Physical Chemistry

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Chapter VI Interaction between Molecules
Physical Chemistry
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6-1 Intermolecular interaction
Weak interactions between molecules
1873---van der Waals 1910 Noble Prize in Physics
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• (1) Electrostatic interaction

Keesom 1912 Dipole-dipole interactions
between two polar molecules
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• (2) Dipole-Induced-dipole interaction

Debye 1920-1921 A molecule with permanent
dipole can induce a dipole of a neighboring
polarizability molecule (polarizability a). The
interaction of the induced dipole with the
permanent dipole can be written as
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For similar molecules, if ?1?2?, ?1?2?, then
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• (3) Dispersion interaction

Transient dipole interactions 1930----London
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(4) Total Energy of intermolecular interactions
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Partition of van der Waals interaction
Dipole moment
Polarizability
molecule
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The interaction energy of AB can be obtained
using Variational Principle (Quantum mechanics)
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• (5) Intermolecular potential energy

(n8-16)
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n6, m12
Lennard-Jones potential energy
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Lennard-Jones potential energy curve
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Van der Waals radius
Primary alkane
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6-2 Intermolecular interactions of gas
For ideal gas
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(1) Real gas and van der Waals equation
For ideal gas, Z1
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Ideal gas
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Virial equation of state
B, C, D the second, third and fourth Virial
coefficient
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Van der Waals equation
Vm V / n
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(2) Critical and supercritical
Critical point Critical pressure Critical volume
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(3) Corresponding state law
Reduced variables
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nitrogen
methane
propane
ethylene
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At the critical point, pr, Tr and Vr all equal to
1
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6-3 Intermolecular interactions in liquid
1. The structure of liquid and radial
distribution function J(R)
The structure of liquid is the spatial
distribution and arrangement of liquid molecules
Long range---- disordered Short range--- ordered
but components vary all the time
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The structure of liquid can usually be described
by the radial distribution function, J(R) or the
pair-correlation function g(R)
---average particle density of liquid
J(R)dR is the probability of finding another
particle within a spherical shell with radius R
and thickness dR defined by the center particle.
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Peak is close to the minimum of U2(R)
wide-shell structure
g2(R) 0 due to molecular repulsion
Typical radial distribution curve of simple liquid
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The structure of liquid mainly depends on the
density of liquid and is less perturbed by
temperature
Ar
The J(R) curves of liquid Ar at different
temperatures
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Ar
The J(R) curves of liquid Ar at different
densities
As the increase of liquid density, the
population of short-range ordered structures also
increases
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if define the coordination number of liquid
(Z) as the number of particles at the first
coordination spherical shell, then Z is
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2. Measurement and calculation of J(R)
The radial distribution function can be
determined experimentally by X-ray or neutron
diffraction
Diffraction intensity can be correlated with
diffraction angle
where
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Theoretical calculation of J(R)
VNpotential energy
Molecular dynamics Monte Carlo simulation
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6-4 Supermolecule chemistry and molecular
assembly
Supermolecule chemistry is concerned with
molecular assemblies with special structure and
function, consisting of two or more chemical
species glued together through intermolecular
interaction.
1. Supermolecule
several components donor and acceptors
a vast number of components
2. Supermolecule assembly
such as films, colloids
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Intermolecular interactions in supermolecule

• Electrostatic
• Hydrogen bonding
• Metal-ligand interaction
• ?-? stacking
• Induced dipole-induced dipole interaction
• Hydrophobic effect

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Molecular recognition Supermolecular
self-assembly
Donor and acceptor selectivity
Ordered structure
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Crown ether and cryptand
Molecular recognition between crown ether and NH4
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Hydrogen bonding recognition and self-assembly