CHEM 430
IR SPECTROSCOPY
1
Infrared and Raman
Spectroscopy
11-1
INTRODUCTION
• The method provides a rapid and simple method for observing the functional
group species present in an organic molecule
• The spectrum is a plot of the percentage of IR radiation that passes through the
sample (% transmission) versus some function of the wavelength of the
radiation related to covalent bonding
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-1
INTRODUCTION
Instrumentation.
Modern IR spectrometers are based on the Michelson interferometer
• Fourier transform infrared ( FT– IR) spectrometers: The absorption spectrum is
obtained by means of Fourier transformation of an interferogram.
• Dispersive infrared spectrometers: Earlier instruments based on
monochromators that disperse the radiation from an IR source into its
component wavelengths - spectrum is obtained by measuring the amount of
radiation absorbed by a sample as the wavelength is varied.
• Raman spectroscopy provides information complementary to that obtained
from IR spectroscopy.
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The quantum mechanical energy levels observed in IR spectroscopy are those of
molecular vibration
• When we say a covalent bond between two atoms is of a certain length, we are
citing an average because the bond behaves as if it were a vibrating spring
connecting the two atoms
• For a simple diatomic molecule, this model is easy to visualize:
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• There are two types of bond vibration:
1. Stretch – Vibration or oscillation along the line of the bond
H
H
C
C
H
H
asymmetric
symmetric
2. Bend – Vibration or oscillation not along the line of the bond
H
H
H
C
C
C
H
H
scissor
rock
in plane
H
C
H
H
twist
wag
out of plane
5
5
Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• Each stretching and bending vibration occurs with a characteristic frequency
• Typically, this frequency is on the order of 1.2 x 1014 Hz
(120 trillion oscillations per sec. for the H2 vibration at ~4100 cm-1)
• The corresponding wavelengths are on the order of 2500-15,000 nm or 2.5 – 15
microns (mm)
• When a molecule is bombarded with electromagnetic radiation (photons) that
match the frequency of one of these vibrations (IR radiation), it is absorbed and
the bonds begin to stretch and bend more strongly (emission and absorption)
• When this photon is absorbed the amplitude of the vibration is increased NOT
the frequency
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The result of the spectroscopic process is a spectrum of the various stretches
and bends of the covalent bonds in an organic molecule
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The x-axis of the IR spectrum is in units of wavenumbers, n, which is the
number of waves per centimeter in units of cm-1 (Remember E = ħn or E =
ħc/l)
• This unit is used rather than wavelength (microns) because wavenumbers are
directly proportional to the energy of transition being observed –
chemists like this, physicists hate it
High frequencies and high wavenumbers equate higher energy
is quicker to understand than
Short wavelengths equate higher energy
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• This unit is used rather than frequency as the numbers are more “real” than the
exponential units of frequency
• IR spectra are observed for what is called the mid-infrared: 400-4000 cm-1
• The peaks are Gaussian distributions of the average energy of a transition
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• So how does the IR detect different bonds?
• The potential energy stretching or bending vibrations of covalent bonds follow
the model of the classic harmonic oscillator (Hooke’s Law)
Potential Energy (E)
Remember:
E = ½ ky2
where: y is spring displacement
k is spring constant
Interatomic Distance (y)
CHEM 430 – NMR Spectroscopy
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10
Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
Aside: Physically here are the movements we are discussing:
• Stretching vibration: a typical C-C bond with a bond length of 154 pm, the
displacement is averages 10 pm:
154 pm
10 pm
• Bending vibration: For C-C-C bond angle a change of 4° is typical, which
corresponds to an average displacement of 10 pm.
4o
10 pm
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The energy levels for these vibrations are quantized as we are
considering quantum mechanical particles
• Only discrete vibrational energy levels exist:
Potential Energy (E)
rotational transitions – (in microwave region)
Vibrational transitions, n
Interatomic Distance (r)
• Note there is no energy level below n = 0, at any temperature above
absolute zero there is always the first vibrational energy level
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• However, the application of the classical vibrational model fails apart for
two reasons:
1. As two nuclei approach one another through bond vibration,
potential energy increases to infinity, as two positive centers begin
to repel one another
2. At higher vibrational energy levels, the amplitude of displacement
becomes so great, that the overlapping orbitals of the two atoms
involved in the bond, no longer interact and the bond dissociates
• We say that the model is really one of an aharmonic oscillator, for which
the simple harmonic oscillator model works well for low energy levels
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• Here is the derivation of Hooke’s Law we will apply for IR theory:
• Vibrational frequency given by:
n : frequency
K: force constant – bond strength
m: reduced mass = m1m2/(m1+m2)
• Reduced mass is used, as each atom in the covalent bond oscillates about
the center of the two masses
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• What does this mean for the different covalent bonds in a molecule?
Let’s consider reduced mass, m, first:
• The C-H and C-C single bonds differ by only 16 kcal/mole:
99 kcal · mol-1 vs. 83 kcal · mol-1 (similar K)
• Due to the reduced mass term, these two bonds of similar strength show up
in very different regions of the IR spectrum:
C─C
C─H
1200 cm-1 m = (12 x 12)/(12 + 12) = 6
3000 cm-1 m = (1 x 12)/(1 + 12)
= 0.92
(0.41)
(0.95)
• A smaller atom therefore gives rise to a higher wavenumber (and  n and E)
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• What does this mean for the different covalent bonds in a molecule?
When greater masses are added, the trend is similar (K’s here are different)
C─I
C─Br
C─Cl
C─O
C─C
C─H
500 cm-1
600 cm-1
750 cm-1
1100 cm-1
1200 cm-1
3000 cm-1
A smaller atom therefore gives rise to a higher wavenumber (and  n and E)
and a larger atom gives rise to lower wavenumbers (and  n and E)
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• What does this mean for the different covalent bonds in a molecule?
Let’s consider bond strength, K:
• A C≡C bond is stronger than a C=C bond is stronger than a C-C bond
wavenumber, cm-1
DHf
• From IR spectroscopy we find: C≡C
~2100
200
C=C
~1650
146
C—C
~1200
83
Note the good correlation with the heats of formation for each bond!
Stronger bonds give higher wavenumbers (and  higher n and E)
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The y-axis of the IR spectrum is in units of transmittance, T, which is the
ratio of the amount of IR radiation transmitted by the sample (I) to the
intensity of the incident beam (I0); % Transmittance is T x 100
T = I / I0
%T = (I / I0) X 100
• IR is different than other spectroscopic methods which plot the y-axis as
units of absorbance (A). A = log(1/T)
• As opposed to chromatography or other spectroscopic methods, the area of
a IR band (or peak) is not directly proportional vs. concentration of other
functionalities, it can be used vs. itself if standardized!!!
