Water

Even though water is a relatively simple molecule, the third most abundant molecule in the Universe (after H2 and CO), and the most abundant molecule on Earth, it’s science is extremely complicated, let’s have a look

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» Water molecule

Atomic formula

Water is a tri atomique molecule made of 1 Oxygen et 2 Hydrogens.

Hydrogen

Hydrogen is the must abundant atoms in the Universe (by far). 1 proton, 1 electron, couldn’t be more simple

• Isotopes:

  • Deuterium (D)

  • Tritium (T)

• ISM abundances

Oxygen

Oxygen however is a bit more heavy and it’s isotopes are keys to unveil history of the Solar System []

• Isotopes

  • O¹⁶

  • O¹⁷

  • O¹⁸

• ISM abundances

Molecular structure

OH covalent bonds

Fig. 20 Explain: source Wikimedia

Geometry: HOH angle = 104.48 deg (Hoy 1979) - different than in tetrahedral configuration (109.47). Each molecule is electrically neutral but also polar due to the Oxygen atom having a higher electronegativity than the two Hydrogens. Length

Electron density: The Oxygen has 2 lone pair of electrons.

Strength

Oxygen lone electron pairs

Fig. 21 Explain: source Wikimedia

tetraedral

Note

• The lone electron pair may not be similar (2s vs 2p orbitals), unless they are degenerated.

How would that affect the strength (geometry) of the Hydrogen bonds ?

Molecular Orbitals for water

Fig. 22 Explain: source

Properties

Water is known for it’s anomalous properties including a So let’s have a look to it’s molecular properties to see how they arise

Molecular vibration

Polarizability leads to an induced Dipole moment that makes the water molecule IR active.

3 fundamental atomic vibration modes:

• (v1) Symmetric Stretching
• (v3) Assymetric Stretching
• (v2) Bending

Fig. 23 Explain: source Wikimedia

Fig. 24 Explain: source Wikimedia

Hydrogen bonding

One of the most crucial propertie of water is it’s ability to form Hydrogen bonds (with itself or other molecules/atoms)

Note

• How is Hydrogen Bonding affecting the molecular vibration shown above is one of the key research questions we are trying to adress

Solvatation

More ?

Phase Diagram

Water can exist in all three 3 physical states under Earth conditions. We are quite familiar with liquid water, however I would like to point your attention on the solid phase of water, ice. Many polymorphs, the one you know being Hexagonal ice (Ih).

Phase lines on this phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist in any relative proportions (having the same Gibbs free energy and identical chemical potential)

Fig. 25 Explain: source Wikimedia

Water models

Now we have seen some of water properties let’ have a look at how scientists model the water molecule to push further their investigations. A key point is that:

• There is no universal water model that can explain all the water properties

[] reviewed the different models created over time (up to 2015) and classified them in 4 groups:

• 1
• 2
• 3
• 4

Fig. 26 Explain: source []

The Lennard-Jones relationship

• Models for individual molecules to perform Molecular Dynamics simulations.

» Gas

Gas: weakly interacting molecules

Thermodynamic properties

» Cluster

Hydrogen Bonding

With two other water molecules

• DDAA
• …

Small clusters

Large clusters

Network Description

How do we describe an assembly of water molecules

• Protonated Water Cluster (PW)

Modeling Clusters

Thole-type model

• 1999
• 2002
• Hydrogen-bond pattern to characterize water network
• The dipole moment of a constituent molecule in a water cluster is enhanced depending on the local HB network around the water molecule

» Liquid

Note

• description of the science in liquid water …
• Low T vs High T liquid - subject of polemic

Constant breaking and reorganisation of individual hydrogen bonds on a picosecond timescale. However

Ice

» Overview

Ice rules (or Bernal–Fowler rules): basic principles that govern arrangement of atoms in water ice

• Each oxygen is covalently bonded to two hydrogen atoms
• The oxygen atom in each water molecule forms two hydrogen bonds with other water molecules

Hexagonal Ice

Ih

» Water ice polymorphism

» Amorphous vs Crystaline Ice

Comparison

Crystaline

• Polymorph: 18 ?
• Ice rules

Amorphous

• Polymorph: 5

Amorphous Solid Water

» Crystalisation

At molecular scales (few molecules up to 100 ?) - Matrix isolation techniques