1. Infrared (IR)

2. ?????????????

3. Spectroscopy “seeing the unseeable”
Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.
Chemists can use portions of the EMS to selectively manipulate the energies contained within a molecule, to uncover detailed evidence of its chemical structure and bonding.

4. electromagnetic relationships: λv = c λ µ 1/v E = hv E µ v
  E = hc/λ E µ 1/λ
λ = wave length
v = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant (3.99X10-13 kj*s/mol
or 9.54 X kcal*s/mol λ c

5. The IR Region Just below red in the visible region (380 nm-780nm)
Wavelengths usually mm
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters (104/mm = cm-1)
Wavenumbers are proportional to frequency and energy
Higher wavenumber higher energy

6. Vibration and rotation
Electromagnetic Spectrum
X-ray
diffractometry
Ultraviolet - Visible
Infrared
Microwave
NMR
Pi and non-bonding
electrons
Molecular
Vibration and rotation
Molecular
Rotation
nucleus
Proton Spin
valence
electrons
Mass Spectrometry

8. Infrared radiation
λ = 2.5 to 17 μm (wavelenght)
v = 4000 to 600 cm-1 (frequency)
 These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.
IR is used to tell:
1. what type of bonds are present
2. some structural information

9. Energy, frequency, and wavenumber are directly proportional to each other.
Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional group
What happens when a sample absorbs IR energy?
stretching and bending of bonds
(typically covalent bonds)
Evibration increases momentarily IR
-O-H -O
(3500 cm-1) —H

10. Molecules are made up of atoms linked by chemical bonds
Molecules are made up of atoms linked by chemical bonds. The movement of atoms and chemical bonds like spring and balls (vibration)

11. There are two main vibrational modes :
Vibrations
What is a vibration in a molecule?
Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds
There are two main vibrational modes :
Stretching - change in bond length (occurs at higher frequencies; cm1)
Stretching vibration

13. 2. Bending - change in bond angle ( occurs at
lower frequency; cm1)

14. Fingerprint of Molecule
No two molecules will give exactly the same IR spectrum (except enantiomers).
Simple stretching: cm-1.
Complex vibrations: cm-1, called the “fingerprint region.”

15. IR Spectrum Regions

16. IR-Active IR-Inactive The bond undergoing the vibration must be polar
A polar bond is usually IR-active.
The greater the polarity of the bond, the more intense the absorption
The bond’s vibration must cause a change in dipole
Asymmetrical stretching/bending change the dipole moment of a molecule.
IR-Inactive
A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

17. Interpretation of IR Spectrum
Use Correlation tables
Looking for presence/absence of functional groups
A polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule will absorb weakly or not at all

18. Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher frequencies:
C-C cm-1
C=C cm-1
CC cm-1 (weak or absent if internal)
Conjugation lowers the frequency:
isolated C=C cm-1
conjugated C=C cm-1
aromatic C=C approx cm-1

19. Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency.
sp3 C-H, just below 3000 cm-1 (to the right)
sp2 C-H, just above 3000 cm-1 (to the left)
sp C-H, at 3300 cm-1

20. Figure 12.15: The four regions of the infrared spectrum: single bonds to hydrogen, triple bonds, double bonds, and fingerprint.

23. FEATURES OF AN IR SPECTRUM
An IR spectrum is a plot of per cent transmittance (or absorbance) against wavenumber (frequency or wavelength). A typical infrared spectrum is shown below on the next slide
A 100 per cent transmittance in the spectrum implies no absorption of IR radiation. When a compound absorbs IR radiation, the intensity of transmitted radiation decreases. This results in a decrease of per cent transmittance and hence a dip in the spectrum. The dip is often called an absorption peak or absorption band.
Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb infrared radiation at different characteristic wavenumbers

24. Undecane (alkane)
wavelenght
frequency

25. 1-hexene (alkene)
wavelenght
frequency

26. 1-hexyne (alkyne)

27. O-H and N-H Stretching
Both of these occur around 3300 cm-1, but they look different
Alcohol O-H, broad with rounded tip
Secondary amine (R2NH), broad with one sharp spike
Primary amine (RNH2), broad with two sharp spikes
No signal for a tertiary amine (R3N)

28. An Alcohol IR Spectrum

29. An Amine IR Spectrum

30. Carbonyl Stretching
The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1
Usually, it’s the strongest IR signal
Carboxylic acids will have O-H also
Aldehydes have two C-H signals around and 2800 cm-1

31. A Ketone IR Spectrum

32. O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding

33. An Aldehyde IR Spectrum

34. Variations in C=O Absorption
Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1
The C=O group of an amide absorbs at an even lower frequency, cm-1
The C=O of an ester absorbs at a higher frequency, ~ cm-1
Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency

35. An Amide IR Spectrum

36. methyl n-propyl ether
no O--H
C-O ether

37. Summary of IR Absorptions
Chapter 12
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38. PRACTICE
Identify the possible functional group or possible functional groups

39. Identify the functional group
Alcohol
Ether
Ketone
Copyright © Houghton Mifflin Company.All rights reserved.

40. Practice
Identify the possible functional group or possible functional groups

41. Identify the functional group
Alcohol
Ether
Ketone
Copyright © Houghton Mifflin Company.All rights reserved.

42. Index of Hydrogen Defficiency
The sum of the number of rings and pi bonds in a molecule
The first step in problems to determine structure
Calculated from the molecular formula
IHD = (Hreference-Hmolecule)/2
The molecular formula of a reference hydrocarbon is CnH2n+2

43. Calculate the hydrogen deficiency for 1-hexene
The molecular formula is C6H12
The molecular formula for the reference hydrocarbon is C6H14
The IHD of 1-hexene is
(14-12)/2= 1

44. Practice Calculate the IHD of isopentyl acetate
The molecular formula is C7H14O2

45. Practice answer Calculate the IHD of isopentyl acetate
The molecular formula is C7H14O2
The molecular formula of the reference hydrocarbon is C7H16
IHD = (16-14)/2 = 1