Intermolecular Forces and the Experiment

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Intermolecular Forces and the Experiment

• Dr. Shawn Sendlinger
• Department of Chemistry
• North Carolina Central University

2
Results from WebMO (PM3)

• Dipole moments (D) Electrostatic potentials

0.001
1.739
1.526
2.781
3
Nonpolar Interactions

• London Dispersion Forces always present
• Depends on the polarizability
• The ease with which the electron distribution in
  an atom or molecule can be distorted
• Larger molecules have greater polarizabilities
  (more electrons). Dispersion forces a MW

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London Dispersion Forces

• Also depends on molecular shape

Neopentane C5H12
n-Pentane C5H12
B.P. 282.7 K
B.P. 309.4 K
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Dipole-Dipole Forces

• Present between polar molecules
• This force increases with increasing polarity
  (for molecules of equal mass and size).

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Hydrogen Bonding

• Present when highly polar bonds exist
• N-H O-H F-H
• Intermolecular attraction between d H atom of
  one molecule and the d- heteroatom (N, O, or F)
  in another molecule

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Interactive Visualization

• Molecular Workbench
• Download at http//mw.concord.org/modeler/index.h
  tml
• We will look at portions of the Intermolecular
  Forces activity

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Pairwise Interactions

• London forces always present
• Relative interaction strength

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Properties Intermolecular Forces

• Boiling Points Nonpolar vs. Polar
• Oil and Water

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Other Effects

• Increasing polarity
• Shape effects

3-hexanol 1.59 D
1,3-hexanediol 3.01 D
1,3,5-hexanetriol 4.84 D
(PM3)
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Hydrogen Bonding

• Phase changes
• Hydrogen bonding in DNA

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Designing an Antibody

• Shape and charge in unison

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Gecko Toes

• An application of dispersion forces

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Penny Experiment Conclusions

• Good macroscopic demonstration of intermolecular
  forces
• Uses of computation
• Static (WebMO)
• Calculate and visualize dipole moments
• Visualize charge distribution (electrostatic
  potential)
• Dynamic (Molecular Workbench)
• Tool for interactive investigation of the forces
  at the molecular level
• Provides further applications (antibody design)
  and explanations (gecko toes)

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• Computation is great. . . . . . .
• . . . . . but dont forget to work out
  occasionally!

Moodle
WebMO
Molecular Workbench