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180 4 Comprehensive Insights into Infrared Spectroscopy
Notably, salt plates are hygroscopic and should be avoided with wat er-soluble
samples, necessitating special cells like BaF
and AgCl for such analyses.
2
4.9.3 Gas Samples
Gas samples in IR spectroscopy are managed within specialized gas cells, featuring
IR-transparent windows. Gas cells come in various configurations like multipass and
long-path cells, enabling multiple interactions with the gas sample, enhancing
sensitivity. IR spectra of gases reveal molecular composition, aiding qualitative
and quantitative analysis. Gas-phase IR spectroscopy finds applications in analyzing
atmospheric gases, pollutants, and chemicals. Various gas sampling techniques,
including gas chromatography-IR (GC-IR), allow flexible introduction of gases.
GC-IR is valuable for analyzing complex gas mixtures. This technique plays a
pivotal role in environmental air quality monitoring, gas emission identification,
process control, and offers critical insights in numerous fields, providing powerful
analytical capabilities.
4.10 Types of IR Spectroscopy
IR spectroscopy encompasses various techniques, each tailored for specific
applications and analytical requirements. Here are some of the primary types of IR
spectroscopy:
4.10.1 Dispersive IR Spectroscopy
Dispersive IR spectroscopy is a traditional technique for analyzing the interaction of
matter with IR radiation (Fig.
4.7). It uses a dispersive element like a prism or
diffraction grating to separate different IR wavelengths, creating a spectrum of
intensity versus wavelength. This method provides high spectral resolution, making
it valuable for fine structural analysis. While it is slower compared to FT-IR
spectroscopy, it has found applications in chemistry, materials science, and environmental science for qualitative and quantitative analysis. In recent years, FT-IR has
gained prominence due to its speed and sensitivity, but dispersive IR spectroscopy
remains a valuable tool for specific applications requiring high spectral resolution
and detailed structural insights.
4.10.2 FT-IR Spectroscopy
FT-IR spectroscopy is a widely used analytical technique for studying the interaction
of matter with infrared radiation. In FT-IR spectroscopy, an interferometer is used to
collect data across a broad range of infrared frequencies simultaneously (Fig.
4.8).
Fig. 4.7 Schematic representation of dispersive IR spectroscopy. This diagram illustrates the
components and functioning of a dispersive IR spectrometer. The IR source emits radiation,
which is directed toward a beam-splitter via mirrors. The beam is split into two paths: one directed
toward the reference cell and the other toward the sample cell. After passing through the cells, the
beams are reflected by mirrors and recombined. A chopper modulates the combined beam, which is
directed to a diffraction grating through slits. The grating disperses the beam into its component
wavelengths. The detector records the intensity of each wavelength, and the amplified signal is
transmitted to a recorder, which displays the IR spectrum
Fig. 4.8 Schematic representation of Fourier-transform infrared (FT-IR) spectroscopy. This diagram illustrates the working principle of an FT-IR spectrometer, incorporating a Michelson
interferometer. The IR source emits radiation, which is split into two beams by a beam splitter.
One beam reflects off a fixed mirror, while the other is directed to a moving mirror. The two beams
recombine, causing interference patterns that encode information about the sample. The combined
beam passes through a window to the working electrode, and the resulting signal is detected. A
reference laser ensures precision in the measurement. The detector collects the final interferogram,
which is processed to generate the IR spectrum. (Adapted from [Encyclopedia of Electrochemistry,
Vol. 3 (Eds.: A. J. Bard, M. Stratmann, E. J. Calvo), Wiley Publishers, 2003])
182 4 Comprehensive Insights into Infrared Spectroscopy
Table 4.2 Difference between dispersive IR spectroscopy and FT-IR spectroscopy
Sr.# Dispersive IR spectroscopy FT-IR spectroscopy
1. There are numerous moving parts, causing
mechanical slippage
2. Calibration against the reference spectra is
needed to measure
3. Stray light gives spurious readings Stray light does not affect the detector
4. Only a small amount of IR beam is
allowed to pass to increase the resolution
5. Only radiation of a narrow frequency
range falls on the detector at one time
6. Scanning speed is slow Scanning speed is high
the frequency
Mirror is the only part that moves during
the experiment
Use of laser provides greater frequency
accuracy (up to 0.01 cm
A much larger beam is used at all times.
