absorption spectroscopy
TRANSCRIPT
Absorption and Mass Spectroscopy
Presenter: Cynthia Echefu
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
AAMC: Must know about IR
• intramolecular vibrations and rotations • recognizing common
characteristic group absorptions, fingerprint region
Infrared spectrum
violate , indigo, blue, green, yellow, orange, red
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm
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.
Wave number (1/ƛ) (cm-1) 4000
500
3500 3000 2500 2000 1500 1000
High vibration frequency
Low vibration frequency
Infrared Spectrum
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..
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.
Wave number (cm-1) Functional Group Wave numberFingerprint 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)
Must Know
Alcohol (broad)Acid (sharp)
Ketone (sharp)Amine (slightly sharp)
Alkyne (C-H)(sharp)
AAMC: Must know about UV
• π -electron and nonbonding electron transitions
• conjugated systems
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).
Bonding, non-bonding and anti-bonding
• Electron excitation
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
AAMC: Must know about visible
• absorption in visible region yielding complementary color
• effect of structural changes on absorption
Visible Spectroscopy
violate , indigo, blue, green, yellow, orange, red
Remember:ROY G. BIV
Range: 400nm - 700nm
Absorption in visible region
Think of the flame test in high school chemistry
AAMC: Must know about mass spec.
•Mass-to-charge ratio (m/z) •Molecular ion peak
Mass spectroscopy
Used to identify chemical formula and molecular mass, by bombarding with electrons
and fragmenting sample.
m/z =18 (H20)+ since O + H + H = 16 + 1 + 1 = 18 amum/z=17 (HO)+ since O + H = 16 + 1 = 17 amum/z=16 (O)+ since O = 16 amum/z=1 (H)+ since H = 1 amu
Any questions?Comments?
Suggestions?
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 spectroscopyb) IR spectroscopyc) Ultraviolet spectroscopyd) Mass Spectroscopy
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 spectroscopyb) IR spectroscopyc) Ultraviolet spectroscopyd) Mass Spectroscopy
Question 2
A stretch at 3400 cm-1 in an IR spectrum indicates the presence of what functional group?
a) Alkyneb) Ketonec) Alkened) Alcohol
Question 2
A stretch at 3400 cm-1 in an IR spectrum indicates the presence of what functional group?
a) Alkynesb) Ketonec) Alkenesd) Alcohol
Question 3
Which peak is referred to as the parental peak?a) 44b) 87c) 72d) 13
13
Question 3
Which peak is referred to as the parental peak?a) 44b) 87c) 72d) 13
13
References• Dr. Flowers MCAT• Google Images: IR spectroscopy, visible
spectroscopy, • http://mcat-review.org/molecular-structure-sp
ectra.php• http://www.users.muohio.edu/gungbw/functi
onalgroups.html• http://chemicalinstrumentation.weebly.com/ir
-spectrometry.html
Answers: Visible spectrum passage
28. D29. C30. C31. D32. B33. A