A complete Guide for IR Spectroscopy.pptx

A Complete Guide For
IR Spectroscopy
By
Dr. Tejas Pavagadhi
Indian Institute of Teacher Education, Gandhinagar

I. Introduction
A. Spectroscopy is the study of the interaction of matter with the electromagnetic
radiation
1. Electromagnetic radiation displays the properties of both particles and waves
2. The particle component is called a photon
3. The energy (E) component of a photon is proportional to the frequency. Where
h is Planck’s constant and υ is the frequency in Hertz (cycles per second)
E ∝ υ, ∴ E = hυ
4. The term “photon” is implied to mean a small, mass less particle that contains a
small wave-packet of EM radiation/ light. we will use this terminology in the
course
Vibrational Spectroscopy
IITE : Nurturing teachers of tomorrow 2

I. Introduction
5. Because the speed of light, c, is constant, the frequency, υ, (number of cycles
of the wave per second) can complete in the same time, must be inversely
proportional to how long the oscillation is, or wavelength:
6. Amplitude, A, describes the wave height, or strength of the oscillation
7. Because the atomic particles in matter also exhibit wave and particle
properties EM radiation can interact with matter in two ways:
• Collision – particle-to-particle – energy is lost as heat and movement
• Coupling – the wave property of the radiation matches the wave property
of the particle and “couple” to the next higher quantum mechanical
energy level
υ =
c = 3 x 1010 cm/s
___
l
c
 E = hυ =
___
l
hc

I. Introduction
8. The entire electromagnetic spectrum is used by chemists:
UV
X-rays IR
g-rays Radio
Microwave
Energy (kcal/mol)
300-30 300-30 ~10-4
> 300 ~10-6
Visible
Frequency, υ in Hz
~1015 ~1013 ~1010 ~105
~1017
~1019
Wavelength, nm
10 nm 1000 nm 0.01 cm 100 m
~0.01 nm
~.0001 nm
nuclear
excitation
core
electron
excitation
electronic
excitation
molecular
vibration
molecular
rotation
Nuclear Magnetic
Resonance NMR
(MRI)

Night Vision Goggles
IR in Everyday Life

Humans, at normal body temperature, radiate
most strongly in the infrared, at a wavelength of
about 10 microns (A micron is the term
commonly used in astronomy for a micrometer or
one millionth of a meter). In the image to the left,
the red areas are the warmest, followed by
yellow, green and blue (coolest).
The image to the right shows a cat in the infrared.
The yellow-white areas are the warmest and the
purple areas are the coldest. This image gives us a
different view of a familiar animal as well as
information that we could not get from a visible light
picture. Notice the cold nose and the heat from the
cat's eyes, mouth and ears.

Theory

• Vibrational radiation lies between the visible and microwave portions of the
electromagnetic spectrum.
• Vibrational waves have wavelengths longer than visible and shorter than
microwaves, and have frequencies which are lower than visible and higher than
microwaves.
• The Vibrational region is divided into: near, mid and far-infrared.
– Near-infrared refers to the part of the infrared spectrum that is closest to visible
light.
– Far-infrared refers to the part that is closer to the microwave region.
– Mid-infrared is the region between these two.

I. Introduction
C. The IR Spectroscopic Process
1. The quantum mechanical energy levels observed in IR spectroscopy are those of
molecular vibrations
2. We perceive this vibration as heat
3. When we say a covalent bond between two atoms is of a certain length, we are
citing an average because the bond behaves as it is a vibrating spring connecting
two atoms
4. For a simple diatomic molecule, this model is easy to visualize:

Types of Molecular Vibrations (called modes of vibration).
Stretching Vibrations
change in bond length
 Symmetric
 Asymmetric
Bending Vibrations(Deformation vibration)
change in bond angle
In plane bending
 scissoring
 rocking
Out of plane bending
 wagging
 twisting/torsion

I. Introduction
5. There are two types of bond vibration:
• Stretch – Vibration or oscillation along the line of the bond
• Bend – Vibration or oscillation not along the line of the bond
H
H
C
H
H
C
scissor
asymmetric
H
H
C
C
H
H
C
C
H
H
C
C
H
H
C
C
symmetric
rock twist wag
in plane out of plane
Vibrational
Spectroscopy

C. The IR Spectroscopic Process
6.As a covalent bond oscillates – due to the oscillation of the dipole of the
molecule – a varying electromagnetic field is produced
7.The greater the dipole moment change through the vibration, the more intense
is the EM field that is generated