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• The intensity of an IR band is affected by two primary factors:
• Whether the vibration is one of stretching or bending
• Electronegativity difference of the atoms involved in the bond:
• For both effects, the greater the change in dipole moment in a given
vibration or bend, the larger the peak.
• The greater the difference in electronegativity between the atoms involved
in bonding, the larger the dipole moment
Typically, stretching will change dipole moment more than bending
CHEM 430 – NMR Spectroscopy
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Infrared and Raman
Spectroscopy
11-2
VIBRATIONS OF MOLECULES
The IR Spectroscopic Process.
• It is important to make note of peak intensities to show the effect of these
factors:
• Strong (s) – peak is tall, transmittance is low
• Medium (m) – peak is mid-height
• Weak (w) – peak is short, transmittance is high
• * Broad (br) – if the Gaussian distribution is abnormally broad
(* this is more for describing a bond that spans many energies)
Exact transmittance values are rarely recorded
CHEM 430 – NMR Spectroscopy
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II.
Infrared Group Analysis
A. General
1.
The primary use of the IR spectrometer is to detect functional groups
2.
Because the IR looks at the interaction of the EM spectrum with actual
bonds, it provides a unique qualitative probe into the functionality of a
molecule, as functional groups are merely different configurations of
different types of bonds
3.
Since most “types” of bonds in covalent molecules have roughly the
same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show
up in similar regions of the IR spectrum
4.
Remember all organic functional groups are made of multiple bonds and
therefore show up as multiple IR bands (peaks)
There are 4 principle regions:
Bonds to H
Triple bonds
O-H single bond
N-H single bond
C-H single bond
C≡C
C≡N
Double bonds
Single Bonds
C=O
C=N
C=C
C-C
C-N
C-O
Fingerprint
Region
4000 cm-1
2700 cm-1
2000 cm-1
1600 cm-1
21
400 cm-1
We will pick up next time with peak intensities, width of bands and
some simple symmetry rules, as well as instrument design
Monday we should finally get to functional groups where we will
apply in depth the general topics we have discussed in the
introductory material
No Problem set for today!
But take this time to review some organic:
- bond strengths – both inter and intra-molecular
- bond distances for more organic-y bonds
- hybridization models
- Periodic table and properties – you should know the
position and EN’s of H, B, C, N, O, F, Si, P, S,
Cl, Br and I
22
IR Spectroscopy
I.
Introduction
F. The IR Spectrum
4. The intensity of an IR band is affected by two primary factors:
•
Whether the vibration is one of stretching or bending
•
Electronegativity difference of the atoms involved in the bond:
For both effects, the greater the change in dipole moment in a
given vibration or bend, the larger the peak
The greater the difference in electronegativity between the atoms
involved in bonding, the larger the dipole moment
Typically, stretching will change dipole moment more than bending
5.
It is important to make note of peak intensities to show the effect of
these factors:
•
Strong (s) – peak is tall, transmittance is low
•
Medium (m) – peak is mid-height
•
Weak (w) – peak is short, transmittance is high
•
* Broad (br) – if the Gaussian distribution is abnormally broad
•
(*this is more for describing a bond that spans many energies)
Exact transmittance values are rarely recorded
23
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
We have learned:
•
That IR radiation can “couple” with the vibration of covalent bonds,
where that particular vibration causes a change in dipole moment
•
The IR spectrometer irradiates a sample with a continuum of IR
radiation; those photons that can couple with the vibrating bond
elevate it to the next higher vibrational energy level (increase in A)
•
When the bond relaxes back to the n0 state, a photon of the same
n is emitted and detected by the spectrometer; the spectrometer
“reports” this information as a spectral band centered at the n of
the coupling
•
The position of the spectral band is dependent on bond strength
and atomic size
•
The intensity of the peak results from the efficiency of the
coupling; e.g. vibrations that have a large change in dipole moment
create a larger electrical field with which a photon can couple
24more
efficiently
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
Remember, most interesting molecules are not diatomic, and mechanical or
electronic factors in the rest of the structure may effect an IR band
From a molecular point of view (discounting phase, temperature or other
experimental effects) there are 10 factors that contribute to the position,
intensity and appearance of IR bands
1. Symmetry
2. Mechanical Coupling
3. Fermi Resonance
4. Hydrogen Bonding
5. Ring Strain
6. Electronic Effects
7. Constitutional Isomerism
8. Stereoisomerism
9. Conformational Isomerism
10. Tautomerism (Dynamic Isomerism)
25
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
1. Symmetry H2O
•
For a particular vibration to be IR active there must be a change in
dipole moment during the course of the particular vibration
•
For example, the carbonyl vibration causes a large shift in dipole
moment, and therefore an intense band on the IR spectrum
-
+
C
•
vibration
C
O
-
+
O
For a symmetrical acetylene, it is clear that there is no permanent
dipole at any point in the vibration of the CC bond. No IR band
appears on the spectrum
vibration
C
C
C
C
26
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
1. Symmetry H2O
•
Most organic molecules are fortunately asymmetric, and bands are
observed for most molecular vibration
•
The symmetry problem occurs most often in small, simple
symmetric and pseudo-symmetric alkenes and alkynes
H3C
CH3
C
H3C
H2C
C
•
H3C
C
C
CH3
C
H2
C
CH3
H3C
H3C
symmetric
C
CH3
psuedo-symmetric
C
H3C
C
CH3
CH3
Since symmetry elements “cancel” the presence of bonds where no
dipole is generated, the spectra are greatly simplified
27
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
1. Symmetry H2O
•
Symmetry also effects the strength of a particular band
•
The symmetry problem occurs most often in small, simple
symmetric and pseudo-symmetric alkenes and alkynes
H3C
CH3
C
H3C
H2C
C
•
H3C
C
C
CH3
H3C
C
C
H2
C
CH3
H3C
H3C
symmetric
C
CH3
psuedo-symmetric
C
CH3
CH3
Since symmetry elements “cancel” the presence of bonds where no
dipole is generated, the spectra are greatly simplified
28
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
2. Mechanical Coupling
•
In a multi-atomic molecule, no vibration occurs without affecting
the adjoining bonds
•
This induces mixing and redistribution of energy states, yielding
new energy levels, one being higher and one lower in frequency
•
Coupling parts must be approximate in E for maximum interaction
to occur (i.e. C-C and C-N are similar, C-C and H-N are not)
•
No interaction is observed if coupling parts are separated by more
