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 mediaVacuum 1

Mylar 3

Glass 5-10

Benzene 4

Water 80Air (1 atm) 1.0006Air (102 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 viewVan der Waals representation

Electron density (side view)

Electron density (end view)

R = 2.976 (+0.000, -0.030) 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.

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142-5147.

1YPC rendered in PyMol

Average Root Mean Square Deviation of the ca positions for the protein vs. time (a measure of overall protein structure)

Nonpolar Surface Accessible Surface Area vs. time (a measure of protein “openness”

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142-5147.

Figure 1

Figure 3

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142-5147.

water

urea

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142-5147.

Figure 5