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IR spectroscopy
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Adaptado de:
Organic Chemistry, 6th Edition; L. G.
Wade, Jr.
Organic Chemistry, William H. Brown
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Infrared Spectroscopy
IR spectrum of 3-methyl-2-butanone
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Infrared Spectroscopy scale
The vibrational IR extends from 2.5 x 10-6 m (2.5 µm)
to 2.5 x 10-5 m (25 µm)
the frequency of IR radiation is commonly expressed in
wavenumbers
wavenumber(ν) : the number of waves per centimeter,
with units cm-1 (read reciprocal centimeters)
expressed in wavenumbers, the vibrational IR extends from
4000 cm-1 to 400 cm -1
-2
-1
ν = 10 m•cm
= 4000 cm-1
-6
2.5 x 10 m
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ν =
10-2 m•cm -1
-5
= 400 cm-1
2.5 x 10 m
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Molecular vibrations
Covalent bonds vibrate at only
certain allowable frequencies.
http://www.askthenerd.com/ocol/SPEC/IR/IRF.HTM
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Stretching Frequencies
Frequency decreases with increasing
atomic mass.
Frequency increases with increasing
bond energy.
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Vibrational Modes
Nonlinear molecule with n atoms
usually has 3n - 6 fundamental
vibrational modes.
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Fingerprint of Molecule
Whole-molecule vibrations and bending
vibrations are also quantized.
No two molecules will give exactly the
same IR spectrum (except enantiomers).
Simple stretching: 1600-3500 cm-1.
Complex vibrations: 600-1400 cm-1,
called the “fingerprint region.”
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IR-Active and Inactive
A polar bond is usually IR-active.
A nonpolar bond in a symmetrical
molecule will absorb weakly or not at all.
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An Infrared Spectrometer
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FT-IR Spectrometer
Has better sensitivity.
Less energy is needed from source.
Completes a scan in 1-2 seconds.
Takes several scans and averages
them.
Has a laser beam that keeps the
instrument accurately calibrated.
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IR spectra interpretation
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Molecular vibrations
Fundamental stretching and bending
vibrations for a methylene group
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Some metylene vibrations
http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html
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Molecular Vibrations
Consider two covalently bonded atoms as two vibrating
masses connected by a spring
the total energy is proportional to the frequency of
vibration
the frequency of a stretching vibration is given by an
equation derived from Hooke’s law for a vibrating spring
ν
= 4.12
K
µ
K = a force constant, which is a measure of the bonds’
strength; force constants for single, double, and triple
bonds are approximately 5, 10, and 15 x 105 dynes/cm
µ = reduced mass of the two atoms, (m1m2)/(m1 + m2), where
m is the mass of the atoms in grams
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Molecular Vibrations
ν
= 4.12
K
µ
From this equation, we see that the
position of a stretching vibration
is proportional to the strength of the vibrating
bond
is inversely proportional the masses of the
atoms connected by the bond
The intensity of absorption depends primarily on
the polarity of the vibrating bond
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Carbon-Carbon Bond Stretching
Stronger bonds absorb at higher
frequencies:
C-C
1200 cm-1
C=C 1660 cm-1
C≡C
<2200 cm-1 (weak or absent if
internal)
Conjugation lowers the frequency:
isolated C=C
1640-1680 cm-1
conjugated C=C
1620-1640 cm-1
aromatic C=C
approx. 1600 cm-1
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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
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Correlation Tables
Characteristic IR absorptions for the
types of bonds and functional groups
we deal with most often
Bon d
O-H
N-H
C-H
C=C
C=O
C-O
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Stretching
Frequ ency (cm -1) Intens ity
3200-3650
w eak to s trong
mediu m
3100-3550
2700-3300
w eak to medium
1600-1680
w eak to medium
1630-1820
strong
1000-1250
strong
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Hydrocarbon
Alk ane
C-H
CH3
C-C
Alk ene
C-H
C=C
Alk yn e
C-H
C C
Arene
C-H
C=C
C-H
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Vib ration
Stretchin g
Bend ing
Bend ing
(N ot useful
Frequen cy
-1
(cm )
Intens ity