than two bonds
•
Coupling requires that the vibration be of the same symmetry
29
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
2. Mechanical Coupling
•
For example, the calculated and observed n for most C=C bonds is
around 1650 cm-1
•
Butadiene (where the two C=C systems are separated by a
dissimilar C-C bond) the bands are observed at 1640 cm-1 (slight
reduction due to resonance, which we will discuss later)
•
In allene however, mechanical coupling of the two C=C systems
gives two IR bands – at 1960 and 1070 cm-1 due to mechanical
coupling
H
H
C
C
C
H
H
•
For purposes of this course, when we discuss the group
frequencies, we will point out when this occurs
30
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
3. Fermi Resonance
•
A Fermi Resonance is a special case of mechanical coupling
•
It is often called an “accidental degeneracy”
•
In understanding this, for many IR bands, there are “overtones” of
the fundamental (the n’s you are taught) at twice the wavenumber
•
In a good IR spectrum of a ketone (2-hexanone, here) you will see
a C=O stretch at 1715 cm-1 and a small peak at 3430 cm-1 for the
overtone
overtone
fundamental
31
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
3. Fermi Resonance
•
Ordinarily, most overtones are so weak as not to be observed
•
But, if the overtone of a particular vibration coincides with the band
from another vibration, they can couple and cause a shift in group
frequency and introduce extra bands
•
If you first looked at the IR (working “cold”) of benzoyl chloride,
you may deduce that there were two dissimilar C=O bonds in the
molecule
O
C
Cl
32
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
3. Fermi Resonance
•
In this spectrum, the out of plane bend of the aromatic C-H bonds
occurs at 865 cm-1; the overtone of this band coincides with the
fundamental of C=O at 1730 cm-1
•
The band is “split” by Fermi resonance (1760 and 1720 cm-1)
O
C
Cl
33
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
3. Fermi Resonance
•
Again, we will cover instances of this in the discussion of group
frequencies, but this occurs often in IR of organics
•
Most observed:
Aldehydes – the overtone of the C-H deformation mode at
1400 cm-1 is always in Fermi resonance with the stretch of
the same band at 2800 cm-1
O
C
H
-
The N-H stretching mode of –(C=O)-NH- in polyamides
(peptides for the biologists and biochemists) appears as two
bands at 3300 and 3205 cm-1 as this is in Fermi resonance
34
with the N-H deformation at 1550 cm-1
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
4. Hydrogen Bonding
•
One of the most common effects in chemistry, and can change the
shape and position of IR bands
•
Internal (intramolecular) H-bonding with carbonyl compounds can
serve to lower the absorption frequency
CH3
O
1680 cm-1
O
H
O
CH3
1724 cm-1
O
O
35
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
4. Hydrogen Bonding
•
Inter-molecular H-bonding serves to broaden IR bands due to the
continuum of bond strengths that result from autoprotolysis
•
Compare the two IR spectra of 1-propanol; the first is an IR of a
neat liquid sample, the second is in the gas phase – note the shift
and broadening of the –O-H stretching band
Gas phase
Neat liquid
36
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
4. Hydrogen Bonding
•
Some compound, in addition to intermolecular effects for the
monomeric species can form dimers and oligomers which are also
observed in neat liquid samples
•
Carboxylic acids are the best illustrative example – the broadened
O-H stretching band will be observed for the monomer, dimer and
oligomer
H
O
Monomer
O
O
H
O
O
O
O
H
H
O
Dimer
O
H
O
Oligomer
O
O
H
37
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
5. Ring Strain
•
Certain functional group frequencies can be shifted if one of the
atoms hybridization is affected by the constraints of bond angle in
ring systems
•
Consider the C=O band for the following cycloalkanones:
O
O
1815
•
1775
O
O
O
1750
1715
1705 cm-1
We will discuss the specific cases for these shifts during our
coverage of group frequencies
38
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
6. Electronic Effects - Inductive
•
The presence of a halogen on the a-carbon of a ketone (or electron
w/d groups) raises the observed frequency for the p-bond
•
Due to electron w/d the carbon becomes more electron deficient
and the p-bond compensates by tightening
O
C


X

39
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
6. Electronic Effects - Resonance
•
One of the most often observed effects
•
Contribution of one of the less “good” resonance forms of an
unsaturated system causes some loss of p-bond strenght which is
seen as a drop in observed frequency
O
C
C C
vs.
O
C
C C
O
O
1684 cm-1
C=O
1715 cm-1
C=O
40
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
6. Electronic Effects - Resonance
•
In extended conjugated systems, some resonance contributors are
“out-of-sync” and do not resonate with a group
•
Example:
O
C
H3C
H2N
X
X=
O
C CH3
Strong resonance contributor
NH2
CH3
Cl
NO2
1677
1687
1692
1700
O
N
vs.
O
cm-1
O
C
CH3
Poor resonance contributor
(cannot resonate with C=O)
41
IR Spectroscopy
I.
Introduction
G. The IR Spectrum – Factors that affect group frequencies
6. Electronic Effects - Sterics
•
Consider this example:
O
O
CH3
C=O: 1686 cm-1
C=O: 1693 cm-1
•
In this case the presence of the methyl group “misaligns” the
conjugated system, and resonance cannot occur as efficiently
•
The effects of induction, resonance and sterics are very casespecific and can yield a great deal of information about the
electronic structure of a molecule
42
IR Spectroscopy
III. Group Frequencies and Analysis
A. Introduction
1. When approaching any IR spectrum be sure to use the larger-to-smaller
region approach- do not immediately focus on any one single peak (even
–OH or C=O)
2.
From the Hooke’s Law derivation we are using we find that the IR can be
conveniently be divided into four major regions:
Bonds to H
Triple bonds
O-H
N-H
C-H
Double bonds
C≡C
C≡N
Single Bonds
C=O
C=N
C=C
C-C
C-N
C-O
C-X
“Fingerprint
Region”
4000 cm-1
2700 cm-1
2000 cm-1
1600 cm-1
400 cm-1
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IR Spectroscopy
III. Group Frequencies and Analysis
A. Introduction
3. If supporting information is available – molecular formula, chemical
inferences – (i.e. this was the product of an oxidation reaction), assume
this information is correct and the analysis of the IR should support it
(later in your careers you can doubt information given to you)
4.
If a molecular formula is available, do an HDI!
5.
Many texts list various methods for approaching an IR spectrum; use the
method that works best for you and stick to it.
6.
The most common mistakes in spectral analysis are those of “jumping the
gun” to a conclusion (usually based on some small, insignificant peak) or
taking a random haphazard approach to the spectrum (gee, here is an
IR, oh, let’s start looking for phosphorus this time)
Be methodical, develop a scheme and stick to it!
