Delocalized Electrons

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Delocalized Electrons

• Resonance Structures
• always s bonds between nuclei, local electrons
• p bonds move depending on the structure
• The p electrons are delocalized
• Use LE model for s, MO model for p electrons
• Benzene

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Sigma Bonds Hybrid Orbitals, Localized Electrons
Pi Bonds Molecular Orbitals, Delocalized
Electrons
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Spectroscopy

• Quantized energy levels of any sort
• EM radiation is applied to system
• Frequencies absorbed correspond to difference
  between energy levels

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Electronic Spectroscopy (UV-VIS)

• Energy levels electrons in different levels
  (atomic orbitals, molecular orbitals)
• The frequencies are in the UV or Visible region

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Uses of UV-Vis

• Kinetics following a reaction
• Finding concentration, if you have a standard or
  know absorption coefficient
• Proteins concentration (abs of tyrosine)

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Vibrational Spectroscopy (IR)

• Energy levels molecular vibrations
• Molecular identification
• Some peaks can tell specific bonds
• Other regions act like a fingerprint

http//en.wikipedia.org/wiki/Infrared_spectroscopy
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Vibrational Spectroscopy (IR)
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Rotational Spectroscopy (Microwave)

• Energy levels molecular rotations
• Only works in gases, where rotations are well
  quantized.
• In liquids, rotations are quenched by collisions.
• Uses
• Physical Chemistry structure of gases
• Astrophysics Composition of interstellar medium

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http//www.sciencedaily.com/releases/2008/09/08091
9075007.htm
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Nuclear Magnetic Resonance (NMR)

• Energy Levels Nuclear alignment with and against
  a strong magnetic field
• Energy corresponds to radio frequency
• Uses
• Small molecule structure determination
• Protein structure and binding information
• Magnetic Resonance Imaging (MRI)

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1D spectrum of Ethanol (CH3CH2OH)
2D Spectra can give structures of large molecules
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MRI

• This same information in a spatial manner gives
  images

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Intermolecular Forces

• Attractions between molecules
• If IMF are low relative to KE, substance is a gas
• If comparable to KE, liquid
• If high relative to KE, solid
• Higher IMF higher melting point and boiling
  point

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Dispersion Forces

• The default (and weakest) IMF
• Caused by fluctuations in electron distribution
  within molecules

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Dispersion Forces

• Strength increases with size of molecule
• Ex Which has a higher boiling point
• He or Ar
• CH4 or C10H22

• The default (and weakest) IMF
• Caused by fluctuations in electron distribution
  within molecules

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Dipole-Dipole Forces

• For molecules with a net dipole moment (polar)
• end of one molecule attracts end of another
• Stronger than dispersion

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Hydrogen Bonding

• A super dipole-dipole force, strongest IMF
• Polar molecules with HF HO or HN bond