C.The IR Spectroscopic Process
8.When a wave of infrared light encounters this oscillating EM field generated by
the oscillating dipole of the same frequency, the two waves couple, and IR light
is absorbed
9.The coupled wave now vibrates with twice the amplitude
ir beam from spectrometer
EM oscillating wave
from bond vibration
“coupled” wave

D. The IR Spectrum
1. Each stretching and bending vibration occurs with a characteristic frequency as
the atoms and charges involved are different for different bonds
The y-axis on an IR spectrum is in units
of
% transmittance (%T)
In regions where the EM field of an osc.
bond interacts with IR light of the same
υ – transmittance is low (light is
absorbed)
In regions where
no osc. bond is
interacting with IR
light, transmittance
nears 100%

D. The IR Spectrum
2. The x-axis of the IR spectrum is in units of wavenumbers, υ (nu bar) which is the
number of waves per centimeter in units of cm-1 (Remember E = hυ or E = hc/λ)

D. The IR Spectrum
3. 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
4. This unit is used rather than frequency as the numbers are more “real” than
the exponential units of frequency
5. IR spectra are observed for the mid-infrared: 4000- 400 (690)cm-1
6. The peaks are Gaussian distributions of the average energy of a transition

D. The IR Spectrum
7. In general:
Lighter atoms will allow the oscillation to be faster – higher energy
This is especially true of bonds to hydrogen – C-H, N-H and O-H
Stronger bonds will have higher energy oscillations
Triple bonds > double bonds > single bonds in energy
Energy of oscillation

E. The IR Spectrum – The detection of different bonds
7. As opposed to chromatography or other spectroscopic methods, the area of a
IR band (or peak) is not directly proportional to concentration of the
functional group producing the peak
8. 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

E. The IR Spectrum – The detection of different bonds
9. It is important to make note of band/peak intensities to show the effect of
these factors:
• Strong (s) – band/peak is tall
• Medium (m) – band/peak is mid-height
• Weak (w) – band/peak is short
• * 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

II. Vibrational Group Analysis
A. General
1. The primary use of the IR is to detect functional groups
2. Because the IR looks at the interaction of the EM Radiations 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)

3.) Typical IR spectrum for Organic Molecule
- many more bands then in UV-vis, fluorescence or phosphorescence
- bands are also much sharper
- pattern is distinct for given molecule
• except for optical isomers
- good qualitative tool
• can be used for compound identification
• group analysis
- also quantitative tool
• intensity of bands related to amount of compound present
- spectra usually shown as percent transmittance (instead of absorbance)
vs. wavenumber (instead of l) for convenience
Hexane
Hexene
Hexyne

G) Theory of IR Absorption
1.) Molecular Vibrations(Hooke’s law)
i.) Harmonic Oscillator Model:
- approximate representation of atomic stretching
- two masses attached by a spring
E = ½ ky2
where:
y is spring displacement
k is spring constant

Vibrational frequency given by:
where:
υ: frequency
k: force constant (measure of bond
stiffness)
m: reduced mass = m1m2/m1+m2
We can get k:
Single bonds:
k ~ 3x102 to 8 x102 N/m (Avg ~ 5x102)
double and triple bonds ~ 2x and 3x k for
single bond.
So, vibration occur in order:
single < double < triple
1/ 2 /
k m
 


ii.) Anharmonic oscillation:
- harmonic oscillation model good at low energy levels (n0, n1, n2, …)
- not good at high energy levels due to atomic repulsion & attraction
• as atoms approach, coulombic repulsion force adds to the bond force
making energy increase greater then harmonic
• as atoms separate, approach dissociation energy and the harmonic
function rises quicker
Harmonic oscillation
Anharmonic oscillation
Because of anharmonics: at low DE, Dn =±2, ±3 are observed which cause the appearance of overtone
lines at frequencies at ~ 2-3 times the fundamental frequency. Normally Dn = ± 1

iv.) Number of Vibrational Modes:
- for non-linear molecules, number of types of vibrations: 3N-6
- for linear molecules, number of types of vibrations: 3N-5
- why so many peaks in IR spectra
- observed vibration can be less than predicted because
• symmetry ( no change in dipole)
• energies of vibration are identical
• absorption intensity too low
• frequency beyond range of instrument
http://www.chem.purdue.edu/gchelp/vibs/co2.html
See web site for 3D animations of vibrational modes for a variety of molecules
Examples:
1) HCl: 3(2)-5 = 1 mode
2) CO2: 3(3)-5 = 4 modes
- - moving in-out of plane
+

