Infrared Spectroscopy Part 1

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Infrared Spectroscopy Part 1

• Lecture Supplement
• Take one handout from the stage

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Midterm Exam 1

• Date Monday April 30
• Time 500-650 PM
• Topics Molecular Structure Introduction and
  Review ? Carbohydrates
• Location last name A-H in Franz 1178
• last name I-Sh in Haines 39
• last name Si-Z in PAB 1425
• Question and Answer Session
• Lecture time, Monday April 30
• Submit questions to harding_at_chem.ucla.edu
• Label as Question for QA
• Deadline for possible inclusion noon Sunday
  April 29
• Extra Office Hours (Steve J) Sunday 2-4 PM,
  Young Hall 3077F

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Infrared Spectroscopy (IR)Molecular Vibrations

• Molecular Vibrations
• Bonded atoms move around in space
• Very fast one vibration cycle 10-15 seconds

• Stretching (H-Cl)
• Atoms move along bond axis

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Molecular Vibrations

• Vibration energy
• ? vibration energy ? average bond length

Excited state higher energy
Ground state lower energy
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Molecular Vibrations

• Vibration energy
• Vibrational energy is quantized (only certain
  energy values are possible)

n stretching frequency
For bond vibrations DE dependent on bond
5 kcal mol-1 lower energy than red
light photons infrared photons
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The Infrared Spectrum
Spectrum plot of photon energy versus photon
quantity
Typical infrared spectrum
Number of photons absorbed
Stretching frequency Proportional to photon energy
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Molecular Structure from IR Spectrum

• How does spectrum give information about
  molecular structure?
• Structure controls number of photons absorbed
• Structure controls stretching frequency

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Structure versus Photon Quantity
From quantum mechanics
Chance of photon absorption controlled by change
in dipole moment (m)
Useful approximation Consider only one bond
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Absorption Intensity versus Bond Dipoles

• Bond dipole (magnitude of electronegativity
  difference) x (bond length)
• ? DEN ? dipole
• ? bond length ? dipole
• ? bond dipole ? absorption
• In practical terms
• Highly polar bond ? strong peak
• Symmetrical (nonpolar) or nearly symmetrical bond
  ? peak weak or absent

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Absorption Intensity versus Bond Dipoles
CO peak strong
Examples
CC peak absent (or maybe weak)
CC peak present but weak

• Caution!
• Weak peaks not always discernable
• Be careful when excluding symmetrical functional
  groups base on absence of peak

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Structure versus Stretching Frequency

• Hookes Law (1660)
• Stretching frequency of two masses on a spring

? bond order
C-C CC C?C
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Characteristic Stretching FrequenciesThe Five
Zones
IR spectrum divided into five zones (groups) of
important absorptions
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Characteristic Stretching FrequenciesThe Five
Zones
attached to benzene ring attached to alkene
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Characteristic Stretching FrequenciesThe Five
Zones
CO frequencies 20-40 cm-1 lower when conjugated
to a pi bond
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Characteristic Stretching FrequenciesThe Five
Zones

• Complete table Thinkbook, inside front cover
• What do I have to know?
• Functional groups in each zone ? Learn by working
  lots of problems
• Do not memorize stretching frequencies table
  given on exam

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Guided Tour of Functional GroupsTerminal Alkyne
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Guided Tour of Functional GroupsTerminal Alkene
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Guided Tour of Functional GroupsAlcohol
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Guided Tour of Functional GroupsKetone
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Infrared Spectroscopy Part 2

• Lecture Supplement
• Take one handout from the stage

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Infrared Spectroscopy Part 1 Summary

• Infrared photons cause excitation of molecular
  vibrations
• Photon absorption probability higher with more
  polar bonds
• Energy of photons absorbed depends on

Bond order Masses of atoms bonded

• IR spectrum divided into five zones
• Each zone analyzed for absence or presence of
  functional groups
• Stretching frequency, peak shape both important

Alcohol O-H usually gives broad peak CO stretch
gives strong peak
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Guided Tour of Functional GroupsKetone (again)
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Guided Tour of Functional GroupsAldehyde
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Guided Tour of Functional GroupsKetone with
Alkene Conjugation
Conjugation with pi bond lowers CO stretch by
20-40 cm-1
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Guided Tour of Functional GroupsEster
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Guided Tour of Functional GroupsCarboxylic Acid
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Guided Tour of Functional GroupsBenzene Ring
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1
Step 1 Calculate DBE DBE C - (H/2) (N/2)
1 6 - (12/2) (0/2) 1 1
One ring or one pi bond
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1

• Step 2 Analyze IR Spectrum
• Zone 1 (3700-3200 cm-1)

Alcohol O-H N-H ?C-H
Absent - no N in formula Absent - not enough DBE
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1

• Zone 2 (3200-2700 cm-1)

Aryl/vinyl sp2 C-H Alkyl sp3 C-H Aldehyde
C-H Carboxylic acid O-H
Probably not (not enough DBE)
Absent - no 2700 cm-1 Absent - not broad enough
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1

• Zone 3 (2300-2000 cm-1)

Alkyne C?C Nitrile C?N
Absent - no peaks not enough DBE Absent - no
peaks not enough DBE
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1

• Zone 4 (1850-1650 cm-1)

CO
Possibilities ketone ester
- not enough oxygens
aldehyde - no 2700 cm-1 peak
carboxylic acid - zone 2 not broad
amide - no nitrogen
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Five Zone IR Spectrum AnalysisExample 1 C6H12O2
1700 cm-1

• Zone 5 (1680-1450 cm-1)

Benzene ring Alkene CC
Absent - no peak 1600 cm-1 not enough
DBE Absent - no peak 1600 cm-1 not enough DBE
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Five Zone IR Spectrum AnalysisExample 2 C8H7N
Step 1 Calculate DBE DBE C - (H/2) (N/2)
1 8 - (7/2) (1/2) 1 6
Six rings and/or pi bonds Possible
benzene ring
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Five Zone IR Spectrum AnalysisExample 2 C8H7N

• Step 2 Analyze IR Spectrum
• Zone 1 (3700-3200 cm-1)

Absent - no oxygen in formula Absent - peaks too
small Absent - peaks too small
Alcohol O-H N-H ?C-H
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Five Zone IR Spectrum AnalysisExample 2 C8H7N

• Zone 2 (3200-2700 cm-1)

Aryl/vinyl sp2 C-H Alkyl sp3 C-H Aldehyde
C-H Carboxylic acid O-H
Present - peaks gt 3000 cm-1
Present - peaks lt 3000 cm-1
Absent - no 2700 cm-1 no CO in zone 4 Absent -
not broad enough CO in zone 4
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Five Zone IR Spectrum AnalysisExample 2 C8H7N

• Zone 3 (2300-2000 cm-1)

Alkyne C?C Nitrile C?N
Possible Possible
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Five Zone IR Spectrum AnalysisExample 2 C8H7N

• Zone 4 (1850-1650 cm-1)

CO Absent - no peak no oxygen in formula
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Five Zone IR Spectrum AnalysisExample 2 C8H7N

• Zone 5 (1680-1450 cm-1)

Benzene ring Alkene CC
Present - peaks 1600 cm-1 and 1500 cm-1