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.

n to p* (weak, often not observed) 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.