The Greenhouse Effect

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The Greenhouse Effect
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Terrestrial Emissions

• Terrestrial emissions have energies corresponding
  to the infrared (IR) region of the
  electromagnetic spectrum

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Visible UV Radiation

• Certain energies cause electrons to be excited
  into higher energy orbitals
• Very energetic photons (uv to x-ray) may cause
  electrons to be ejected from molecule (ionization)

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IR Radiation

• Lower in energy than visible radiation
• Does not possess enough energy to eject electrons
• Absorbed IR radiation can excite vibrations in
  molecules

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Molecular Responses to Radiation
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What Are Molecular Vibrations?

• Consider a diatomic molecule as being 2 balls
  connected by a spring
• When the molecule vibrates, the balls move
  towards and away from each other at a certain
  frequency
• The energy of the system is related to the amount
  the spring stretches or compresses

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• The frequency of the vibration is proportional to
  the square root of the ratio of the spring force
  constant to the masses on the string.
• the lighter the masses on the spring, the higher
  the vibrational frequency
• the tighter the spring, the higher the
  vibrational frequency
• Vibrational frequencies for stretching bonds in
  molecules are related to
• bond strength
• masses of atoms

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• Molecules differ from balls springs because
  their vibrational frequencies are quantized
• Only certain energies are allowed
• only photons of certain energies will excite
  molecular vibrations

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Number of Vibrational Modes

• different types of vibrations in an N atom
  molecule
• linear molecule
• 3N - 5
• nonlinear molecule
• 3N - 6
• Diatomic molecule
• (3 x 2) - 5 1 (stretching of bond)
• CO2
• (3 x 3) - 6 4

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CO2 Vibrations

• (A) (B) stretching of chemical bonds
• (A) symmetrical (not IR active - no change in
  dipole moment
• (B) asymmetrical (IR active - change in dipole
  moment)
• (C) (D) bending vibrations
• one in plane of paper one out of plane of paper
• equal in energy

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IR Spectrum of CO2
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General Trends

• The stronger the bond, the more energy required
  to excite the stretching vibration
• frequency decreases in the order
• triple bond gt double bond gt single bond
• The heavier an atom, the lower the frequencies of
  vibrations involving that atom

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General Trends

• Polar molecules and molecules with polar bonds
  capture IR photons more efficiently than nonpolar
  molecules
• All molecules with three or more atoms absorb IR
  radiation because they all have some vibrations
  that change the polarity of the molecul

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Chart of Characteristic Vibrations