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Lecture 21

Lecture 21. UV/Vis Pick up Lecture Problem 7. This Week in Lab: Work on 1st Synthetic Next Week in Lab: Ch 9 Final Report Due Synthetic #2 PreLab Due. UV/Vis Spectroscopy. Basic Idea: Expose compound to UV/Vis radiation UV: 200-400 nm Vis: 400-800 nm

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Lecture 21

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  1. Lecture 21 • UV/Vis • Pick up Lecture Problem 7 • This Week in Lab: • Work on 1st Synthetic • Next Week in Lab: • Ch 9 Final Report Due • Synthetic #2 PreLab Due

  2. UV/Vis Spectroscopy Basic Idea: Expose compound to UV/Vis radiation UV: 200-400 nm Vis: 400-800 nm 2. Absorption of radiation causes electronic excitations at specific wavelengths (lmax) Main Use: Detects the presence of pi systems in a compound. Unlike NMR, IR, and MS, UV/Vis is NOT used to determine pieces (specific bonds) of a molecule.

  3. Electronic Transitions Types of Electronic Transitions: The length of the arrow is roughly proportional to the energy difference between two levels. The longer arrow means a higher Energy transition. Higher energy means lower wavelength of radiation.

  4. UV/Vis Spectroscopy Transitions observed by UV/Vis: p to p* (strong) n to p*(weak, often not observed) n to p* transition is lower energy (higher wavelength) p to p* transition is higher energy (lower wavelength)

  5. UV/Vis of Acetone p --> p* Shorter wavelength, stronger absorption n --> p* Longer wavelength, weaker absorption

  6. The more conjugation, the lower the energy, the higher the Wavelength (lmax value). p to p* transitions

  7. Quantitative Data from UV/Vis Extinction Coefficients or Molar Absorptivity (): A numerical value of the intensity of the absorptions. Calculate  by using the Beer-Lambert Law: A (Absorbance) =  b c Where b = path length of cell (usually 1.0 cm) c = concentration of the solute When reporting data, chemists only report lmax’s and ‘s from UV/Vis data.

  8. Plant Pigments (Chapter 9) Color wheel: complementary colors

  9. Plant Pigments (Chapter 9) • What you’re looking for: • Example: Chlorophyll: • Observe the visible color: green • Absorption of red light (~ 620 to 680 nm); should be low % • transmittance

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