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