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
bonds holding molecules together
many-fold stronger than the
intermolecular forces of attraction
molecules.
covalent
and are
weaker
between
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
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
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 kJ 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.
1YPC rendered in PyMol
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.