v.) IR Active Vibrations:
- In order for molecule to absorb IR radiation:
• vibration at same frequency as in light
• but also, must have a change in its net dipole moment
as a result of the vibration
Examples:
1) CO2: 3(3)-5 = 4 modes
- -
+
m = 0; IR inactive
m > 0; IR active
m > 0; IR active
m > 0; IR active
d-
d-
2d+
d-
d-
2d+
d-
d-
2d+
d-
d-
2d+
degenerate –identical energy single IR peak

Fewer and more experimental peaks than calculated
• Fewer peaks
• Symmetry of the molecule (inactive)
• Energies of two or more vibrations are identical or nearly identical
• Undetectable low absorption intensity
• Out of the instrumental detection range
• More peaks
• Overtone
• Combination bands
28
IITE : Nurturing teachers of tomorrow

Fundamental Peaks and Overtones
• Fundamental transition: The excitation from the ground state ʋ0 to the
first excited state ʋ1 is called the fundamental transition. It is the most
likely transition to occur.
• Fundamental absorption bands are strong compared with other bands
that may appear in the spectrum due to overtone, combination, and
difference bands.
• Overtone bands: Result from excitation from the ground state to higher
energy states ʋ2 , ʋ3 and so on.
• These absorptions occur at approximately integral multiples of the
frequency of the fundamental absorption.
• If the fundamental absorption occurs at frequency ʋ, the overtones will
appear at about 2ʋ, 3ʋ and so on.
• Overtones are weak bands and may not be observed in all
samples/analysis
29

•Coupling modes:
• Vibrating atoms may interact with each other. Two
vibrational frequencies may couple to produce a new
frequency 3 = 1 + 2 .
• The band at 3 is called a combination band.
• If two frequencies couple such that 3 = 1 - 2 , the band
is called a difference band.
• Not all possible combinations and differences occur.
30

Rules for Vibrational coupling
• Coupling of different vibrations shifts frequencies
• Energy of a vibration is influenced by coupling
• Coupling is likely when
– common atom in stretching modes
– common bond in bending modes
– common bond in bending + stretching modes
– similar vibrational frequencies
• Coupling is not likely when
– atoms separated by two or more bonds
– symmetry inappropriate
31

• Fermi Resonance
• Fermi resonance results in the splitting of two vibrational bands that have
nearly the same energy and symmetry in both IR and Raman
spectroscopies.
• The two bands are usually a fundamental vibration and either an
overtone or combination band.
• The wave functions for the two resonant vibrations mix according to the
harmonic oscillator approximation, and the result is a shift in frequency
and a change in intensity in the spectrum.
• As a result, two strong bands are observed in the spectrum, instead of the
expected strong and weak bands. It is not possible to determine the
contribution from each vibration because of the resulting mixed wave
function.
32

Fermi Resonance…
Because the vibrations have nearly the same frequency, the interaction will be affected
if one mode undergoes a frequency shift from deuteration or a solvent effect while the
other does not.
The molecule most studied for this type of resonance (even what Fermi himself used to
explain this phenomena), is carbon dioxide, CO2.
Another typical example of Fermi resonance is found in the vibrational spectra of
benzoyl chloride shown partly in the C=O region displays two bands in carbonyl
region at 1790 cm-1 and 1745 cm-1.
if this were an unknown compound, one might be misled to conclude that the
compound has two non adjacent carbonyl groups in the molecule.
However the lower frequency band is in fact, due to the overtone of the CH bending
mode at 875 cm-1 in fermi resonance with the C=O stretching fundamental frequency.
33

34
Instrumentation

Components of IR Instruments
• Light source
• Sampling techniques (solid, liquid and gaseous samples)
• Monochromator
• Detector and recording devices

