Spectroscopy: IR
Renee Y. Becker
Valencia Community College
CHM 2011
• Radiant energy is proportional to its frequency
(cycles/s = Hz) as a wave (Amplitude is its
height)
• Different types are classified by frequency or
wavelength ranges
• γ-rays (gamma rays)- greatest energy and highest
frequency. Emitted from some radioactive nuclei. Can
cause biological damage.
• X-rays- lower in energy than γ -rays. Can cause
biological damage in high doses.
• Ultraviolet (UV) light- can cause sunburn and even skin
cancer.
• Visible light- speaks for itself !
• Infrared radiation (IR) - heat!
• Microwaves- we cook with them and also used in radar.
• Radio waves- lowest frequency. Radio and TV
transmissions and NMR spectroscopy.
• The mode of propagation of electromagnetic radiation is
the wave.
• The wave is characterized by its wavelength (),
frequency (), and amplitude.
• Frequency () units: s-1 or hertz (Hz)
• The intensity of a wave is proportional to the square of
its amplitude.
• Electromagnetic radiation travels at constant velocity in a
vacuum: 3.00 x 1010 cm/s (speed of light).
• Wavelength x
Frequency = Speed
• (cm) x (s-1) = c
(cm/s)
c


  c
• Thanks to Max Planck and Al Einstein:
hc
E  h 

E = Energy of 1 photon (1 quantum)
h = Planck’s constant (6.62 x 10-34 J·s = 1.58 x 10-34cal·s)
 = Frequency (s-1)
 = Wavelength (cm)
c = Speed of light (3.00 x 1010 cm/s)
• The energy of a photon varies directly with the
frequency  and inversely with the wavelength
• High frequencies and short wavelengths =
higher energies
• Low frequencies and long wavelengths = lower
energies
Infrared Spectroscopy of Organic Molecules
• IR region lower energy than visible light (below
red – produces heating as with a heat lamp)
• 2.5 x 10-4 cm to 2.5 x 10-3 cm region used by
organic chemists for structural analysis
• IR energy in a spectrum is usually measured as
wavenumber (cm-1), the inverse of wavelength
and proportional to frequency
• Specific IR absorbed by organic molecule
related to its structure
• The IR spectrum covers the range from 4000
cm-1 to 400 cm-1
• This represents energy ranges from 48.0 - 4.80
kJ/mol (11.5 - 1.15 kcal/mol).
• IR energy absorption corresponds to specific modes,
corresponding to combinations of atomic movements,
such as bending and stretching of bonds between
groups of atoms called “normal modes”
• Energy is characteristic of the atoms in the group and
their bonding
• Corresponds to vibrations and rotations
Interpreting Infrared Spectra
• Most functional groups absorb at about the same energy
and intensity independent of the molecule they are in
• Characteristic higher energy IR absorptions in Table
12.1 can be used to confirm the existence of the
presence of a functional group in a molecule
• IR spectrum has lower energy region characteristic of
molecule as a whole (“fingerprint” region; 1300 to 625
cm-1)
• See samples in Figure 12-13
Regions of the Infrared Spectrum
• 4000-2500 cm-1 N-H, CH, O-H (stretching)
– 3300-3600 N-H, O-H
– 3000 C-H
• 2000-1500 cm-1 double
bonds (stretching)
– C=O 1680-1750
– C=C 1640-1680 cm-1
• 2500-2000
CC and
C  N (stretching)
• Below 1500 cm-1
“fingerprint” region
cm-1
Differences in Infrared Absorptions
• Molecules vibrate and rotate in normal modes,
which are combinations of motions (relates to
force constants)
• Bond stretching dominates higher energy modes
• Light objects connected to heavy objects vibrate
fastest: C-H, N-H, O-H
• For two heavy atoms, stronger bond requires
more energy: C  C, C  N > C=C, C=O, C=N >
C-C, C-O, C-N, C-halogen
Infrared Spectra of Hydrocarbons
• C-H, C-C, C=C, C  C have characteristic peaks
– absence helps rule out C=C or C  C
IR: Alcohols and Amines
• O–H 3400 to 3650 cm1
– Usually broad and intense
• N–H 3300 to 3500 cm1
– Sharper and less intense than an O–H
1-butanol
butylamine
IR: Aromatic Compounds
• Weak C–H stretch at 3030 cm1
• Weak absorptions 1660 - 2000 cm1 range
• Medium-intensity absorptions 1450 to 1600 cm1
• See spectrum of phenylacetylene, Figure 12.15
IR: Carbonyl Compounds
• Strong, sharp C=O peak 1670 to 1780 cm1
• Exact absorption characteristic of type of
carbonyl compound
– 1730 cm1 in saturated aldehydes
– 1705 cm1 in aldehydes next to double bond or
aromatic ring
C=O in Ketones
• 1715 cm1 in six-membered ring and acyclic
ketones
• 1750 cm1 in 5-membered ring ketones
• 1690 cm1 in ketones next to a double bond or
an aromatic ring
3-hexanone
C=O in Esters
• 1735 cm1 in saturated esters
• 1715 cm1 in esters next to aromatic ring or a
double bond
Carboxylic Acids
1700 - 1725 cm
O
R
-1
(strong)
OH
-1
2500 - 3300 cm
(very broad and intense)
Pentanoic acid
Identify the functional groups in compounds
that are responsible for the following
absorptions:
• A compound with a strong absorption at 1710 cm-1
• A compound with a strong absorption at 1540 cm-1
• A compound with a strong absorption at 1720 cm-1
and at 2500-3100 cm-1
How might you use IR spectroscopy to
distinguish between the following pairs of
isomers?
• CH3CH2OH and CH3OCH3
• Cyclohexane and 1- hexene