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Infrared (IR) Spectroscopy
•IR deals with the interaction of infrared radiation with matter.
The IR spectrum of a compound can provide important
information about its chemical nature and molecular structure.
•Most commonly, the spectrum is obtained by measuring the
absorption of IR radiation, although infrared emission and
reflection are also used.
•Widely applied in the analysis of organic materials, also useful
for polyatomic inorganic molecules and for organometallic
compounds.
Overview
1. Electromagnetic radiation
2. Vibrations
3. Principle of IR experiment
4. IR spectrum
5. Types of vibration
6. CGF/Fingerprint regions
7. IR activity of vibrations
8. Interpretation of IR spectra
9. Instrumentation
10. Sample preparation
Electromagnetic Radiation
The propagation of electromagnetic
radiation in a vacuum is constant for all
regions of the spectrum (= velocity of
light):
c=×
1 Å = 10 –10 m 1 nm = 10 –9 m 1 m = 10 –6 m
Another unit commonly used is the wavenumber, which is linear with energy:
Work by Einstein, Planck and Bohr indicated that electromagnetic radiation can be
regarded as a stream of particles or quanta, for which the energy is given by the
Bohr equation:
The Electromagnetic Spectrum
Infrared region
LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1
NEAR INFRARED: 0.8 -2.5 m, 12500 - 4000 cm-1
MID INFRARED: 2.5 - 50 m, 4000 - 200 cm-1
FAR INFRARED: 50 - 1000 m, 200 - 10 cm-1
Divisions arise because of different optical materials and
instrumentation.
Molecular spectra
There are three basic types of optical
spectra that we can observe for
molecules:
1.Electronic or vibronic spectra (UVvisible-near IR)
(transitions between a specific vibrational and rotational level
of one electronic state and a vibrational and rotational level of
another electronic state)
2.Vibrational or vibrational-rotational
spectra (IR region)
(transitions from the rotational levels of one vibrational level
to the rotational levels of another vibrational level in the same
electronic state)
3.Rotational spectra (microwave region)
(transitions between rotational levels of the same vibrational
level of the same electronic state)
Vibrational spectra (I): Harmonic oscillator model
• Infrared radiation in the range from 10,000 – 100 cm –1 is
absorbed and converted by an organic molecule into energy of
molecular vibration
–> this absorption is quantized:
A simple harmonic oscillator is a mechanical system consisting of
a point mass connected to a massless spring. The mass is under
action of a restoring force proportional to the displacement of
particle from its equilibrium position and the force constant f (also
k in followings) of the spring.
• The vibrational energy V(r) can be calculated using the (classical) model of the
harmonic oscillator:
• Using this potential energy function in the Schrödinger equation, the vibrational
frequency can be calculated:
The vibrational frequency is increasing with:
• increasing force constant f = increasing bond strength
• decreasing atomic mass
• Example: f cc > f c=c > f c-c
Vibrational spectra (II): Anharmonic oscillator model
Fig. 12-1
The actual potential energy
of vibrations fits the
parabolic function fairly
well only near the
equilibrium internuclear
distance. The Morse
potential function more
closely resembles the
potential energy of
vibrations in a molecule for
all internuclear distancesanharmonic oscillator
model.
EVIB = ( En+1 – En ) =h osc
• The energy difference between
the transition from n to n+1
corresponds to the energy of the
absorbed light quantum
• The difference between two
adjacent energy levels gets
smaller with increasing n until
dissociation of the molecule
occurs (Dissociation energy ED )
Note:
Weaker transitions called “overtones” are sometimes observed. These correspond to
=2 or 3, and their frequencies are less than two or three times the fundamental
frequency (=1) because of anharmonicity.
Typical energy spacings for vibrational levels are on the order of 10-20 J. from the
Bolzmann distribution, it can be shown that at room temperature typically 1% or less of
the molecules are in excited states in the absence of external radiation. Thus most
absorption transitions observed at room temperature are from the =0 to the =1 level.
Vibrational spectra (III): Rotation-vibration transitions
The vibrational spectra appear as bands rather than lines. When vibrational
spectra of gaseous diatomic molecules are observed under high-resolution
conditions, each band can be found to contain a large number of closely
spaced components— band spectra. The structure observed is due to that
a single vibrational energy change is accompanied by a number of
rotational energy changes. The form of such a vibration-rotation spectrum
can be predicted from the energy levels of a vibrating-rotating molecule.
