1
Use of IR spectroscopy
4000
O
H
C
C
N
H
C
N
C
H
C
O
bonds to
hydrogen
triple
bonds
O
H
change in scale
energy to cause vibration
3000
2000
1500
N
N
H
H
H
3500 - NH
2950 - CH
2270 - C N
1700 - C=O
1000 cm-1
C
C
C
O
C
O
C
F
C
Cl
double
bonds
single
bonds
•
•
•
•
4 interesting areas to IR spectra (left)
1500-400cm-1 = fingerprint region
Individual to each molecule
But not much useful information!
2
X-H Region
-1
C–H
2900 - 3000 cm
-1
N–H
3300 - 3400 cm
O–H
3500 - 3600 cm-1
Ph
N
Me
Reduced mass μ very similar for all
So order a result of bond strength
OH > NH > CH
Ph
H
N
H
H
R
R
N
H
H
anti-symmetric stretch
3400cm-1
N
H
H
symmetric stretch
3300cm-1
• 2 N-H bonds in aniline act as one 'unit'
• Two possible vibrations so two peaks
3
X-H: Hydrogen bonding
t-Bu
O
O
t-Bu
Me
Ph
H
O
H
H
O
Ph
H
H
O
O
H
H
O
H
O
Ph
can't
approach
• H-bonds vary in strength and length
• They cause a weakening of X-H bonds
• The range of strengths leads to a range of absorptions
4
Butylated hydroxytoluene BHT
The steric bulk of the two tert-butyl groups prevents two molecules
of BHT from hydrogen bonding with each other and we get a sharp
peak in the IR spectrum
5
Double bond region
C=O
carbonyl group
1900 – 1500 cm-1
strong
C=C
alkene
1640 cm-1
weak
O–N=O
nitro group
1500 & 1300 cm-1
2 strong peaks
• Practically the most useful
•
region in IR
Certainly the most information
RNO2
≡
O
N
O
≡
O
R
NO2
O
N
O
R
anti-symmetric
1550 cm-1
• Two peaks
• Symmetric
N
O
R
O
N
O
R
symmetric
1350 cm-1
as two forms of stretch (like primary amine)
stretch weaker absorption as less change in dipole
6
Alkenes
wavenumber (cm-1)
structure
1640 (m, sh)
m = medium
sh = sharp
1655 (m, sh)
More substituents the
stronger the bond &
higher absorption
1660 - 1675 (w)
• More substituents =
stronger bond
• But less change in
dipole moment so
reduced intensity
• Symmetrical alkenes
close to no absorption
R
R
R2
R
R2
R
H
H
H
H
R2
R
R2
R
R2
R3
H
R3
R4
comment
• sp2 centres form stronger bonds as more s character
overlap
-1
• C-H of alkene >3000 cm
• C-H of alkyl <3000 cm-1
better orbital
7
Examples of alkenes
O
• Limonene - peaks weak as little
•
•
O
•
•
change in dipole moment
Internal alkene higher wavenumber
due to substitution
But less intense as less change in
dipole moment
Isolated C=O & C=C in normal
positions (above)
Conjugated / resonance lowers
both as more single bond character
BUT more intense as polarised
8
Carbonyl group (C=O) in IR
O
R
O
H
R
aldehydes
RCHO
1720 – 1725 cm-1
R1
ketones
RCOR1
1710 – 1715 cm-1
Aldehydes have stronger bonds than ketones - why?
Carbonyl functionality most useful in IR (in my opinion)
O
O
O
• As C=O gets more strained so bond
•
1715 cm-1
1745 cm-1
1780 cm-1
•
gets stronger! But more reactive
Angle less than 120˚ so needs more
p character
Leaves more s character in C=O so
shorter and stronger bond
O
• Why weaker than normal?
• Delocalisation results in single
bond character
O
≡
1690 cm-1
O
9
Carbonyl group (C=O) in IR II
Carboxylic acid derivatives
O
R
X
1815 cm-1
O
>
X = Cl
Electronegative Cl drags electrons close making !
bond shorter and stronger
R
>
Cl
Stronger than ketone due to electronegative oxygen.
But delocalisation of lone pair weakens ! bond.
X = OH
O
1745cm-1
R
X = NH2
1650 cm-1
O
OH
R
O !–
"
R
OH
!+
OH
Amides have even weaker ! bond due to increased
delocalisation. Nitrogen is less electronegative so
donates lone pair more readily.
O
R
O
NH2
R
O !–
"
NH2
R
!+
NH2
10
Carbonyl group: examples
O
O
Pr
Cl
1802cm-1
Pr
OH
1712cm-1
O
Pr
NH2
1662cm-1
O
OH
1706 &
1655cm-1
11
Fingerprint region
• 1500 cm-1 or less is fingerprint region - unique to a molecule
• But dull C–C, C–N & C–O roughly same reduced mass and bond
strength - so nothing distinctive
• Deformations (or bending) occurs at low energy as well
• Very few are useful
Summary of absorptions in IR spectra
Position of
band
reduced mass
light atoms - high
frequency
bond strength
strong bond - high
frequency
Strength of
band
change in dipole
moment
large dipole - strong band
Width of band
hydrogen bonding
strong H bond - weak
band
12
Correlation table