Intermolecular Forces Intermolecular forces are weak shortrange attractive

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 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 Air (1 atm) 1. 0006 Air (102 atm)1. 0548 Mylar 3 Benzene 4 Glass Water 5 -10 80

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.

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

Induced Dipole-Induced Dipole Interactions Induced Dipole-Dipole Interactions

Ion-Dipole Interaction

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

Geometry of the Hydrogen Bond 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 k. J mol-1) is broad and extends towards β = 0°.

Extended Hydrogen Bonding Structures

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

Relative Motion of Water Molecules

Structure of Ice

Space Filling Model of Liquid and Solid Water Liquid water 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.

1 YPC rendered in Py. Mol

Figure 1 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 3 water urea Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142 -5147.

Figure 5 Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA 100, 5142 -5147.