–> “vibrational-rotational bands”
A vibrational absorption transition from  to +1 gives rise to three sets
of lines called branches:
Lower-frequency P branch: =1, J=-1;
Higher-frequency R branch: =1, J=+1;
Q branch: branch: =1, J=0.
Spectrum of the Rotating Oscillator
P
R
• The selection rules allow only transitions with  = +1 and J = ±1
(the transition with J = 0 is normally not allowed except those with
an odd number of electrons (e.g. NO)).
The IR absorption spectrum can be obtained with gas-phase or
with condensed-phase molecules. For gas-phase molecules
vibration-rotation spectra are observed, while in condensed
phases, the rotational structure is lost.
For most routine analytical applications of infrared
spectrometry, spectra are obtained with condensed-phase
samples. Hence, the discuss here centers around the vibrational
transitions observed with molecules present as pure liquid, as
solutions, or in the solid state.
Molecular vibrations
• How many vibrations are possible (=fundamental vibrations)?
A molecule has as many degrees of freedom as the total degree of
freedom of its individual atoms. Each atom has three degrees of freedom
(corresponding to the Cartesian coordinates), thus in an N-atom
molecule there will be 3N degree of freedom.
In molecules, movements of the atoms are constrained by interactions
through chemical bonds.
Translation - the movement of the entire molecule while the positions
of the atoms relative to each other remain fixed: 3 degrees of
translational freedom.
Rotational transitions – interatomic distances remain constant but the
entire molecule rotates with respect to three mutually perpendicular
axes: 3 rotational freedom (nonlinear), 2 rotational freedom (linear).
Vibrations – relative positions of the atoms change while the average
position and orientation of the molecule remain fixed.
Fundamental Vibrations
Vibration Types
• There are two different types of vibrational modes:
Vibrations can either involve a change in bond length
(stretching) or bond angle (bending)
The bending vibrations are often subdivided into scissoring,
rocking, wagging, and twisting.
Principle of IR experiments
•E-vector in electromagnetic radiation has frequency 
•Molecular vibrations involving change in dipole moment set up
fluctuating electric field
Vibrational energies: fundamental (= one quantum)
•Energy transferred to molecule by resonance when vibration
frequency is the same as that of the electromagnetic radiation
IR  SAMPLE  SAMPLE*
(MOLECULE, GS)
(VIB.)
Selection Rules
The energy associated with a quantum of light may be transferred to the
molecule if work can be performed on the molecule in the form of
displacement of charge.
Selection rule:
A molecule will absorb infrared radiation if the change in vibrational
states is associated with a change in the dipole moment () of the
molecule.
µ = qr
q: electrical charge, r: directed distance of that charge from some
defined origin of coordinates from the molecule.
Dipole moment is greater when electronegativity difference between the
atoms in a bond is greater. Some electronegativity values are:
H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16
• Vibrations which do not change the dipole moment are Infrared Inactive
(homonuclear diatomics).
Why not 3N-6/3N-5 bands in IR spectrum?
• The theoretical number of fundamental vibrations (absorption
frequencies) will seldom be observed
–> overtones (multiples of a given frequency), combination (sum of
two other vibrations) or difference (the difference of two other
vibrations) tones increase the number of bands
–> the following effects will reduce the number of theoretical
bands:
• frequencies which fall outside the measured spectral region (4004000 cm –1 )
• bands which are too weak
• bands are too close and coalesce
• occurrence of a degenerate band from several absorptions of the
same frequency
• lack of change in molecular dipole
Infrared Spectrum
of Carbon Dioxide
Vibrational Modes for a CH2 Group
Absorption Regions
Group frequencies
With certain functional or structural groups, it has been found
that their vibrational frequencies are nearly independent of the
rest of the molecule – group frequencies.
Carbonyl group 1650 to 1740 cm-1
various aldehydes and ketones
For many groups involving only two atoms, the approximate
frequency of the fundamental vibration can be calculated from
a simple harmonic oscillator model.
Calculations show that for most groups of interest, characteristic
frequencies of stretching vibrations should lie in the region 4000 to
1000 cm-1. In practical, the region from 4000 to 1300 cm-1 is often
called the group frequency region.
