1. Intermolecular Forces

2. Intermolecular forces are weak, short-range attractive forces between atoms or molecules.
Intermolecular forces ultimately derive from the electrostatic properties of molecules.
Although intermolecular forces are weak, they result in significant effects on the physical properties of molecules because these forces are additive.

3. Intramolecular bonds refer to the covalent bonds holding molecules together and are many fold stronger than the weaker intermolecular forces of attraction between molecules.
The strength of intermolecular forces between molecules is inversely proportional to the thermal energy of the system.

4. Vibrational and Rotational Motion of Ethane

5. Relative Motion of Water Molecules

6. Summary of Intermolecular Forces
Coulombic interactions
Van der Waals interactions (London dispersion forces)
Hydrogen bonds
Hydrophobic effect

7. Coulomb's Law
o is the permittivity of the medium, also known as the dielectric constant

8. Dielectric constants of common media
Dielectric constants are related to the polarity and polarizability of the medium, that is the ability of the medium to diminish the force between two point charges at a constant distance d.
Vacuum (o = 1) has little effect on Coulombic interactions
Water (o = 80) significantly dampens Coulombic interactions
Dielectric constants of common media
Vacuum
Glass
Air (1 atm)
Water
Air (102 atm)1.0548
Mylar
Benzene

9. Van der Waals interactions arise from weak electrostatic forces that act over a short distance, generally near the point of physical contact. These forces ultimately rely on the inherent repulsive force of the outer electron clouds of molecules and its inherent polarizability.

10. The London-Jones Thought Experiment
Infinite distance– no interaction + -
Attraction due to induced dipoles in outer electron shells
Strong repulsion as outer electron shells begin to overlap
Van der Waals radius
Atomic radius

11. Induced Dipole-Induced Dipole Interactions
Induced Dipole-Dipole Interactions
Dipole-Dipole Interactions

12. Ion-Dipole Interaction

13. The Structure of Water Conventional view Van der Waals representation
Electron density
(side view)
Electron density
(end view)

14. Geometry of the Hydrogen Bond
R = (+0.000, ) Å, α = 6 ± 20°, β = 57 ± 10°; α is the donor angle and β is the acceptor angle. The dimer (with slightly different geometry) dipole moment is 2.6 D. Although β is close to as expected if the lone pair electrons were tetrahedrallly placed (109.47°/2), the energy minimum (~21 kJ mol-1) is broad and extends towards β = 0°.

15. Extended Hydrogen Bonding Structures

16. Flicker Clusters
Flicker clusters are short-lived local areas of order water within an otherwise disordered bulk solution.

17. Relative Motion of Water Molecules

18. Structure of Ice

19. Space Filling Model of Liquid and Solid Water
Liquid water
Solid water

20. Hydration Spheres Surrounding Ions

21. Hydrophobic Effect
The hydrophobic effect is an entropically-driven association of hydrophobic molecules that is a direct consequence of the polar nature of water and it propensity for hydrogen bonding.