1. Lecture 7 Mass Spectrometry UV/Vis Spectroscopy
Sample Problems - handout
Due:
Lecture Problem 5

2. Mass Spectrometry Molecular ions and fragments can rearrange under MS
conditions.
For example, the benzylic cation can rearrange to a
tropollium ion:

3. Mass Spectrometry Alpha-cleavage fragments
Seen in amines, alcohols, and carbonyl-containing compounds

4. Mass Spectrometry
Loss of carbon monoxide:

5. UV/Vis Spectroscopy Basic Idea: Expose compound to UV/Vis radiation
UV: nm
Vis: 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.

6. 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.

7. 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)

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

9. The more conjugation, the lower the energy, the higher the
Wavelength (lmax value).

10. Spectra are absorbance versus wavelength in nanometers (nm) - lmax Examples:

11. 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.