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CHEM 30B SPECTROSCOPY. QUESTIONS AND ANSWERS PROVIDED. STUDY GUIDE FOR EXAM PREP 2024
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CHEM 30B SPECTROSCOPY. QUESTIONS AND ANSWERS PROVIDED. STUDY GUIDE FOR EXAM PREP 2024
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CHEM 30B SPECTROSCOPY. QUESTIONS AND ANSWERSPROVIDED. STUDY GUIDE FOR EXAM PREP 2024
1 . What is spectroscopy?: the use of electromagnetic radiation to probe
and characterize matter
2. What is electromagnetic radiation?: a form of energy that
exhibits wavelike behavior as it travels through space; electric
component in 1 plane, magnetic component in perpendicular
plane
3. What determines the intensity of an IR peak (amount of
energy that is absorbed)? State the relationship.: - the change
in the dipole moment
- large change in the dipole moment = larger amount of energy is
absorbed and exhibits strong absorption peaks
- ALSO dependent on the quantity of identical bonds (more identical
bonds = higher intensity)
4. What types of bonds don't absorb infrared radiation? Why?:
nonpolar bonds because there is no change in the dipole
moment
5. What is wavenumber? What is the unit?: - inverse of wavelenc
- proportional to energy and frequency - unit: cm A-1
6. What types of bonds are polar (i.e. what are the atoms)? :
polar bonds have 2 atoms bonded together with different
electronegativities
7. What are the 2 factors that determine the wavenumber at
which IR light is absorbed? State the relationships.: 1. bond
strength
--> stronger bonds will vibrate at higher frequencies or higher
wavenumbers 2. atomic mass
--> atoms with less mass vibrate at higher frequencies or higher
wavenumbers
8. What is % transmission on an IR spectrum? : % of light NOT
being absorbed 9. What is included in the 4 zones of an IR
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, CHEM 30B SPECTROSCOPY. QUESTIONS AND ANSWERS
PROVIDED. STUDY GUIDE FOR EXAM PREP 2024
spectrum? List from highest to lowest wavenumber and give the
general wavenumber range.: 1 . N-H, O-H, C-H
3500-2700 cm A-1
2. CaC, CaN (i.e. alkynes, nitrides) -> 2300-2100 cm A-1
3. c=N, C=C, C=O 1800-1600 cm A-1
4. C-C, C-O, C-N, C-X -> 1400-600 cm A-1 (known as the fingerprint
region) 10. What are the characteristics of 0-14 IR peaks? Why? :
exceptionally broad
(NEVER sharp) and intense
- broad because of hydrogen bonding
1 1 . What are the characteristics of N.H IR peaks?: broad and
intense
12. What does it mean for a peak to be sharp in IR?: sharp =
narrow
13. When hydrogen bonding is present in a functional group, what
does it do to the IR peak? Why?: hydrogen bonding creates
exceptionally broad peaks because H-bonding distorts bond
lengths, and a variable bond length = variable bond strength =
different wavenumber absorptions
14. What is the difference between primary, secondary, and
tertiary amines in an IR spectrum?: primary- gives twin set of
peaks that are close together b/c of 2
secondary- gives 1 peak tertiary- no
N-H signal because there is no N-H
bond
15. What is the difference between the IR peaks for sp, sp2, and sp3
C-H bonds? Why? : sp gives highest wavenumber peak, then sp2,
then sp3
2/18
, CHEM 30B SPECTROSCOPY. QUESTIONS AND ANSWERS
PROVIDED. STUDY GUIDE FOR EXAM PREP 2024
-> because with more s-character, bonds get shorter and stronger, so
frequency goes up
16. What is the trick to identifying C-H IR vibrations?: draw a vertical
dotted line at 3000 cm A-1 mark
- sp2 and sp C-H stretches are to the left of the line, sp3 C-H stretches
are to the right of the line
*exception: aldehydes have sp2 C-H bond but DON'T get 3100 cm A-1
peak, get 2700 cm A-1 peak instead
17. What is the difference between the IR absorption peaks for
symmetrical alkenes/alkynes and asymmetrical
alkenes/alkynes?: asymmetrical alkenes/alkynes will have
more intense peak because of higher dipole moment
18. What are the characteristics of C=O IR
peaks?: - sharp - intense
- around 1750 cm A-1
- always sticks out
19. Why do the C=O IR frequencies vary so much? Give an
example.: vary because of varying resonance effects in different
types of C=O bonds
- aldehydes and ketones have no resonance delocalization
- esters have minor resonance delocalization
- amides have significant resonance delocalization, so carbonyl only
has partial double bond character -> weaker bond, so slightly lower
frequency absorption 20. Which is more polar: cis or trans isomer?
Why? : cis is more polar because bulky groups are on the same
side, so the electron cloud is on the same side 21 . What is the
typical IR absorption pattern for benzene?: 1. peak at 1600 cmA-1
2. 1-2 peaks at 1500 and 1450 cm A-1
3/18
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