44
IR Spectroscopy
III. Group Frequencies and Analysis
Before we begin – Each functional group will be described as follows:
Group
General – What is most recognizable? What makes it different from similar
groups?
Group Frequencies (cm-1):
Bond
observed
n in cm-1
type of vibration
Exceptions and things to watch
Scale on bottom summarizes band positions and strengths
Strong Medium -
Weak -
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IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkanes
General – due to the small electronegativity difference between C and H,
hydrocarbon bands are of medium intensity at best and give simple
spectra
Group Frequencies (cm-1):
C-H
3000-2800
Stretch
-CH2-
~1465
Methylene bend (scissor)
-CH3
~1375
Methyl bend (sym)
-(CH2)4-
~720
Rocking motion 4 or more
–CH2- (long chain band)
C-C
Strained ring systems may have
higher n
Not interpretively useful,
small weak peaks
46
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkanes – Dodecane – C12H26
47
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkanes – Cyclopentane – C5H10
48
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkanes
Additional – If the 1400-1350 region is free of interference, the presence of
certain alkyl groups can be discerned:
Methylene
Methyl
H
H
H
C
C
H
H
Scissor
1465
C
H
H
Bendasymm
1450
usually overlap
H
Bendsymm
1375
C
1380
1370
CH3
CH3
gem-dimethyl
CH3
1390
1370
C
CH3
t -butyl
CH3
49
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkanes
Additional – Example: Compare 2,2-dimethylpentane vs. 2-methylhexane:
vs.
50
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
General – slightly more complex than alkanes; asymmetric C=C is observed as
well as the sp2-C-H stretch. Still, bands are weak to medium in intensity
Group Frequencies (cm-1):
=C-H
3095-3010 Stretch
- Diagnostic for unsaturation- may be
aromatic as well
=C-H
1000-650
- Can be used to determine degree
of substitution
C=C
1660-1600 Stretch
Out-of-plane (oop) bend
- Can be reduced by resonance
- Symmetrical C=C do not absorb
- trans- weaker than cis-
51
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes – 1-octene – C8H16
Note – you still have alkane present!
52
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes – trans-4-octene – C8H16
Note – absence of C=C band, shouldering of C-H band
53
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes – cis-2-pentene – C5H10
Note – shouldering of C-H band
54
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes – cyclopentene – C5H8
Note – increased complexity due to ring vibrations
H
H
55
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
Substitution – The out of plane =C-H bend produces strong bands but
interference can come from aromatic rings (similar oop) and C-Cl bonds
(~700)
1000
900
800
700
monosubstituted
R
cis-1,2
R
trans-1,2
R
R
R
R
1,1-disubstitued
R
R
trisubstituted
tetrasubstituted
R
R
R
R
R
R
none, with weak C=C
56
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
Substitution – The monosubstitued band is very reliable; and the variance
induced by electronic effects is observed
1000
monosubstituted (R-)
monosubstitued
w/lone pair group
(ex. –Cl, -F, -OR)
monosubstitued
w/conj. group
(ex. C=O, CN)
900
R
800
700
overtone usually observed
G
G
The shifts are similar for 1,1-disubstitued systems
57
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
Rings – Incorporation of a double bond endocyclic or exocyclic to a ring
may shift the observed band
Endocyclic: Ring strain shifts the C=C band to lower n (ex. cyclopropene)
nC=C 1650
1646
1611
1566
1656
The adjacent C-C bond couples with the C=C system – if the resulting
component vector is along the line of the C=C bond an increase in n
occurs – this reaches a minima at 90o for cyclobutene (no net component
along C=C bond) and rises again with cyclopropene
58
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
Rings –
Endocyclic: If C=C at a ring fusion, absorption is reduced as if one further
carbon was removed from the ring:
nC=C 1611
The presence of additional alkyl groups on the ring dramatically raises
nC=C
R
nC=C 1656
1788
1641
R
1883
R
nC=C 1566
R
R
R
1675
59
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkenes
Rings –
Exocyclic: these C=C bonds give an increase in absorption n with
decreasing ring size:
nC=C 1940
1780
1678
1657
1651
As the angle between the two C-C bonds is reduced – more p character is
required (sp = 180°, sp2 = 120°, sp3 = 109.5°, “sp>3” = <109°
The p character of the double bond is reduced, but the stronger s bond
is strengthened to a greater degree
Think of the allene example (“2-membered ring”) as an extreme example
60
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkynes
General – can be symmetric, psuedo-symmetric or internal – greatly reducing
the number of observed bands
Group Frequencies (cm-1):
C-H
~3300
Stretch
- Diagnostic for terminal alkyne
CC
~2150
Stretch
- Can be reduced by resonance
-Symmetrical and psuedo-sym. CC
do not absorb
C-H
900-700
Bend
(Text does not list)
Possible not to observe any bands
for the CC system
61
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkynes – 1-hexyne – C6H10
HC C
Nice terminal, asymmetric, well behaved alkyne
62
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkynes – 3-hexyne – C6H10
C C
A not-so-nice, internal, symmetrical alkyne
63
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Alkynes – 1-hexyne – C6H10
HC C
Nice terminal, asymmetric, well behaved alkyne
64
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings
General – not true alkenes; most of the small bands associated with them are
not of diagnostic value; electronic effects of a single group on the ring
can change the observed bands drastically
Group Frequencies (cm-1):
-C-H
3050-3010 Stretch
C-H
900-690
Out of plane (oop) bend
Can be used to determine
substitution pattern
2000-1667 Overtone and combination
If observed, similar too oop
1600-1400 Ring stretch – observed as
Greatly dependent on substituents
bands
=C-H
Also for alkenes
two doublets (1600, 1580,
1500 & 1450)
65
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings – toluene – C7H8
CH3
Typical mono-substituted (EDG) ring
66
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings – o-xylene – C8H10
CH3
CH3
Typical ortho-substituted (EDG) ring
67
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings – m-xylene – C8H10
Typical meta-substituted (EDG) ring
CH3
CH3
68
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings – p-xylene – C8H10
CH3
Typical para-substituted (EDG) ring
CH3
69
IR Spectroscopy
III. Group Frequencies and Analysis
H2C
CH3
B. The Hydrocarbons
C
Mononuclear aromatic rings – a-methylstyrene – C9H10
Conjugated mono-substituted ring
70
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings
Substitution – The aromatic out of plane =C-H bend produces strong bands
but interference can come from alkenes (similar oop) and C-Cl (~700)
Consider this region to only be reliable for alkyl-, alkoxy-, halo-, amino-,
and acetyl substituted rings
Interpretation is often unreliable for nitro-, carboxylic- and sulfonic
groups
The overtone of these bands is the dominant source of the combination
and overtone bands observed at 2000-1667
71
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings
Substitution –
900
800
700
600
mono
ortho
meta
para
1,2,4
1,2,3
1,3,5
72
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Mononuclear aromatic rings
Substitution – The aromatic combination and overtone bands are a set of
