Structure Determination: Mass Spectrometry and Infrared Spectroscopy

1
Structure Determination Mass Spectrometry and
Infrared Spectroscopy
2
Topics to discuss

• What is Spectroscopy?
• Mass Spectrometry
• What is it?
• How does it work and why is it useful?
• IR Spectroscopy
• What is it?
• Why is it useful?
• How to interpret an IR spectrum
• Common IR absorptions
• Using Spectroscopy to Identify a Molecule

3
Intro to Spectroscopy

• Q What is Spectroscopy?
• Q How does it work?

4
Intro to Spectroscopy

• Q Why do we need it?
• Q How many kinds are there?

5
Mass Spectrometry (MS)

• Q What is it?

6
Mass Spectrometry (MS)

• Q How does it work, and why is it useful?

7
The Mass Spectrum

• Q What is a mass spectrum?
• Q What is important?

8
Mass-Spectral Behavior of Some Common Functional
Groups

• Functional groups cause
• Alcohols, amines, ketones and aldehydes are all
  functional groups that

9
Spectroscopy of the Electromagnetic Spectrum

• Radiant energy is proportional to
• Different types are classified by

10
Absorption Spectra

• Organic compound exposed to electromagnetic
  radiation can
• Changing wavelengths to determine
• Energy absorbed is distributed

11
Infrared Spectroscopy (IR)

• Q What is it?
• Q Why is it useful?

12
Infrared Spectroscopy (IR)

• Q How does it work?

13
Infrared Spectroscopy (IR)

• Q What is an IR spectrum?

14
Interpreting Infrared Spectra

• Most functional groups absorb at
• In this example, the peak is centered at
  1750cm-1. Whenever we see a peak at 1750cm-1, we
  know that it represents

15
Interpreting Infrared Spectra

• Different functional groups will yield different
  absorption spectra which arise from differences
  in
• In IR spectroscopy, various functional groups
  vibrate at unique frequencies, resulting in
• Thus, we can use the wavelength of absorption to
  determine

16
Regions of the Infrared Spectrum

• 4000-2500 cm-1 N-H, C-H, O-H (stretching)
• 2500-2000 cm-1 CºC and CºN (stretching)

• 2000-1500 cm-1 double bonds (stretching)
• Below 1500 cm-1 fingerprint region

17
Infrared Spectra of Hydrocarbons

• C-H, C-C, CC, CºC have characteristic peaks

18
Infrared Spectra of Some Common Functional Groups

• Learn the most common ones. With all others, you
  can use a IR frequency correlation table.
• Most common in organic compounds
• sp2 C-H stretch
• sp3 C-H stretch
• CC stretch
• C-O stretch
• CO stretch
• O-H stretch

19
IR Alcohols and Amines

• OH 3400 to 3650 cm?1
• NH 3300 to 3500 cm?1

20
IR Aromatic Compounds

• Weak CH stretch at
• Weak absorptions in
• Medium-intensity absorptions at

21
IR Carbonyl Compounds

• Strong, sharp CO peak at
• Exact absorption characteristic of type of
  carbonyl compound

22
Using Spectroscopy to Identify a Molecule

• 1. Recall the type of information that each
  spectrum provides
• 2. Use the spectra to verify and refute the
  existence of structural features
• 3. Piece together the structural info to arrive
  at a possible answer.
• 4. Cross check your answer with each spectrum to
  verify it.

23
A Combined Spectral Problem

• Typically, you will be provided with the
  molecular formula of an unknown compound and
  different types of spectra to use to determine
  the structure of an unknown.
• The key is to learn an approach to these types of
  problems which works best for you, and use the
  same systematic approach every time.

24
A Combined Spectral Problem

• First step should always be

25
A Combined Spectral Problem

• Next, look quickly at the IR spectrum.
• Notice the broad peak centered at 3400cm-1.

26
Summary

• What is Spectroscopy?
• Mass Spectrometry
• What is it?
• How does it work and why is it useful?
• IR Spectroscopy
• What is it?
• Why is it useful?
• How to interpret an IR spectrum
• Common IR absorptions
• Using Spectroscopy to Identify a Molecule