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Intermolecular Forces

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Intermolecular Forces. Intermolecular forces are weak, short-range attractive forces between atoms or molecules. Intermolecular forces ultimately derive from the electrostatic properties of molecules.

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Presentation Transcript
slide2
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.

slide3
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.

slide6
Summary of Intermolecular Forces

Coulombic interactions

Van der Waals interactions (London dispersion forces)

Hydrogen bonds

Hydrophobic effect

slide7
Coulomb's Law

o is the permittivity of the medium, also known as the dielectric constant

slide8
Vacuum (o = 1) has little effect on Coulombic interactions

Water (o = 80) significantly dampens Coulombic interactions

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.

Dielectric constants of common media

Glass 5-10

Vacuum 1

Air (1 atm) 1.0006

Water 80

Air (102 atm)1.0548

Mylar 3

Benzene 4

slide9
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.
slide10
Attraction due to induced dipoles in outer electron shells

-

+

-

+

Strong repulsion as outer electron shells begin to overlap

Van der Waals radius

Atomic radius

The London-Jones Thought Experiment

Infinite distance– no interaction

slide11
Induced Dipole-Induced Dipole Interactions

Induced Dipole-Dipole Interactions

Dipole-Dipole Interactions

slide13
The Structure of Water

Conventional view

Van der Waals representation

Electron density

(side view)

Electron density

(end view)

slide14
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°.

slide16
Flicker Clusters

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

slide21
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.

slide24
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. USA100, 5142-5147.

slide25
Figure 3

water

urea

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA100, 5142-5147.

slide26
Figure 5

Bennion and Daggett (2003) Proc. Natl. Acad. Sci. USA100, 5142-5147.

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