weak absorptions that occur from 2000-1667. This is often obscured by
C=O
The general shape of the
pattern is used for
determining substitution
pattern; typically only a
neat liquid sample gives
an intense enough set of
bands for analysis
mono
ortho
meta
para
1,2,3
1,3,5
1,2,4
73
IR Spectroscopy
III. Group Frequencies and Analysis
B. The Hydrocarbons
Polynuclear and Hetero- aromatic rings
General – All bands for these aromatic systems are similar to the mononuclear
systems; shifts should be assumed, and analysis would be case-by-case
N
N
N
S
H
S
O
74
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Alcohols
General – the best recognized group on carefully selected spectra, but Hbonding effects can drastically change the position, intensity and shape of
the O-H band
Group Frequencies (cm-1):
O-H
3650-3600
Stretch
Seen in dilute solution or gas
phase spectra
O-H
3400-3300
Stretch
The “classic” H-bonded band,
seen in addition to the free band
in solution
C-O-H
1440-1220
Bend
Often obscured by -CH3 bend
C-O
1260-1000
Stretch
Can be used to determine 1o, 2o,
3o or phenolic structure
(free)
(H-bond)
75
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Alcohols – 1-octanol
HO
Neat liquid sample gives classic spectrum
76
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Alcohols – 1-octanol
HO
Same sample in dilute CCl4 solution (solvent bands deleted for clarity)
77
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Phenols – p-cresol
OH
Presence of aromatic bands, sharper -OH
78
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Alcohols –
Substitution – Using the position of the C-O stretching band, it is possible to
suggest a 1o, 2o, 3o or phenolic structure to the alcohol; but these should
be considered as base values, that may be changed by the effects of
conjugation or an adjacent ring system
OH
phenol
base value
1220
OH
OH
tertiary
1150
secondary
1100
primary
1050
nC-O 1070
nC-O 1017
nC-O 1070
OH
HC
C
OH
nC-O 1060
nC-O 1030
79
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Ethers
General – like alkynes, the simplicity of the spectra may allow them to pass
unnoticed – deduce from molecular formula if one should be present
Group Frequencies (cm-1):
C-O
1300-1000
Stretch (asymm.)
Absence of C=O and O-H will
confirm it is not ester or alcohol
Simple alkyl ethers usually one
band at 1120, aryl alkyl ethers
give two bands – 1250 & 1040
80
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Ethers – diispropyl ether
O
Spectrum dominated by all other functionality
81
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Ethers –
Additional Types
Aryl and vinyl ethers – The effect of conjugation gives the C-O bond a small
amount of double bond character, raising the observed n
H2C
C
H
O
R
H2C
C
H
O
R
Furthermore, strongly asymmetric systems (aryl alkyl and vinyl alkyl
ethers) may show an additional weak C-O band for the symmetric stretch
at 1040 and 850 respectively
82
IR Spectroscopy
III. Group Frequencies and Analysis
C. sp3 Oxygen – Alcohols, phenols and ethers
Ethers –
Additional Types
Epoxides – Most important bands are the ring deformation bands at nasym 950815 and nsym 880-750
Weaker “breathing mode” band is present at 1280-1230
O
Acetals and Ketals – Give four or five unresolved bands in the 1200-1020
region
R
R
O
O
R
H
R
R
O
R
O
R
83
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
Amines – Once presence is determined, the substitution at nitrogen is easy
to determine; only the 3° amine may present a problem
Group Frequencies (cm-1):
N-H
3650-3600
(2 bands)
1640-1560
Stretch (sym. and asym.)
3400-3300
(1 band)
1500
Stretch
N-H
~800
Oop bend
N-N
1350-1000
Stretch
(-NH2)
N-H
(-NHR)
Bend
For alkyl amines, very weak –
for aromatic 2° amines, stronger
Bend
Remember 3° amines have no
N-H bands
84
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
1° Amine – tert-butylamine
NH2
Two band –NH2 peak appears as small “w”
85
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
2° Amine – dibutylamine
H
N
Note weakness of –NH- band (can be mistaken as C=O overtone, if carbonyl
is present)
86
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
3° Amine – tributylamine
N
Difficult to discern from alkane – molecular formula for confirmation almost
requisite
87
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
Ammonium Salts
Almost certainly never encountered in neat samples, but an important
component of amino acids and many pharmaceuticals
Group Frequencies
N-H
3300-2600
Stretch
1° salts are at the higher n end
of this band, 3° salts at the
lower end
Additional band sometimes obs.
at 2100
N-H
1600-1500
Bend
1° as two bands (sym. And
asymm.), 2° at the upper end of
this range, 3° absorbs weakly
88
IR Spectroscopy
III. Group Frequencies and Analysis
D. sp3 Nitrogen – Amines
Ammonium Salts – anilinium hydrochloride
H
H
N
H
Cl
Spectrum is of a KBr disc sample:
89
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Along with alcohols, the most ubiquitous group on the IR spectrum.
Although it is easy to determine if the C=O is present, deducing the exact
functionality and factors that influence the position of the band provide
the challenge
Base C=O Frequencies (cm-1):
C=O
1810
Stretch (sym.)
Anhydride band 1
1800
Acid Chloride
1760
Anhydride band 2
1735
Ester
1725
Aldehyde
1715
Ketone
1710
Carboxylic Acid
1690
Amide
90
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – The carbonyl C=O frequency is very sensitive to the effects we
went over previously – a quick recap
Electronic Effects: Inductive vs. Resonance:
On first inspection, the ester, amide and acid halide/anhydride all possess
lone pairs of electrons that can resonate with the C=O (which should
lower n)
O
O
O
R
NH2
O
C N O F
S Cl
O
Cl
91
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In the case of an oxygen or chlorine being adjacent to the carbonyl, each
of these atoms resist the positive charge in the contributing resonance
structure, and the inductive effect becomes a stronger factor
O
O
O
O
C N O F
S Cl
O
O
This inductive effect draws in s electrons from the C=O, which
strengthens the p bond – these carbonyls appear at higher n
92
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In the case of nitrogen, it is less electronegative than oxygen and has a
greater acceptance of the positive charge in the contributing resonance
structure, so the carbonyl is lowered in n
O
O
N
H
N
H
O
C N O F
S Cl
N
H
The inductive effect of nitrogen compared to an sp2 carbon is negligible
by comparison
93
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
Likewise in aldehydes and ketones there is the inductive donation of
electrons to the s bond of the carbonyl which slightly weakens and
reduces the n of the p bond (and explains the small difference between
aldehydes and ketones)
O
C N O F
S Cl
The inductive effect of nitrogen compared to an sp2 carbon is negligible
by comparison
94
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Electronic Effects: Inductive vs. Resonance:
In addition, we discussed this effect in regards to a-halogenated
carbonyls as one of the effects that can change group n
C N O F
S Cl
The inductive effect of chlorine will draw s electrons through a-carbon,
weakening the C=O s and strengthening the p
95
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Electronic Effects - Resonance:
Not only is the C=O n lowered by the effects of conjugation, the peak
may also be broadened or split by the contribution of the two electronic
conformers
O
C
R
C
C
s-cis
C
R
C
C
O
s-trans
The s-cis absorbs at higher n than the s-trans. Why?