Instrumentation
1.) Conventional/Dispersive Instruments
- normal IR instrument similar to UV-vis
- main differences are light source & detector
Detector

i) Light Source:
- must produce IR radiation
- can’t use glass since absorbs IR radiation
- several possible types
a) Nernst Glower
- rare earth metal oxides (Zr, Ce, Th) heated electrically
- apply current to cylinder, has resistance to current flow generates heat
(1200o – 2200o C).
- causes light production similar to blackbody radiation
- range of use ~ 670 – 10,000cm-1
- need good current control or overheats and damaged
b) Globar
- similar to Nernst Glower but uses silicon carbide rod instead of rare earth oxides
- similar usable range
V
Zr, Ce, Th

c) CO2 Laser
- CO2 laser gas mixture consists of 70% He, 15% CO2, and 15% N2
- a voltage is placed across the gas, exciting N2 to lowest vibrational levels.
- the excited N2 populate the asymmetric vibrational states in the CO2 through
collisions.
- infrared output of the laser is the result of transitions between rotational
states of the CO2 molecule of the first asymmetric vibrational mode to
rotational states of both the first symmetric stretch mode and the second
bending mode
- small range but can choose which band used & many compounds have IR
absorbance in this region
d) Others
- mercury arc (l > 50 mm) (far IR)
- tungsten lamp (4000 -12,800cm-1) (near IR)

ii) Sample Cell
- must be made of IR transparent material (KBr pellets or NaCl)
NaCl plates

IR spectroscopy is used for the characterization of solid, liquid or gas
samples. Material containing sample must be transparent to the IR
radiation. So, the salts like NaCl, KBr are only used.
1. Sampling of solids
Various techniques used for preparing solid samples are as follows
a) Mull technique: In this technique, the finely crushed sample is
mixed with Nujol (mulling agent) in n a marble or agate mortar, with
a pestle to make a thick paste. A thin film is applied onto the salt
plates. This is then mounted in a path of IR beam and the spectrum is
recorded.
b) Pressed pellet technique – In this technique, a small amount of
finely ground solid sample is mixed with 100 times its weight of
potassium bromide and compressed into a thin transparent pellet
using a hydraulic press. These pellets are transparent to IR radiation
and it is used for analysis.
c) Thin(Case) film technique – If the solid is amorphous in nature
then the sample is deposited on the surface of a KBr or NaCl cell by
evaporation of a solution of the solid and ensured that the film is not
too thick to pass the radiation.

2. Sampling of liquids
Liquid sample cells can be sandwiched using liquid sample
cells of highly purified alkali halides, normally NaCl. Other salts
such as KBr and CaF2 can also be used. Aqueous solvents
cannot be used because they cannot dissolve alkali halides.
Organic solvents like chloroform can be used. The sample
thickness should be selected so that the transmittance lies
between 15-20%. For most liquids, the sample cell thickness is
0.01-0.05 mm. Some salt plates are highly soluble in water, so
the sample and washing reagents must be anhydrous
3. Sampling of gases
The sample cell is made up of NaCl, KBr etc. and it is similar to
the liquid sample cell. A sample cell with a long path length (5 –
10 cm) is needed because the gases show relatively weak
absorbance.

iii) monochromator
a. Prism
b. Grating
-Reflective grating is common
-can’t use glass prism, since absorbs IR

Grating Monochromators:
• Grating monochromators disperse ultraviolet, visible, and
infrared radiation typically using replica gratings, which
are manufactured from a master grating.
• A master grating consists of a hard, optically flat, surface
that has a large number of parallel and closely spaced
grooves. The construction of a master grating is a long,
expensive process because the grooves must be of
identical size, exactly parallel, and equally spaced over the
length of the grating (3–10 cm).
• A grating for the ultraviolet and visible region typically
has 300–2000 grooves/mm, however 1200–1400
grooves/mm is most common. For the infrared region,
gratings usually have 10–200 grooves/mm.

iv) Detectors:
- two types of detectors mainly used in common IR instruments
a) Thermal Detectors
1.) Thermocouple
- two pieces of dissimilar metals fused together at the ends
- when heated, metals heat at different rates
- potential difference is created between two metals that varies with their
difference in temperature
- usually made with blackened surface (to improve heat absorption)
- placed in evacuated tube with window transparent to IR (not glass or
quartz)
- IR “hits” and heats one of the two wires.
- can use several thermocouples to increase sensitivity.
V
- +
h metal1 metal2
IR transparent
material (NaCl)

