for example, is due largely to hydrogen bonding between the base
pairs, which link one complementary strand to the other and enable replication.
In proteins, hydrogen bonds form between the backbone
oxygens and amide hydrogens.
Hydrogen bonding between guanine and cytosine, one of
two types of base pairs in DNA.
Water Ionization
Water molecules ionize endothermically due to electric field fluctuations caused by
nearby dipole librations resulting from thermal effects, and favorable localized
hydrogen bonding. Ions may separate but normally recombine within a few fs.
Rarely (about once every eleven hours per molecule at 25°C, or less than once a
week at 0°C) the localized hydrogen bonding arrangement breaks before allowing the
separated ions to return, and the pair of ions (H+, OH-) hydrate independently and
continue their separate existence for about 70 µs.
hydroxide is another molecule
resulting from a dissociation of a
water; exists in solvated forms
Hydronium does not exist in a free
state: it is extremely reactive and is
solvated by water: H3O+(H2O)6
sometimes in H3O+(H2O)20 (magic
number cluster). It is called magic
because of its increased stability
due to high symmetry
dodecahedral cage
Water phase anomalies
1. Water has unusually high melting point.
2. Water has unusually high boiling point.
3. Water has unusually high critical point.
4. Solid water exists in a wider variety of stable (and metastable) crystal and
amorphous structures than other materials.
5. The thermal conductivity of ice reduces with increasing pressure
6. The structure of liquid water changes at high pressure
7. Supercooled water has two phases and a second critical point at about -91°C.
8. Liquid water is easily supercooled but glassified with difficulty.
9. Liquid water exists at very low temperatures and freezes on heating.
10. Liquid water may be easily superheated.
11. Hot water may freeze faster than cold water; the Mpemba effect.
12. Warm water vibrates longer than cold water.
Water density anomalies
1. The density of ice increases on heating (up to 70 K)
2. Water shrinks on melting
3. Pressure reduces ice's melting point
4. Liquid water has a high density that increases on heating (up to 3.984°C).
5. Pressure reduces the temperature of maximum density.
6. There is a minimum in the density of supercooled water.
7. Water has a low coefficient of expansion (thermal expansivity).
8. Water's thermal expansivity reduces increasingly (becoming negative) at low to.
9. Water's thermal expansivity increases with increased pressure.
10. The number of nearest neighbors increases on melting.
11. The number of nearest neighbors increases with temperature.
12. Water has unusually low compressibility.
13. The compressibility drops as temperature increases up to 46.5°C.
14. There is a maximum in the compressibility-temperature relationship.
15. The speed of sound increases with temperature up to 74°C.
16. The speed of sound may show a minimum.
17. 'Fast sound' is found at high frequencies and shows an discontinuity at higher
pressure.
18. NMR spin-lattice relaxation time is very small at low temperatures.
19. The refractive index of water has a maximum value at just below 0°C.
20. The change in volume as liquid changes to gas is very large.
Water material anomalies
1. No aqueous solution is ideal.
2. D2O and T2O differ significantly from H2O in their physical properties.
3. Liquid H2O and D 2O differ significantly in their phase behavior.
4. Solutes have varying effects on properties such as density and viscosity.
5. The solubilities of non-polar gases in water decrease with temperature to a
minimum and then rise.
6. The dielectric constant of water is high.
7. The dielectric constant shows a temperature maximum.
8. Proton and hydroxide ion mobilities are anomalously fast in an electric field.
9. The electrical conductivity of water rises to a maximum at about 230°C.
10. Acidity constants of weak acids show temperature minima.
11. X-ray diffraction shows an unusually detailed structure.
12. Under high pressure water molecules move further away from each other with
increasing pressure.
Water thermodynamic anomalies
1. The heat of fusion of water with temperature exhibits a maximum at -17°C.
2. Water has over twice the specific heat capacity of ice or steam.
3. The specific heat capacity (CP and CV) is unusually high.
4. The specific heat capacity CP has a minimum at 36°C.
5. The specific heat capacity (CP) has a maximum at about -45°C.
6. The specific heat capacity (CP) has a minimum with respect to pressure.
7. The heat capacity (CV) has a maximum.
8. High heat of vaporization.
9. High heat of sublimation.
10. High entropy of vaporization.
11. The thermal conductivity of water is high and rises to a maximum at about 130°C.
Water physical anomalies
1. Water has unusually high viscosity.
2. Large viscosity increase as the temperature is lowered.
3. Water's viscosity decreases with pressure below 33°C.
4. Large diffusion decrease as the temperature is lowered.
5. At low temperatures, the self-diffusion of water increases as the density and
pressure increase.
6. The thermal diffusivity rises to a maximum at about 0.8 GPa.
7. Water has unusually high surface tension.
8. Some salts give a surface tension-concentration minimum; the Jones-Ray effect.
9. Some salts prevent the coalescence of small bubbles.