96
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – Ring Strain Effects:
C=O groups that can be incorporated into a ring are sensitive to this
effect. As ring size decreases more p-character must be used to make
the single bonds take on the smaller angle (re: sp>3 = <109°). The p
component of the C=O is weakened, but the s-bond strengthened,
raising the overall n
Cyclic ketones, esters (lactones), amides (lactams) and anhydrides exhibit
this behavior
O
O
O
O
O
NH
O
O
To clear up confusion – there are two ways to strengthen the C=O
1) Remove s bond character – p bond becomes more stronger (better
overlap) – this is a result inductive w/d
2) Remove p bond character – s bond becomes stronger – ring constraint
97
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
General – H-bonding effects:
C=O groups are reduced in n if some of the electron density is tapped off
to form H-bonds:
This effect can be inter- or intra-molecular:
98
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
1. Ketones – Simplest carbonyl group, for a single carbonyl compound,
implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
Stretch (sym.)
n Base, sensitive to change
C=O
1715
conj.
w/C=C
1700-1675
nC=C reduced to 1644-1617
conj.
w/Ph
1700-1680
nring 1600-1450
C=O
1815-1705
Decreased ring size raises n
O
1300-1100
C
C
Bend
C
99
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
1. Ketones – 2-hexanone
O
Typical aliphatic ketone
100
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
1. Ketones – 4-methylacetophenone
O
Typical aromatic ketone,
101
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
1. Ketones – Simplest carbonyl group, for a single carbonyl compound,
implied by a lack of any other functionality except hydrocarbon
Group Frequencies (cm-1):
Stretch (sym.)
n Base, sensitive to change
C=O
1715
conj.
w/C=C
1700-1675
nC=C reduced to 1644-1617
conj.
w/Ph
1700-1680
nring 1600-1450
C=O
1815-1705
Decreased ring size raises n
O
1300-1100
C
C
Bend
C
102
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
2. Aldehydes – Presence of the unique carbonyl C-H bond differentiates
this group from ketones
Group Frequencies (cm-1):
Stretch (sym.)
n Base, sensitive to change
C=O
1725
conj.
w/C=C
1700-1680
nC=C reduced to 1640
conj.
w/Ph
1700-1660
nring 1600-1450
O
C
R
H
2820,
2720
Stretch
Fermi doublet; Higher n band
often obscured by sp3 C-H
103
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
2. Aldehydes – isovaleraldehyde
O
C
H
Typical aliphatic aldehyde – note appearance of Fermi doublet and C=O
overtone
104
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
2. Aldehydes – anisaldehyde
O
H
O
Typical aromatic aldehyde, note how C=O obscures the combination and
overtone region – oop region would be used to determine substitution
105
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids – Various H-bonding effects lead to messy spectra,
especially in the upper frequency ranges – be aware of the effects of
monomeric, dimeric and oligomeric species on the spectrum
Group Frequencies (cm-1):
C=O
1710
Stretch (sym.)
C-O
1320-1210
Stretch
O-H
3400-2400
Stretch
n Base, sensitive to change;
conjugation gives reduced n
Overlaps C-H region in most
cases; multiple “sub-peaks” can
be seen for the dimeric and
oligomeric species – simplified in
non-polar solution or gas phase
spectra
106
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids – propionic acid
Aliphatic carboxylic acid – neat sample
vs. CCl4 solution (right)
107
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids – o-toluic acid
O
HO
Aromatic carboxylic acid, larger non-polar “end” of the molecule cuts down on
the hydrogen bonding seen with the smaller, previous propionic acid
108
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids - Salts
Salts are expressed as possesing one single and one double bond – the true
picture is one that is isoelectronic with the nitro group, with two bonds to
oxygen with a bond order of 1.5
Group Frequencies:
O
C
1600
1400
Stretch (asymm.)
Stretch (sym.)
O
109
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids - Salts – ammonium benzoate
O
H
O
N
H
H
H
Here is an example of ammonium and carboxylate moieties:
110
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
3. Carboxylic Acids – Amino Acids – L-alanine
O
H3N
CH
C
O
CH3
Amino Acids combine the features of carboxylate and ammonium salts:
111
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
4. Esters – Ester oxygen has an electron withdrawing effect that tends to
draw in electrons within the C=O system, strengthening it compared to
other carbonyls
Group Frequencies (cm-1):
Stretch (sym.)
n Base, sensitive to change
C=O
1735
conj.
C=C
1735-1715
nC=C reduced to 1640-1625
w/Ph
1735-1715
nring 1600-1450
conj. of
sp3 O
1765-1760
O
C
O
C-O
nC=O increases with smaller ring
1850-1740
1300-1000
Stretch, 2 bands
112
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
4. Esters – methyl butyrate
O
O
Simple aliphatic ester
113
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
4. Esters – methyl m-bromobenzoate
Conjugation on the carbonyl end:
O
Br
O
114
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
4. Esters – phenyl acetate
O
O
Conjugation on the sp3 oxygen end:
115
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
5. Amides – Amide nitrogen acts as a conjugating group with C=O,
reducing double bond character; amide nitrogen appears similar to
amime, including the effects of substitution
Group Frequencies (cm-1):
C=O
1685
Stretch (sym.)
n Base, sensitive to change
Can be as low as 1630 w/conj.