The presence of various group vibrations in the IR spectrum is of
great assistance in identifying the absorbing molecule.
Fingerprint region
In the region from  1300 to 400 cm-1, vibrational frequencies are
affected by the entire molecule, as the broader ranges for group
absorptions in the figure below – fingerprint region.
Absorption in this fingerprint region is characteristic of the molecule
as a whole. This region finds widespread use for identification purpose
by comparison with library spectra.
Coupled Interactions
• When two bond oscillators share a common atom, they seldom behave
as individual oscillators (unless the individual oscillation frequencies are
widely different).
The frequency of the asymmetric stretching vibration in CO2 is at a
shorter wavelength (higher frequency) than for a carbonyl group in
aliphatic ketones (around 1715 cm–1 ).
–> there must be strong mechanical coupling or interaction!
Example: C–O stretching band in
Methanol: 1034 cm –1
Ethanol: 1053 cm –1
not an isolated stretching vibration, but rather a coupled symmetric
stretching invloving C–C–O stretching
Requirements for Coupled Interactions
• The vibrations must be of the same symmetry
• The interaction is greatest, when the coupled groups absorb
(individually) near the same frequency --- the same energies of isolated
vibrations.
• Strong coupling between stretching vibrations requires a common atom
between the two groups
• Coupling between bending and stretching vibrations can occur if the
stretching bond forms one side of the changing angle.
• A common bond is required for coupling of bending vibrations.
• Coupling is negligible when groups are separated by one or more carbon
atoms and the vibrations are mutually perpendicular.
Hydrogen Bonding
“A hydrogen bond exists when a hydrogen atom is bonded to two
or more other atoms”
–> not an ordinary covalent bond, since the hydrogen atom
has only one orbital (1s) to engage in covalent bonding
Typical H-bond: Hydrogen is attached to two very
electronegative atoms, usually in a linear fashion and not
symmetrically:
X–H••••••B
=> The s orbital of the proton can effectively overlap with
the p or p orbital of the acceptor group.
Effect of Hydrogen Bonding
• Hydrogen bonding alters the force constant of both groups:
– the X–H stretching bands move to lower frequency
– the stretching frequency of the acceptor group (B) is also reduced,
but to a lesser degree
– The X–H bending vibration usually shifts to a shorter wavelength
IR spectrum
y axis is %T or A
x axis is wavenumber (or wavelength)
Io  sample  I
T = I/Io
%T = 100 I/Io
T transmission / transmittance
A = -log T
A absorbance (no units)
(Note A (but not T)  concentration)
Ordinate Scaling
Absorbance spectrum of polystyrene
–> generally used for quantitative
work
Transmittance spectrum of polystyrene
–> traditionally used for spectral
interpretation
Abscissa Scaling
There is a substantial difference in “appearance”, whether the spectrum
is linear in wavenumber (A, standard) or linear in wavelength (B):
A
B
(both spectra are
recorded with the same
sample)
Instrumentation
Perkin ElmerTM
Spectrum One
BRUKE TENSORTM
Series
• Dispersive instruments: with a monochromator to be used in
the mid-IR region for spectral scanning and quantitative
analysis.
• Fourier transform IR (FTIR) systems: widely applied and
quite popular in the far-IR and mid-IR spectrometry.
• Nondispersive instruments: use filters for wavelength
selection or an infrared-absorbing gas in the detection system
for the analysis of gas at specific wavelength.
Dispersive IR spectrophotometers
Modern dispersive IR spectrophotometers are invariably double-beam
instruments, but many allow single-beam operation via a front-panel
switch.
Simplified diagram of a double beam infrared spectrometer
Double-beam operation allows a stable 100% T baseline in the spectra.
Double-beam operation compensates for atmospheric absorption, for the
wavelength dependence of the source spectra radiance, the optical
efficiency of the mirrors and grating, and the detector instability, which
are serious in the IR region.single-beam instruments not practical.
