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: 1. Expose compound to UV/Vis radiation UV: nm Vis: nm 2. Absorption of radiation causes electronic excitations at specific wavelengths ( max ) 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.

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.

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

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

The more conjugation, the lower the energy, the higher the Wavelength ( max value).  to  * transitions

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 max ’s and  ‘s from UV/Vis data.

Plant Pigments (Chapter 9) Color wheel: complementary colors

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