N-H
~3300
Stretch
O
Similar to amines, but typically
more intense
C
NH
nC=O increases with smaller ring
N-H
1640-1550
Bend
N-H
~800
oop bend
116
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
5. Amides – pivalamide
O
NH2
Primary aliphatic amide
117
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
5. Amides – 2-pyrrolidone
H
N
O
Cyclic secondary amide - lactam
118
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
6. Anhydrides – With acid halides, typically the highest n C=O; appears as
two bands for the symmetric and asymmetric stretching modes
Group Frequencies (cm-1):
O
C=O
1830-1800
Stretch (asym.)
n Base, sensitive to change
conj.
C=C
1778-1740
Stretch (sym.)
Two bands of variable relative
intensity
O
O
nC=O increases with smaller ring
C-O
1300-900
Stretch, multiple bands
119
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
6. Anhydrides – iso-butyric anhydride
O
O
O
Typical anhydride
120
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
7. Acid Halides – Acid bromides and iodides are not often encountered;
acid chlorides are the most prevalent (and useful)
Group Frequencies (cm-1):
1810-1775
conj.
add. band
C-Cl
730-550
Stretch
If below 600, not observed
using NaCl windows
C-Br
650-510
Stretch
Typically too low to obs.
C-I
600-485
Stretch
Typically too low to obs.
w/Ph
Stretch (sym.)
n Base, sensitive to change
C=O
Fermi resonance with
combination and overtone
region of aromatic ring
121
IR Spectroscopy
III. Group Frequencies and Analysis
E. Carbonyls
7. Acid Halides – propionyl chloride
Overtones of low n peaks can confuse some spectra
122
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
1. Nitriles – The “other” triple bond group observed in IR, due to the
higher dipole change during the stretching vibration, this band is more
intense than CCs.
Group Frequencies (cm-1):
CN
2250
Stretch (sym.)
n sensitive to change from
conjugation; usually stronger
than CC
123
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds Carbonyls
1. Nitriles – benzonitrile
124
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
2. Imines and Oximes – Often referred to as “derivatives” of carbonyl
compounds, these groups are not often encountered in routine IR obs.
Group Frequencies (cm-1):
C=N
(imine
and
oxime)
1685-1650
Stretch (sym.)
n sensitive to change from
conjugation; usually stronger
than CC
O-H
(oxime)
3250-3150
Stretch
H-bond effects
N-O
(oxime)
965-930
Stretch
125
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
2. Imines and Oximes – acetone oxime
126
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
3. Isocyanates and Isothiocyanates – Reactive groups, not often
observed in routine qualitative IR
Group Frequencies (cm-1):
N=C=O
~2270
Stretch (sym.)
broad n band – coupled
vibration
N=C=S
~2125
Stretch (1 or 2 bands)
broad n band – coupled
vibration
127
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
3. Isocyanates and Isothiocyanates– tert-butyl isocyanate
128
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
4. Nitro – Useful, easily incorporated group on aromatic rings, less often
encountered on alkyl compounds
Group Frequencies (cm-1):
O
R
N
1600-1530
1390-1300
Stretch (asymm.)
Stretch (sym.)
Aliphatic nitro
1550-1490
1355-1315
Stretch (asymm.)
Stretch (sym.)
Aromatic nitro
O
129
IR Spectroscopy
III. Group Frequencies and Analysis
F. sp2 and sp Nitrogen compounds
4. Nitro – o-nitrotoluene
130
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
1. Thiols and Sulfides – Sulfur, due to its large size shifts most observed
IR bands to lower frequencies – often out of the observed region
Group Frequencies (cm-1):
S-H
(thiol)
C-S-C
2550
Stretch (sym.)
Unique region of IR spectrum
No useful information
131
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
1. Thiol (Mercaptan) – 1,2-ethanethiol
132
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
2. Sulfoxides and Sulfones – Oxidized sulfur, the SO bonds are useful
for determining oxidation state, if the presence of sulfur is known
Group Frequencies (cm-1):
SO
1050
OSO ~1375
~1150
Stretch (sym.)
Stretch (asymm.)
Stretch (sym.)
133
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
2. Sulfoxide (Mercaptan) – di-butyl sulfoxide
Be wary of water in the spectrum of sulfoxides
134
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
2. Sulfone– di-butyl sulfone
135
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
3. Sulfonic Acids, Sulfonamides and Sulfonates – Sulfur equivalent of
the carboxylic acid derivatives; the O or N groups act as we have
observed
Group Frequencies (cm-1):
OSO ~1375
~1150
Stretch (asymm.)
Stretch (sym.)
As for sulfones – groups bound
to sulfur identify the group, just
as with the carboxylic acid
derivatives differ from ketones
S-O
(acid &
Stretch
May appear as several bands
1000-650
sulfonate)
O-H &
N-H
As for the carboxylic acid
derivatives
136
IR Spectroscopy
III. Group Frequencies and Analysis
G. Sulfur
3. Sulfonamides – p-toluenesulfonamide
137
IR Spectroscopy
III. Group Frequencies and Analysis
H. Phosphorus
1. Phosphines – Phosphorus in its lowest oxidation state – many bands
that overlap with other useful regions; exercise caution in interpretation
using IR
Group Frequencies (cm-1):
P-H
2320-2270
990-885
Stretch (sym.)
Bend
PH2
1090-1075
840-810
Bend, two bands
P-CH3
1450-1395
1350-1255
Bend, two bands
P-CH2-
1440-1400
Bend
138
IR Spectroscopy
III. Group Frequencies and Analysis
H. Phosphorus
1. Phosphines – tri-butylphosphine
139
IR Spectroscopy
III. Group Frequencies and Analysis
H. Phosphorus
2. Phosphine Oxides – More common to observe these phosphorus
compounds
Group Frequencies (cm-1):
PO
3.
1210-1140
Stretch (sym.)
Phosphate Esters, Acids and Amides – Often encountered in
biological systems
Group Frequencies (cm-1):
PO
1300-1240
Stretch (sym.)
R-O
1088-920
Stretch, 1 or 2 band
P-O
845-725
Stretch
140
IR Spectroscopy
III. Group Frequencies and Analysis
H. Phosphorus
2. Phosphine Oxides, Phosphate Esters – tri-butylphosphate
141
IR Spectroscopy
III. Group Frequencies and Analysis
I. Halogens
1. Fluorides and Chlorides – Smaller halogen bonds to carbon in
observed frequency range
Group Frequencies (cm-1):
C-F
1400-1000
Stretch (sym.)
