Chem 335 - Winter 1999 - Organic Chemistry II
Portland State University - Dr. Carl C. Wamser

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Chapter 14 - IR & UV Spectra

Infrared Spectroscopy

• probes different molecular vibrations
• absorption occurs when the frequency of the IR wave
  matches a vibrational frequency of the molecule
• molecules typically have many possible vibrations
• bond stretching vibrations are most characteristic
  (detect different kinds of A-B bonds)
• bond bending involves several atoms at once
  more complex vibrations (rock, scissor, wag, twist)
• one exception: a completely symmetrical bond vibration does not absorb IR
  e.g., the C=C double bond stretch in ethylene is IR-silent

IR Spectra

• x-axis:
  wavelength range typically 2 - 15 µm
  wavenumber also used (second x-axis)
  wavenumber = reciprocal wavelength (in cm-1)
• y-axis:
  usually transmittance (0 - 100%)
  (valleys correspond to absorptions)
  "peak" position noted by wavenumber

Regions of an IR Spectrum

• Bonds to H (4000 - 2500 cm-1)
• Triple Bonds (2500 - 2000 cm-1)
• Double Bonds (2000 - 1500 cm-1)
• "Fingerprint" (1500 - 600 cm-1)
• Tables 14.1 and 14.2 will be provided for quizzes and exams

Interpreting an Unknown IR Spectrum

• work from left to right (decreasing wavenumber)
• look for characteristic bond stretches
• try to confirm functional groups with additional absorptions
• if possible, compare to a known spectrum

Ultraviolet/Visible Spectroscopy

• x-axis:
  typically 200 - 800 nm
• y-axis:
  typically absorbance in % or log units
  (peaks correspond to absorptions)
  peak positions noted by lmax

Electronic Absorption Spectra

• uv/vis absorptions correspond to electron transitions
  from one molecular orbital to another
• generally pi bonds are clearly seen in the uv region
• absorptions are called pi->pi* transitions
  (from bonding to antibonding orbitals)
• one pi bond (ethylene) 165 nm (l max)
• conjugated dienes and polyenes absorb at longer wavelengths
  (lower energies, or more closely spaced orbitals)
  • 1,3-butadiene 217 nm
  • benzene 254 nm
  • beta-carotene 483 nm (absorbs violet - looks yellow)
• Table 14.5 will be provided for quizzes and exams

Photochemistry

• what do molecules do once an electron is promoted to a higher energy orbital?
• many possibilities:
  • emit light (fluorescence)
  • lose energy as heat
  • rearrange
  • decompose
• transfer an electron to (or from) another molecule