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Lecture 7 • Mass Spectrometry • UV/Vis Spectroscopy • Sample Problems - handout • Due: • Lecture Problem 5
Mass Spectrometry Molecular ions and fragments can rearrange under MS conditions. For example, the benzylic cation can rearrange to a tropollium ion:
Mass Spectrometry Alpha-cleavage fragments Seen in amines, alcohols, and carbonyl-containing compounds
Mass Spectrometry Loss of carbon monoxide:
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.
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: 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)
UV/Vis of Acetone p --> p* Shorter wavelength, stronger absorption n --> p* Longer wavelength, weaker absorption
The more conjugation, the lower the energy, the higher the Wavelength (lmax value).
Spectra are absorbance versus wavelength in nanometers (nm) - lmax Examples:
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.