Dispersive spectrophotometers Designs
Null type instrument
Components of dispersive spectrophotometers
1. IR source
Nernst Glower
heated rare earth oxide rod
(~1500 K)
1-50 µm
(mid- to far-IR)
Globar
heated SiC rod (~1500 K)
1-50 µm
(mid- to far-IR)
W filament lamp
1100 K
0.78-2.5 µm
(Near-IR)
Hg arc lamp
plasma
50 - 300 µm
(far-IR)
CO2 laser
stimulated emission lines
9-11 µm
2. Detector / transducer
Thermocouple
thermoelectric effect dissimilar metal junction
cheap, slow,
insensitive
Bolometer
Ni, Pt resistance
thermometer (thermistor)
Highly sensitive
<400 cm-1
Pyroelectric
Tri glycine sulfate
piezoelectric material
fast and sensitive
(mid IR)
Photoconducting PbS, CdS, Pb Se light
sensitive cells
fast and sensitive
(near IR)
3. Optical system
• Reflection gratings ( made from various plastics): the groove
spacing is greater (e.g. 120 grooves mm-1). To reduce the effect of
overlapping orders and stray radiation, filters or a preceding prism
are usually employed. Two or more gratings are often used with
several filters to scan a wide region.
• Mirrors but not lenses are used to focus and collimate the IR
radiation. Generally made from Pyrex or another material with low
coefficient of thermal expansion. Front surfaces coated with a
vacuum-deposited thin metal film of Al, Ag, or Au.
•Windows are used for sample cells and to permit various compartment
to be isolated from the environment.
 transparent to IR over the wavelength region
 inert to the various chemicals analyzed
 capable of being shaped, ground, and polished to the desired
optical quality
Fourier Transform Infrared Spectrometer (FTIR)
The Fourier transform method provides an alternatives to the use
of monochromators based on dispersion.
In conversional dispersive spectroscopy, frequencies are separated
and only a small portion is detected at any particular instant, while
the remainder is discarded. The immediate result is a frequencydomain spectrum.
Fourier transform infrared spectroscopy generates time-domain
spectra as the immediately available data, in which the intensity is
obtained as a function of time.
Direct observation of a time-domain spectrum is not immediately
useful because it is not possible to deduce, by inspection,
frequency-domain spectra from the corresponding time-domain
waveform (Fourier transform is thus introduced).
Single-beam FTIR Spectrometer
In one arm of the interferometer, the IR source radiation travels through the beam
splitter to the fixed mirror back to the beam splitter through the sample and to the
detector. In the other arm, the IR source radiation travels to the beam splitter to the
movable mirror, back through the beam splitter to the sample and to the detector. The
difference in pathlengths of the two beams is the retardation . An He-NE laser is used
as a monochromatic reference source. The laser beam is sent through the interferometer
in the opposite direction to that of the IR beam.
Double-beam FTIR
Spectrometer
Interferometer
Michelson interferometer
If moving mirror moves 1/4  (1/2  round-trip) waves are out of phase at beamsplitting mirror - no signal
If moving mirror moves 1/2  (1  round-trip) waves are in phase at beam-splitting
mirror – signal
...
Interferograms
Difference in pathlength called retardation 
Plot  vs. signal - cosine wave with frequency proportional to light
frequency but signal varies at much lower frequency
One full cycle when mirror moves distance /2 (round-trip = )
Frequency of signal:
Substituting =c/
VMM 2VMM
f =
=
/2

f =
If mirror velocity is 1.5 cm/s
VMM velocity of moving mirror
2VMM

c
3cm / s
10
f =

=
10

10
3 10 cm / s
Bolometer, pyroelectric, photoconducting IR detectors can "see“ changes
on 10-4 s time scale!
Computer needed to turn complex interferograms into spectra.
Measuring processes
Advantages of FTIR
• very high resolution (< 0.1 cm –1 )
Two closely spaced lines only separated if one complete "beat" is recorded. As
lines get closer together,  must increase.
(cm1) = 1/
Mirror motion is 1/2 
Resolution governed by distance movable mirror travels
• very high sensitivity (nanogram quantity)
can be coupled with GC analysis (–> measure IR spectra in gas-phase)
• High S/N ratios - high throughput
Few optics, no slits mean high intensity of light
• Rapid (<10 s)
• Reproducible and • Inexpensive
To improve S/N ratio
Usually to improve resolution
decrease slit width but less light
makes spectrum "noisier" - signal
to noise ratio (S/N)
S
= n
N
S
S
=
2
 ( S  Si ) N
n
n # scans
S/N improves with more scans
(noise is random, signal is not!)
