Intermolecular Forces

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.

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.

Vibrational and Rotational Motion of Ethane

Relative Motion of Water Molecules

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

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

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 1 Mylar 3 Glass 5-10 Benzene 4 Water 80Air (1 atm) Air (10 2 atm)1.0548

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.

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

Induced Dipole-Induced Dipole Interactions Induced Dipole-Dipole Interactions Dipole-Dipole Interactions

Ion-Dipole Interaction

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

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°. Geometry of the Hydrogen Bond

Extended Hydrogen Bonding Structures

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

Relative Motion of Water Molecules

Structure of Ice

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

Hydration Spheres Surrounding Ions

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.