1. Absorption and Mass
Spectroscopy
Presenter: Cynthia Echefu

2. Absorption Spectroscopy
Infrared (IR)- Used to determine functional
groups (molecular vibration, which includes
bending, stretching and rotation)
Ultraviolet- Used to identify conjugated double
bonds
Visible – sequence of light color the eye can see

3. AAMC: Must know about IR
• intramolecular vibrations and
rotations
• recognizing common
characteristic group
absorptions, fingerprint region

4. Infrared spectrum
violate , indigo, blue, green, yellow, orange, red

5. Vibrations: bonds can stretch, compress and
bend like a spring. It is this vibration that is
measured in IR-spec.
Rotations: molecules can rotate. Rotations
produce waves mainly in the microwave
region. However, part of the rotation spectra
does overlap with the vibration spectra.
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm

6. Wave number (1/ƛ) (cm-1)
4000 500
3500 3000 2500 2000 1500 1000

7. Infrared Spectrum

8. Common functional group
characteristics
• Anything around 3000 cm-1 involves a
hydrogen atom, be it O-H, N-H, or C-H.
• Anything around 2000 cm-1 and below
does not involve hydrogen, be it
C=O, C=C, C-C, or C-O.
• Remember this: 1700 cm-1 is for the
carbonyl group..

9. Continues…
• Remember this: 3300 cm-1 can be O-
H, N-H, or alkyne C-H.
• Broad peaks are due to hydrogen
bonding (OH and NH).
• Below 1100 cm-1 is called the
fingerprint region.

12. Wave number (cm-1)
Functional Group Wave number
Fingerprint region Less than 1100
Carbon oxygen single bond (c-o) 1100
Carbon single bond (c-c) 1100
Carbon double bond (c=c) 1750
Aldehyde, ketone and acid (c=o) 1700- 1750
Aliphatic hydrogen (non-aromatic) Less than 3000
Aromatic (or multiple bond) Greater than 3000
Alcohol (O-H) 3500 (broad)
Amine (N-H) 3500 (slightly sharp)
Alkynes (C-H) 3500 ( very sharp)

14. Must Know
Alcohol (broad)
Acid (sharp)
Ketone (sharp)
Amine (slightly sharp)
Alkyne (C-H)(sharp)

15. AAMC: Must know about UV
• π -electron and nonbonding
electron transitions
• conjugated systems

16. Ultraviolet Spectroscopy
Electrons absorption UV energy,
which causes the electrons to
transition from a low energy level
(ground state) to a higher energy
level (excited state).

17. Bonding, non-bonding and anti-bonding
• Electron excitation

18. Ultraviolet Spectroscopy
Conjugated vs. isolated double bonds
• Conjugated (single bond between two double
bonds)
C=C-C=C
• Isolated (two or more single bonds between
double bonds)
C=C-C-C=C

19. AAMC: Must know about visible
• absorption in visible region
yielding complementary color
• effect of structural changes on
absorption

20. Visible Spectroscopy
Remember:
ROY G. BIV
violate , indigo, blue, green, yellow, orange, red
Range: 400nm - 700nm

21. Absorption in visible region
Think of the flame test in high school chemistry

22. AAMC: Must know about mass spec.
•Mass-to-charge ratio
(m/z)
•Molecular ion peak

23. Mass spectroscopy
Used to identify chemical
formula and molecular mass,
by bombarding with electrons
and fragmenting sample.

25. m/z =18 (H20)+ since O + H + H = 16 + 1 + 1 = 18 amu
m/z=17 (HO)+ since O + H = 16 + 1 = 17 amu
m/z=16 (O)+ since O = 16 amu
m/z=1 (H)+ since H = 1 amu

26. Any questions?
Comments?
Suggestions?

27. Questions 1
A chemist isolated a compound of interest. H e
needs to verify that his product is a
conjugated diene. What spectroscopic method
can he use?
a) NMR spectroscopy
b) IR spectroscopy
c) Ultraviolet spectroscopy
d) Mass Spectroscopy

28. Questions 1
A chemist isolated a compound of interest. H e
needs to verify that his product is a
conjugated diene. What spectroscopic method
can he use?
a) NMR spectroscopy
b) IR spectroscopy
c) Ultraviolet spectroscopy
d) Mass Spectroscopy

29. Question 2
A stretch at 3400 cm-1 in an IR spectrum
indicates the presence of what functional
group?
a) Alkyne
b) Ketone
c) Alkene
d) Alcohol

30. Question 2
A stretch at 3400 cm-1 in an IR spectrum
indicates the presence of what functional
group?
a) Alkynes
b) Ketone
c) Alkenes
d) Alcohol

31. Question 3
13
Which peak is referred to as the parental peak?
a) 44
b) 87
c) 72
d) 13

32. Question 3
13
Which peak is referred to as the parental peak?
a) 44
b) 87
c) 72
d) 13

33. References
• Dr. Flowers MCAT
• Google Images: IR spectroscopy, visible
spectroscopy,
• http://mcat-review.org/molecular-structure-
spectra.php
• http://www.users.muohio.edu/gungbw/functi
onalgroups.html
• http://chemicalinstrumentation.weebly.com/ir
-spectrometry.html

34. Answers: Visible spectrum passage
28. D
29. C
30. C
31. D
32. B
33. A