2.) Bolometer
- strips of metal (Pt, Ni) or semiconductor that has a large change in
resistance to current with temperature.
- as light is absorbed by blackened surface, resistance increases and
current decreases
- very sensitive i
A
h
b) Photoconducting Detectors
- thin film of semiconductor (ex. PbS) on a nonconducting glass surface
and sealed in a vacuum.
- absorption of light by semiconductor switches it to
conducting state from its non-conducting state
- decrease in resistance  increase in current vacuum
Transparent
to IR
glass
semiconductor
h

c) Pyroelectric Detectors
- pyroelectric (ceramic, lithium tantalate) material get polarized
(separation of (+) and (-) charges) in presence of electric field.
- temperature dependent polarization
- measure degree of polarization related to temperature of crystal
- fast response, good for FTIR

v) Overall Instrument Design
-Need chopper to discriminate source light
from background IR radiation
-Monochromator after sample cell
-Not done in UV-Vis since letting in all hυ
to sample may cause photo degradation
(too much energy)
-IR lower energy
-Advantage that allows monochromator
to be used to screen out more background
IR light.
-Problems:
-Source weak , need long scans
-Detector response slow – rounded
peaks

Calibration of Infrared Spectrophotometers :
The wavelength (or wave number) scale calibration of infrared
spectrophotometers is usually carried out with the aid of a strip of
polystyrene film fixed on a frame.
It consists of several sharp absorption bands, the wavelengths of
which are known accurately and precisely.
Basically, all IR-spectrophotometers need to be calibrated
periodically as per the specific instructions so as to ascertain their
accuracy and precision.

vi) Fourier Transfer IR (FTIR) – alternative to Normal IR
- Based on Michelson Interferometer
Principle:
1) light from source is split by central mirror into 2 beams of equal intensity
2) beams go to two other mirrors, reflected by central mirror, recombine and pass through sample to
detector
3) two side mirrors.one is fixed and other movable
a) move second mirror, light in two-paths travel different distances before recombined
b) constructive & destructive interference
c) as mirror is moved, get a change in signal

vi.) Fourier Transfer IR (FTIR) – alternative to Normal IR
- Based on Michelson Interferometer

Destructive Interference can be created when two waves from the same
source travel different paths to get to a point.
This may cause a difference in the phase between the two waves.
• If the paths differ by an integer multiple of a wavelength, the waves will also be in phase.
• If the waves differ by an odd multiple of half a wave then the waves will be 180 degrees out of
phase and cancel out.
Remember

- observe a plot of Intensity vs. Distance (interferograms)
- convert to plot of Intensity vs. Frequency by doing a Fourier Transform
- resolution Dn = 1/Dd (interval of distance traveled by mirror)

Advantages of FTIR compared to Normal IR:
1) much faster, seconds vs. minutes
2) use signal averaging to increase signal-to-noise* (S/N)
(*Signal-to-noise ratio (abbreviated SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal
to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher
than 1:1 (greater than 0 dB) indicates more signal than noise.)
3) higher inherent S/N – no slits, less optical equipment, higher light intensity
4) high resolution (<0.1 cm-1)
Disadvantages of FTIR compared to Normal IR:
1) single-beam, requires collecting blank
2) can’t use thermal detectors – too slow
In normal IR, scan through frequency range.
FTIR collects all frequencies at once.
/
increase S N number scans


1.Better sensitivity and brightness
- Allows simultaneous measurement over the entire wavenumber range
- Requires no slit device, making good use of the available beam
2.High wavenumber accuracy
- Technique allows high speed sampling with the aid of laser light interference
fringes
- Requires no wavenumber correction
- Provides wavenumber to an accuracy of 0.01 cm-1
3. Resolution
- Provides spectra of high resolution
4. Stray light
- Fourier Transform allows only interference signals to contribute to spectrum.
Background light effects greatly lowers.
- Allows selective handling of signals limiting intreference
5. Wavenumber range flexibility
- Simple to alter the instrument wavenumber range
- CO2 and H2O sensitive
FT-IR Advantages

Specific FT-IR Advantages
Fellget's (multiplex) Advantage
• There are three principal advantages for an FT spectrometer
compared to a scanning (dispersive) spectrometer
• The multiplex or Fellget’s advantage. This arises from the fact
that information from all wavelengths is collected
simultaneously.
• It results in a higher signal to noise for a given scan-time for
observations limited by a fixed detector noise contribution
(typically in the thermal infrared spectral region where a
photodetector is limited by noise

