Spectroscopy

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Spectroscopy

• CH 254
• Summer 1999
• UMass Boston

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Structural Information

• Chemical methods
• functional group tests
• chemical transformations
• Physical measurements
• physical properties
• spectroscopy

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Molecules Energy

• Energy absorption corresponds to transitions
  between fixed energy levels.
• Absorption of energy can be monitored by
  spectrometers.
• Tracings obtained are called spectra.

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Types of Spectroscopy

• Different regions of electromagnetic spectrum
  have different types of spectroscopy associated
  with them.
• Fundamental equation
• E hn hc/l

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The Electromagnetic Spectrum

• Wide range of energy
• cosmic rays to radiowaves
• Online table of the electromagnetic spectrum.

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Types of Spectroscopy in CH 254

• UV-visible spectroscopy
• IR spectroscopy
• NMR spectroscopy

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UV-Visible Spectroscopy
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Instrumentation
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Functional Groups and UV-Vis
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What is UV-Vis Good For?

• Detecting the presence of distinctive groupings
  of atoms (chromophores).
• These are usually conjugated systems.
• The more conjugation, the longer the wavelength
  of the absorption.
• Added substituents (electron donors or
  withdrawers) alter the wavelength.

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Examples

• 1,3-pentadiene lmax 224 nm
• 2,5-dimethyl-2,4-hexadiene lmax 243 nm
• b-carotene lmax 425, 450, 477 nm

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Color Wheel
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IR Spectroscopy

• Atoms in a molecule are constantly in motion
  (stretch, bend, etc.)
• Energy levels correspond to vibrational states.
• Spacing between these states is on the order of
  3-30 microns.
• Use frequency units for IR 400-4000 cm-1 (1 cm-1
  10,000/microns)

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IR Interactions

• Frequency required depends on
• mass of bonded atoms (lighter atoms vibrate at
  higher frequencies)
• stiffness of bond (stiffer bonds vibrate at
  higher frequencies)
• Functional groups have characteristic stretching
  frequencies (see handout).

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Hydrocarbons

• C-H stretch (2800-3300 cm-1)
• Hybridization of carbon affects position
• alkanes (sp3) 2800-3000 cm-1
• alkenes (sp2) 3000-3100 cm-1
• alkynes (sp) 3300 cm-1

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Alkanes and Alkenes
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Alkynes and Nitriles
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Alcohols
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Amines
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Ethers
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Carbonyl Compounds

• CO stretch is very distinctive.
• Many functional groups contain this substructure.
• Supplementary bands are needed to help
  distinguish the functional group.

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Ketones
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Aldehydes
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Carboxylic Acids
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Esters
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Hints for IR

• Work from left to right - look for the major
  absorptions first.
• Useful checkpoints
• 3500 (OH, NH)
• 3000 (is there unsaturation present?)
• 2200 (is there a triple bond?)
• 1700 (is there a carbonyl?)
• 1200 (is there a C-O?)

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More Hints

• Use common sense!
• Look at the molecular formula - if there isnt
  any nitrogen, dont look for NH or CN groups.
• The SODAR calculation is useful - if SODAR is
  zero, there wont be a carbonyl, for instance.