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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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.




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


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


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.


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


Induced Dipole-Induced Dipole Interactions

Induced Dipole-Dipole Interactions

Dipole-Dipole Interactions



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°.



Flicker Clusters

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






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. USA100, 5142-5147.



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.


Figure 3

water

urea

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


Figure 5

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


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