Spectrum calibration
For routine instrument calibration, run the spectrum of
polystyrene film (or indene) at resolution 2 cm-1. Band
positions are available in the literature.
Higher resolution calibrations may be made from gasphase spectra (e.g. HCl gas).
Sample preparation techniques
Infrared spectra may be obtained for gases, liquids or
solids (neat or in solution)
The preparation of samples for infrared spectrometry is often the most
challenging task in obtaining an IR spectrum. Since almost all substances absorb
IR radiation at some wave length, and solvents must be carefully chosen for the
wavelength region and the sample of interest.
Gas samples
• A gas sample cell consists of a cylinder of glass or sometimes a metal.
The cell is closed at both ends with an appropriate window materials
(NaCl/KBr) and equipped with valves or stopcocks for introduction of the
sample.
• Long pathlength (10 cm) cells – used to study dilute (few molecules)
or weakly absorbing samples.
• Multipass cells – more compact and efficient instead of long-pathlength
cells. Mirrors are used so that the beam makes several passes through
the sample before exiting the cell. (Effective pathlength  10 m).
• To resolve the rotational structure of the sample, the cells must be
capable of being evacuated to measure the spectrum at reduced
pressure.
• For quantitative determinations with light molecules, the cell is
sometimes pressurized in order to broaden the rotational structure and
all simpler measurement.
Liquid samples
• Pure or soluted in transparent solvent – not water (attacks windows)
•The sample is most often in the form of liquid films (“sandwiched”
between two NaCl plates)
• Adjustable pathlength (0.015 to 1 mm) – by Teflon spacer
Regions of transparency for common infrared solvents.
The horizontal lines indicate regions where solvent transmits at least
25% of the incident radiation in a 1-mm cell.
Solid samples
• Spectra of solids are obtained as alkali halide discs (KBr), mulls
(e.g. Nujol, a highly refined mixture of saturated hydrocarbons) and
films (solvent or melt casting)
Alkali halide discs:
1. A milligram or less of the fine ground sample mixed with about
100 mg of dry KBr powder in a mortar or ball mill.
2. The mixture compressed in a die to form transparent disc.
Mulls
1. Grinding a few milligrams of the powdered sample with a mortar
or with pulverizing equipment. A few drops of the mineral oil
added (grinding continued to form a smooth paste).
2. The IR of the paste can be obtained as the liquid sample.
Main uses of IR spectroscopy:
1. Fundamental chemistry
Determination of molecular structure/geometry.
e.g. Determination of bond lengths, bond angles of
gaseous molecules
2. Qualitative analysis – simple, fast, nondestructive
Monitoring trace gases: NDIR.Rapid, simultaneous
analysis of GC, moisture, N in soil. Analysis of fragments
left at the scene of a crime
Quantitative determination of hydrocarbons on filters, in
air, or in water
Near-infrared and Far-infrared absorption
The techniques and applications of near-infrared (NIR) and
far-infrared (FIR) spectrometry are quite different from those
discussed above for conventional, mid-IR spectrometry.
Near-infrared: 0.8 -2.5 m, 12500 - 4000 cm-1
Mid-infrared: 2.5 - 50 m, 4000 - 200 cm-1
Far-infrared: 50 - 1000 m, 200 - 10 cm-1
Near-infrared spectrometry
NIR shows some similarities to UV-visible spectrophotometry and
some to mid-IR spectrometry. Indeed the spectrophotometers
used in this region are often combined UV-visible-NIR ones.
The majority of the absorption bands observed are due to
overtones (or combination) of fundamental bands that occur in
the region 3 to 6 m, usually hydrogen-stretching vibrations.
NIR is most widely used for quantitative organic functional-group
analysis. The NIR region has also been used for qualitative
analyses and studies of hydrogen bonding, solute-solvent
interactions, organometallic compounds, and inorganic
compounds.
Far-infrared spectrometry
Almost all FIR studies are now carried out with FTIR
spectrometers.
The far-IR region can provide unique information.
i)
The fundamental vibrations of many organometallic and
inorganic molecules fall in this region due to the heavy atoms
and weak bonds in these molecules.
ii) Lattice vibrations of crystalline materials occur in this region,
iii) Electron valence/conduction band transition in
semiconductors often correspond to far-IR wavelengths.
Infrared Spectrum of CCl4