•Connes Advantage
• The wavelength accuracy or Connes' advantage. The wavelength
scale is calibrated by a laser beam(He-Ne laser ) of known
wavelength that passes through the interferometer.
• This is much more stable and accurate than in dispersive
instruments where the scale depends on the mechanical
movement of diffraction gratings. In practice, the accuracy is
limited by the divergence(deflection/difference) of the beam in
the interferometer which depends on the resolution.
• Therefore, Wavelength accuracy is much higher than dispersive
IR Instrument and thus gives precise information about
absorption

Jacquinot Advantage
• The throughput or Jacquinot's advantage. For a given resolution
and wavelength, this circular aperture allows more light through
than a slit, resulting in a higher signal-to-noise ratio
• This results from the fact that in a dispersive instrument, the
monochromator has entrance and exit slits which restrict the
amount of light that passes through it. The interferometer
throughput is determined only by the diameter of the collimated
beam coming from the source.
• Although no slits are needed, FTIR spectrometers do require an
aperture to restrict the convergence(Union of wavelength) of the
collimated beam in the interferometer. This is because
convergent rays are modulated at different frequencies as the
path difference is varied. Such an aperture is called a Jacquinot
stop.

Applications

Bond Type of Compound Frequency(W.N.) Range, cm-1 Intensity
C-H Alkanes 2850-2970 Strong (s)
C-H Alkenes 3010-3095
675-995
Medium (m)
Strong
C-H Alkynes 3300 Strong
C-H Aromatic rings 3010-3100
690-900
Medium
strong
o-H Monomeric alcohols, phenols
Hydrogen-bonded alchohols, phenols
Monomeric carboxylic acids
Hydrogen-bonded carboxylic acids
3590-3650
3200-3600
3500-3650
2500-2700
Variable
Variable, sometimes
broad
Medium
Broad (Br.)
N-H Amines, amides 3300-3500 medium
C=C Alkenes 1610-1680 Variable (v)
C=C Aromatic rings 1500-1600 Variable
Alkynes 2100-2260 Variable
C-N Amines, amides 1180-1360 Strong
Nitriles 2210-2280 Strong
C-O Alcohols, ethers,carboxylic acids, esters 1050-1300 Strong
C=O Aldehydes, ketones, carboxylic acids, esters,
acid chloride, Amides
1590-1800 Strong
NO2 Nitro compounds 1500-1570
1300-1370
Strong
C C
H
C C H
C C
C N
Abbreviated Table of Group Frequencies for Organic Groups

Fingerprint Region (1500-900 cm-1)
- region of most single bond signals
- many have similar frequencies, so affect each other & give pattern characteristics of
overall skeletal structure of a compound
- exact interpretation of this region of spectra seldom possible because of complexity
- complexity  uniqueness Fingerprint Region

Computer Searches
- many modern instruments have reference IR spectra on file (~100,000 compounds)
- matches based on location of strongest band, then 2nd strongest band, etc
overall skeletal structure of a compound
- exact interpretation of this region of spectra seldom possible because of complexity
- complexity  uniqueness
Bio-Rad SearchIT database
of ~200,000 IR spectra

2.) Quantitative Analysis
- not as good as UV/Vis in terms of accuracy and precision
► more complex spectra
► narrower bands (Beer’s Law deviation)
► limitations of IR instruments (lower light throughput, weaker detectors)
► high background IR
► difficult to match reference and sample cells
► changes in e (A=ebc) common
- potential advantage is good selectivity, since so many compounds have different IR
spectra
► one common application is determination of air contaminants.
Contaminants Concn, ppm Found, ppm Relative error, %
Carbon Monoxide 50 49.1 1.8
Methylethyl ketone 100 98.3 1.7
Methyl alcohol 100 99.0 1.0
Ethylene oxide 50 49.9 0.2
chloroform 100 99.5 0.5

Example : The spectrum is for a substance with an empirical formula of C3H5N. What is the
compound?
Nitrile or
alkyne group
No aromatics
Aliphatic
hydrogens
&
No Aromatic
character
Alkane
Bending
(Deformation)

Vibrational Group Analysis
the four primary regions of the IR spectrum
4000 cm-1 2700 cm-1 2000 cm-1 1500 cm-1
600 cm-1
O-H
N-H
C-H
C≡C
C≡N
C=O
C=N
C=C
Fingerprint
Region
C-C
C-N
C-O
X-H bonds Triple bonds Double bonds Single Bonds