2850 - 3000
Mediu m
1450-1475
Mediu m
1375 and 1450 Weak to medium
for interpretation - too man y b ands
Stretchin g
Stretchin g
3000 - 3100
1600 - 1680
Weak to medium
Weak to medium
Stretchin g
Stretchin g
3300
2100-2250
Mediu m to stron g
Weak
Stretchin g
Stretchin g
Bend ing
3030
1450-1600
690-900
Weak to medium
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Alkanes
IR spectrum of decane
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Alkenes
IR spectrum of cyclohexene
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Alkynes
IR spectrum of 1-octyne
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Alkynes IR Spectra
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Aromatics
IR spectrum of toluene
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Alcohols
Bond
Frequency, cm
-1
Inten sity
O-H (free)
3600-3650
Weak
O-H (H b ond ed)
C-O
3200 - 3500
1000 - 1250
Medium, broad
Medium
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Alcohols
IR spectrum of 1-hexanol
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Ethers
IR spectrum of dibutyl ether
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Ethers
IR spectrum of anisole
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Amines
IR spectrum of 1-butanamine (Fig
12.11)
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CHO, CO, COOH
Carbonyl Group
Vibration
Frequen cy
(cm-1 )
Inten sity
O
RCR'
Ketones
C=O
Stretchin g
1630-1820
Strong
O
RCH
Aldeh yd es
C=O
C-H
Stretching
Stretching
1630-1820
2720
Strong
Weak
Carboxylic acids
C=O
Stretching
O H
Stretching
1700-1725
2500-3300
Strong
Strong (broad)
O
RCOH
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Aldehydes and Ketones
IR spectrum of menthone
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Carbonyl groups
The position of C=O stretching vibration is sensitive to its
molecular environment
as ring size decreases and angle strain increases, absorption shifts
to a higher frequency
O
O
O
O
1715 cm-1
1745 cm-1
1780 cm-1
1850 cm-1
conjugation shifts the C=O absorption to lower frequency
O
O
O
H
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1717 cm
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1700 cm
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acid derivatives
O
RCNH2
Amides
C=O
Stretching
1630-1680
N H
Stretching
3200, 3400
(1°amides have two N -H stretches)
(2°amides have one N -H stretch)
O
RCOR'
Carboxylic esters
C=O
Stretching
2
sp C O
Stretching
3
sp C O
Stretching
O O
RCOCR
Acid anhydrides
C=O
Stretching
RC N
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C O
Stretching
Nitriles
≡N
C≡
Stretching
Strong
M edium
1735-1800
1200-1250
1000-1100
Strong
Strong
Strong
1740-1760 and
1800-1850
900-1300
Strong
2200-2250
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Carboxylic acids
IR spectrum of pentanoic acid
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O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding.
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Esters
IR of ethyl butanoate
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Carbon - Nitrogen Stretching
C - N absorbs around 1200 cm-1.
C = N absorbs around 1660 cm-1
and is much stronger than the C = C
absorption in the same region.
C ≡ N absorbs strongly just above
2200 cm-1. The alkyne C ≡ C signal
is much weaker and is just below
2200 cm-1 .
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A Nitrile IR Spectrum
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Summary of IR
Absorptions
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Strengths and Limitations
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indicated.
The absence of a signal is definite proof that
the functional group is absent.
Correspondence with a known sample’s IR
spectrum confirms the identity of the
compound.
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Raman spectroscopy
http://www.chemsoc.org/ExemplarChem/e
ntries/2004/birmingham_jones/raman.html
#Raman
http://www.kosi.com/raman/resources/tec
hnotes/1101.pdf
IR bands arise from a change in the dipole
moment of a molecule,
Raman bands arise from a change in the
polarizability
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Theoretical Background
Molecule
Absorption (i.e. IR
spectroscopy)
Electromagnetic wave
Scattering (i.e.
Raman Spectroscopy)
Diffraction (i.e. x-ray
diffraction)
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Normal Raman Instrumentation
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IR and
Raman
Comparison of
IR and Raman
for a CO2
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A. Fadini and F.M. Schnepel, Vibrational Spectroscopy
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