Monofluoroalkyl at lower n
Polyfluoroalkyl at upper n
Aryl fluorides up to 1450
C-Cl
785-540
Stretch
Different conformers may give
split peaks
Bromides and Iodides – Not often obs. Due to low n of C-X stretch
•
Group Frequencies (cm-1):
C-Br
650-510
Stretch
Bend is obs. at ~1200
C-I
600-485
Stretch
Bend is obs. at ~1150
142
IR Spectroscopy
Energy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
All spectrometers consist of four basic parts that are coupled with
all four parts of the spectroscopic process - irradiation,
absorption-excitation, re-emission-relaxation and detection.
excited state
AbsorptionExcitation:
Spectrometer
needs to
contain the
sample
hn
rest state
hn hn hn
Detection-reemission :
Spectrometer needs to detect
the photons emitted by the
sample and ascertain their
energy
rest state
Irradiation: Spectrometer
needs to generate photons
143
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
Those four parts are:
1. Source/Monochromator
2. Sample cell
3. Detector/Amplifier
4. Output
•
Dispersive IR spectrometers were the first IR instruments, however
their simplicity and longevity allows them to continue in service –
for most routine organic analyses their speed and resolution is
adequate
•
For the most part, their design is austere and relies on simple
mechanics and optics to generate a spectrum, very similar to
simply rotating a glass prism to see different bands of visible light
144
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
Here is a general schematic:
145
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
Source is a heated nichrome wire which produces a broad band
continuum of IR light (as heat)
•
The beam is directed through both the sample and a reference cell
•
A rapidly rotating sector (beam chopper) continuously switches
between directing the two beams to a diffraction grating
146
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
The diffraction grating slowly rotates, such that only one narrow
frequency band of IR light is at the proper angle to reach the
detector
•
A simple circuit compares the light from the sample and reference
and sends the difference to a chart recorder
147
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
On the older instruments the motor in the chart recorder was
synchronized (& calibrated) to the motor on the diffraction grating
•
Because each spectrum is the result of the tabulation of the
spectroscopic process at each frequency individually, it is said to
record the spectrum in the frequency domain
148
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
1. Dispersive IR Spectrometers
•
Advantages – simple, easy to maintain – last the life of the source
and moving parts
•
Disadvantages – to cover the entire IR band of interest to chemists
it is necessary to use two diffraction gratings
•
At high q, the component frequencies are more spread out, so the
resulting spectra appear to have various regions expanded or
compressed
•
The limit to resolution is 2-4 cm-1
149
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
2. Fourier Transform IR Spectrometers
•
FT-IR is the modern state of the art for IR spectroscopy
•
The system is based on the Michelson interferometer
o Laser source IR light is separated by a beam splitter, one
component going to a fixed mirror, the other to a moving one
and are reflected back to the beam splitter
o
The beam splitter recombines the two to a pattern of
constructive and destructive interferences known as an
interferogram – a complex signal, but contains all of the
frequencies that make up the IR spectrum
150
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
2. Fourier Transform IR Spectrometers
•
The resulting signal is essentially a plot of intensity vs. time
•
Such information if plotted would look like the following:
•
This is meaningless to a chemist – we need this to be in the
frequency domain rather than time….
151
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
2. Fourier Transform IR Spectrometers
•
By applying a mathematical transform on the signal – a Fourier
transform – the resulting frequency domain spectrum can be
observed
•
FT-IRs give three theoretical advantages:
1. Fellgett’s advantage – every point in the interferogram is
information – all wavelenghts are represented
2. Jacquinot’s advantage – the entire energy of the source is
used – increasing signal-to-noise
3. Conne’s advantage – frequency precision – Dispersive
instruments can have errors in the ability to move slits and
gratings reproducibly – FTIR is internally referenced from its
own beam
152
IR Spectroscopy
II. Instrumentation and Experimental Aspects
A. The IR Spectrometer – Dispersive and Fourier Transform
2. Fourier Transform IR Spectrometers
•
Justik’s advantage – does it give me what I need
1. Single-beam instrument – collect a background (air has IR
active molecules!)
•
2.
Fast – all frequencies are scanned simultaneously
3.
No referencing!
4.
Computer based – scaling and editing of the spectrum to
squeeze out the most data; spectra are proportional (no
stretching or squeezing of regions), comparison with spectral
libraries
Disadvantages – expenisve relative to dispersive instruments, and
the components take more expertise and service calls to replace
153
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
1. Sample size – typically the size of the beam – mm’s  mg’s
2.
Non-destructive – sample can be recovered with varying degrees of
difficulty
3.
Liquid samples – the easiest IR spectra are those of “neat” liquid samples
•
Solid samples are too dense for good IR spectra – inter-molecular
coupling of vibrational states occurs and peaks are greatly
broadened
•
In the liquid state full 3-D motion is available, and these effects are
averaged out and diminished
•
The thickness of a sample can be decreased to reduce these effects
further
 Thin film liquid samples are best!
154
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
3. Liquid samples
•
Sample cell cannot possess covalent bonds (SiO2, or glass is out)
•
The most common cell is a pair of large transparent “windows” of
inorganic salts
•
Most common:
NaCl – cheap, transparent from 650 – 4000 cm-1, but fragile
Less common – AgCl, KBr, etc. – if you need transparency below
650, limit is practically 400
•
155
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
4. Solution samples
•
One way solids can be handled is as a solution
•
Key is that the solvent picked will cover the least amount of the
spectrum as possible, as it will also be present
•
Common solvents typically are symmetrical, or have many
halogenated bonds – low cm-1: CCl4, CHCl3, CH2Cl2, etc.
•
The cell in this case is two NaCl (or other) windows with a spacer,
the sample is loaded via a syringe into the cell:
156
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
4. Solution samples
•
A newer method involves the use of a polyethylene matrix, that will
hold allow a solution sample to evaporate, leaving small portions of
the sample embedded in the matrix
•
The samples are “liquid-like”
•
The only interference is that of hydrocarbon
157
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
5. Solid Samples
•
The most common treatment for solid samples is to “mull” them
with thick mineral oil (high MW hydrocarbon) - Nujol®
•
Just like with the polyethylene cards, the molecules of the sample
are held in suspension within the oil matrix
•
Again, the interference is that of hydrocarbon
158
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
5. Solid Samples
•
The connoisseurs method (with no organic interference) is to press
the solid with KBr into a pellet
•
Under high pressure the KBr liquefies and entraps individual
molecules of the sample in the matrix
•
These spectra are the only spectra of solids that are as interference
free as liquids
159
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of p-cresol
Neat Sample
160
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of p-cresol
KBr Pellet
161
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of p-cresol
CCl4 Solution
162
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of m-nitroanisole
Nujol Mull
163
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of m-nitroanisole
KBr Pellet
164
IR Spectroscopy
II. Instrumentation and Experimental Aspects
B. The IR Spectrometer – Experimental aspects
6. Differences in Spectral Appearence
•
Compare the following three IR spectra of m-nitroanisole
CCl4 Solution
165