1. Alkanes – combination of C-H and C-C bonds
• sp3 C-H between 2800-3000 cm-1
• C-C stretches and bends 1360-1470 cm-1
• CH2-CH2 bond 1450-1470 cm-1
• CH2-CH3 bond 1360-1390 cm-1
Octane
(w – s) (m)

2. Alkenes – addition of the C=C and vinyl C-H bonds
• C=C stretch at 1620-1680 cm-1 weaker as
substitution increases
• vinyl C-H stretch occurs at 3000-3100 cm-1
• The difference between alkane, alkene or alkyne C-H
is important!
• If the band is slightly above 3000 it is vinyl sp2 C-H
or alkynyl sp C-H,
if it is below, it is alkyl sp3 C-H
1-Octene
(w – m)
(w – m)

3. Alkynes – addition of the C=C and vinyl C-H bonds
• C≡C stretch 2100-2260 cm-1; strength depends on
asymmetry of bond, strongest for terminal alkynes,
weakest for symmetrical internal alkynes
• C-H for terminal alkynes occurs at 3300-3320 cm-1
• Internal alkynes ( R-C≡C-R ) would not have this band!
1-Octyne
(m – s)
(w-m)

4. Aromatics
• Due to the delocalization of e- in the ring, C-C bond
order is 1.5, the stretching frequency for these
bonds is slightly lower in energy than normal C=C
• These show up as a pair of sharp bands, 1500 &
1600 cm-1, (lower frequency band is stronger)
• C-H bonds off the ring show up similar to vinyl C-H
at 3000-3100 cm-1
Ethyl benzene
(w – m) (w – m)

5. Unsaturated Systems – substitution patterns
• The substitution of aromatics and alkenes can also be discerned(Recognized)
through the out-of-plane (oop)bending vibration region
• However, other peaks often are apparent in this region. These peaks should only
be used for reinforcement of what is known or for hypothesizing as to the
functional pattern.
R
C
H
C
R
C
H
CH2
R
C
H
C
R
C
R
CH2
R
C
R
C
R
H
R
H
R
H
985-997
905-915
cm-1
960-980
665-730
885-895
790-840
R
R
R
R
R
R
R
cm-1
730-770
690-710
735-770
860-900
750-810
680-725
800-860

6. Ethers – addition of the C-O-C asymmetric band and
vinyl C-H bonds
• Show a strong band for the antisymmetric C-O-C
stretch at 1050-1150 cm-1
• Otherwise, dominated by the hydrocarbon
component of the rest of the molecule
Diisopropyl ether
(s)

7. Alcohols
• Strong, broad O-H stretch from 3200-3400 cm-1
• Like ethers, C-O stretch from 1050-1260 cm-1
• Band position changes depending on the alcohols
substitution: 1° 1075-1000; 2° 1075-1150; 3° 1100-1200;
phenol 1180-1260
• The shape is due to the presence of hydrogen bonding
1-butanol
(m– s)
br
(s)

8. Amines - Primary
• Shows the –N-H stretch for NH2 as a doublet
between 3200-3500 cm-1 (symmetric and anti-
symmetric modes)
• -NH2 has deformation band from 1590-1650 cm-1
• Additionally there is a “wag” band at 780-820 cm-1
that is not diagnostic
2-aminopentane
(w) (w)

9. Amines – Secondary
• N-H band for R2N-H occurs at 3200-3500 cm-1
as a single sharp peak weaker than –O-H
• Tertiary amines (R3N) have no N-H bond and
will not have a band in this region
pyrrolidine
(w – m)

Pause and Review
• Inspect the bonds to H region (2700 – 4000 cm-1)
• Peaks from 2850-3000 are simply sp3 C-H in most organic molecules
• Above 3000 cm-1 Learn shapes, not wavenumbers!:
Broad U-shape peak
-O—H bond
V-shape peak
-N—H bond for 2o
amine (R2N—H)
Sharp spike
-C≡C—H bond
W-shape peak
-N—H bond for 1o
amine (RNH2)
3000 cm-1
Small peak shouldered
just above 3000 cm-1
C=C—H or Ph—H

10. Aldehydes
• C=O (carbonyl) stretch from 1720-1740 cm-1
• Band is sensitive to conjugation, as are all
carbonyls (upcoming slide)
• A highly unique sp2 C-H stretch appears as a
doublet, 2720 & 2820 cm-1 called a “Fermi doublet”
Cyclohexyl carboxaldehyde
(w-m)
(s)

