Molecular interactions

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Molecular interactions

• Yongsik lee
• June 2009

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Residual interaction

• Complete valence shells are still able to
  interact with one another
• Van der Waals
• Hydrogen bodning

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Interaction between partial charges

• Coulombs law
• V q1q2/4peor (in vacuum)
• Where qi the partial charge
• Where r the separation
• Where e permittivity of the medium

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frequency response of e

• frequency response of various dielectric
  mechanisms in terms of the real and imaginary
  parts of the permittivity.
• Image used with the consent of Prof. Kenneth A.
  Mauritz

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Polarizability, Relative Permittivity, Refractive
Index
Clausius-Mossotti equation
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• Permittivity e ereo
• Where er is relative permittivity
• Where eo is vacuum permittivity
• At room temperature, er is 78

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Electrical Dipole Moments
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Dipole moment

• µ qR
• Represent by an arrow (vector)
• Length magnitude of dipole moment
• Pointing (-) charge to () charge
• But the other way is still used
• Unit
• D (debye)
• 1 D 3.33564 x 10-30 Cm
• Electric chages e and e by 100 pm 4.8 D

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Polar vs. non-polar molecules
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Estimating µ

• Pauling electronegativities
• µ/D ??
• If an atom attracts the electrons more strongly,
  the more electronegative atom is (-) of µ

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Dispersion of Refractive Index
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Total Dipole moments mol geometry
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Interactions between Dipoles
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a charge interacts with a dipole

• calculate all the interactions as if they were
  merely a series of point charges. We start with
  the Coulombic potential energy of two charges
• We need to add up all the attractions and
  repulsions in this three charge system to arrive
  at a potential of a system pictured above of a
  charge q2 and a dipole µ1 with charges q1.

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Intermolecular forces between µ
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Induced dipole moments

• Temporary induced dipole moment, µ
• Result of the influence of an electric field
• Electron distribution of the molecule
• Polarizable
• Magnitude of µ
• µ a?
• Polarizability (a) depends on the distortion
  caused by E

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Polarizability volumes, a

• Dimensions of volume
• a a/(4peo)
• Difference between HOMO and LUMO

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Dipole-induced dipole interaction

• Induced dipole, µ, follows the changing
  orientation of the permanent dipole, µ
• V -µ12a2/pe0r6

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Induced dipole induced dipole

• Dispersion force (London force)
• the attraction
• between temporarily induced dipoles in non-polar
  molecules
• often disappear within an instant).
• The polarization can be induced either
• by a polar molecule
• by the repulsion of negatively charged electron
  clouds in non-polar molecules
• At any given point in time the electrons may be
  more concentrated in an electron cloud closer to
  one atom, causing a fractional, momentary
  negative charge

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quadrupole

• Electric quadrupole

• Magnetic quadrupole

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Dipole-Dipole interaction (Keesom
interaction) Dipole-Induced dipole
interaction (Debye interaction) Induced
dipole-Induced dipole interaction) (London
interaction, Dispersion) Total Van der Waals
attractive interaction Keesom Debye London
Additional hydrogen-bonding can be considered.
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• Dipole moment? polarizability? ??? ?? VDW? ??
• ? ??? ??? ?? Keesom, Debye, London? ?? ??
• London interaction? ???
• ?? ???

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Molecular structure

• All bonds are stronger than all intermolecular
  forces.
• Systems that use bonding only will have the
  strongest structures.
• Systems that use bonding and intermolecular
  forces will have weaker structures, where the
  strength will depend upon the type of
  intermolecular force being used.
• Systems that use intermolecular forces and no
  bonding will have the weakest structures known
  examples of these would be Noble Gases.

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Overall Potential Energy
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Various Models for Overall Potential
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Morse potential
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Morse potential

• The Morse potential
• Philip M. Morse
• a convenient model for the PE of a diatomic
  molecule
• a better approximation for the vibrational
  structure of the molecule
• the quantum harmonic oscillator
• Property
• bond breaking, such as the existence of unbound
  states
• the anharmonicity of real bonds
• the non-zero transition probability for overtone
  and combination bands

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Lennard-Jones potential

• between a pair of neutral atoms or molecules.
• John Lennard-Jones(1924)

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Lennard-Jones 12,6 Potential
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Potential energy surface