11. Ketones
• Simplest of the carbonyl compounds as far as IR
spectrum – carbonyl only
• C=O stretch occurs at 1705-1725 cm-1
3-methyl-2-pentanone
(s)

12. Esters
• C=O stretch at 1735-1750 cm-1
• Strong band for C-O at a higher frequency than
ethers or alcohols at 1150-1250 cm-1
Ethyl pivalate
(s)
(s)

13. Carboxylic Acids:
• Gives the messiest of IR spectra
• C=O band occurs between 1700-1725 cm-1
• The highly dissociated O-H bond has a broad band from 2400-
3500 cm-1 covering up to half the IR spectrum in some cases
4-phenylbutyric acid
(w – m)
br
(s) (s)

14. Acid anhydrides
• Coupling of the anhydride though the ether oxygen
splits the carbonyl band into two with a separation
of 70 cm-1
• Bands are at 1740-1770 cm-1 and 1810-1840 cm-1
• Mixed mode C-O stretch at 1000-1100 cm-1
Propionic anhydride
O
O O
(s) (s)

15. Acid halides
• Clefted band at 1770-1820 cm-1 for C=O
• Bonds to halogens, due to their size (see Hooke’s
Law derivation) occur at low frequencies, only Cl is
light enough to have a band on IR, C-Cl is at 600-
800 cm-1
Propionyl chloride
Cl
O
(s)
(s)

16. Amides
• Display features of amines and carbonyl compounds
• C=O stretch at 1640-1680 cm-1
• If the amide is primary (-NH2) the N-H stretch occurs
from 3200-3500 cm-1 as a doublet
• If the amide is secondary (-NHR) the N-H stretch
occurs at 3200-3500 cm-1 as a sharp singlet
pivalamide
NH2
O
(m – s) (s)

17. Nitro group (-NO2)
• Proper Lewis structure gives a bond order of 1.5
from nitrogen to each oxygen
• Two bands are seen (symmetric and asymmetric) at
1300-1380 cm-1 and 1500-1570 cm-1
• This group is a strong resonance withdrawing group
and is itself vulnerable to resonance effects
2-nitropropane
N
O
O
(s) (s)

18. Nitriles (the cyano- or –C≡N group)
• Principle group is the carbon nitrogen triple
bond at 2100-2280 cm-1
• This peak is usually much more intense than
that of the alkyne due to the electronegativity
difference between carbon and nitrogen
propionitrile
N
C
(s)

Effects on IR bands
1. Conjugation – by resonance, conjugation lowers the energy of a double or triple
bond. The effect of this is readily observed in the IR spectrum:
• Conjugation will lower the observed IR band for a carbonyl from 20-40 cm-1
provided conjugation gives a strong resonance contributor
• Inductive effects are usually small, unless coupled with a resonance
contributor (note –CH3 and –Cl above)
O
O
1684 cm-1
1715 cm-1
C=O C=O
C
H3C
O
X X = NH2 CH3 Cl NO2
1677 1687 1692 1700 cm-1
H2N C CH3
O
Strong resonance contributor
vs.
N
O
O
C
CH3
O
Poor resonance contributor
(cannot resonate with C=O)

Effects on IR bands
2. Steric effects – usually not important in IR spectroscopy, unless they reduce the
strength of a bond (usually ) by interfering with proper orbital overlap:
• Here the methyl group in the structure at the right causes the carbonyl
group to be slightly out of plane, interfering with resonance
3. Strain effects – changes in bond angle forced by the constraints of a ring will
cause a slight change in hybridization, and therefore, bond strength
• As bond angle decreases, carbon becomes more electronegative, as well
as less sp2 hybridized (bond angle < 120°)
O
C=O: 1686 cm-1
O
C=O: 1693 cm-1
CH3
O O O O O
1815 cm-1
1775 cm-1
1750 cm-1
1715 cm-1
1705 cm-1

Effects on IR bands
4. Hydrogen bonding
• Hydrogen bonding causes a broadening in the band due to the creation of
a continuum of bond energies associated with it
• In the solution phase these effects are readily apparent; in the gas phase
where these effects disappear or in lieu of steric effects, the band appears
as sharp as all other IR bands:
Gas phase spectrum of
1-butanol
Steric hindrance to H-bonding
in a di-tert-butylphenol
• H-bonding can interact with other functional groups to lower frequencies
OH
C=O; 1701 cm-1
O
O
H

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