Data collection is comparatively easy
All frequency of radiation falls on the
detector simultaneously
-1
)
This data is then transformed from the time domain to the frequency domain
using a mathematical technique called the Fourier transform. The resulting spectrum
represents the intensity of the IR radiation as a function of wavenumber, which is
directly related to the frequency of molecular vibrations in the sample. Key
characteristics of FT-IR spectroscopy include:
• Speed: FT-IR spectroscopy is significantly faster than traditional dispersive IR
spectroscopy, as it can collect a wide range of frequencies in a single measure-
ment, making it ideal for rapid data acquisition.
• Sensitivity: FT-IR spectroscopy offers high sensitivity, making it suitable for the
analysis of trace components in a sample.
• Versatility: It is used in various fields, including chemistry, materials science,
biology, and environmental science, for qualitative and quantitative analysis of
organic and inorganic compounds.
• Sample types: FT-IR can analyz
e a
wide range of sample types, including solids,
liquids, and gases, and can be applied to various sample forms, including thin
films and powders.
• Applications: FT-IR spectroscopy has diverse applications, including identifying
chemical compounds, studying molecular structures, monitoring chemical
reactions, and characterizing materials.
The difference between the dispersive IR and FT-IR spectroscopy is described in
4.2:
Table
4.10.3 Near-IR Spectroscopy
NIR spectroscopy is a non-destructive analytical technique that examines the interaction between matter and near-infrared light. It covers the region of the electromagnetic spectrum adjacent to the visible light range, typically from about
780 nanometers to 2500 nanometers. NIR spectroscopy provides valuable
4.11 Regions of IR Spectrum 183
information about the chemical composition and properties of materials. Key
features of NIR spectroscopy include:
• Nondestructive: One of the main advantages of NIR spectroscopy is its nonde-
structive nature. It allows for the analysis of samples without altering or damaging
them, making it suitable for quality control and process monitoring.
• Versatility: NIR spectroscopy is versatile and can be applied to a wide range of
sample types, including solids, liquids, and even gases. It is used in various
industries, including agriculture, pharmaceuticals, food, and materials science.
• Quantitative analysis: NIR spectroscopy is often used for quantitative analysis,
such as determining the concentration of specific components within a sample. It
is particularly valuable for assessing the composition of complex mixtures.
• Rapid data acquisition: NIR spectroscopy is known for its rapid data acquisition,
making it suitable for high-throughput applications.
• Applications: This technique is widely
applied
for purposes like assessing the
quality of agricultural products, monitoring chemical processes, and analyzing
pharmaceutical formulations.
4.11 Regions of IR Spectrum
The regions of the IR spectrum associated with different types of chemical bonds can
be broadly categorized as follows:
Single Bonds (e.g., C-C, C-H, O-H, N-H) Single bonds, which include most
organic compounds, are typically associated with stretching vibrations in the region
of about 2800–3000 cm
around 3200– 3800 cm
vibrations of single bonds are usually found at lower wavenumbers (around
1400–1600 cm
-1
-1
(2.8–3.0 micrometers in wavelength) for C-H bonds and
-1
(3.2–3.8 micrometers) for O-H and N-H bonds. Bending
).
Double Bonds (e.g., C=C, C=O) Double bonds, such as those in alkenes (C=C)
and carbonyl groups (C=O), are associated with stretching vibrations at approximately 1600–1800 cm
vibrations of double bonds are typically found at lower wavenumbers, around
1600–1400 cm
Triple B
-1
e.g., C C) Triple bonds, like those in alkynes (C C), exhibit
onds (
stretching vibrations in the region of about 2100–2300 cm
triple bonds are usually found at lower wavenumbers, around 700–900 cm
-1
for C=C and 1600–1700 cm
-1
for C=O. Bending
.
-1
. Bending vibrations of
-1
.
Infrared spectroscopy is a powerful tool for identifying and characterizing differ-
ent types of chemical bonds based on their characteristic vibrational frequencies in
the IR spectrum. The primary regions in IR spectra help interpret the spectra of
184 4 Comprehensive Insights into Infrared Spectroscopy
Fig. 4.9 Interpretation of IR spectroscopy regions for functional group identification. This figure
illustrates the primary regions of the IR spectrum, emphasizing their importance in identifying
different molecular bonds based on their characteristic absorption frequencies. The IR spectrum is
divided into four major regions: The Bonds to Hydrogen region (4000–2700 cm
stretching vibrations of bonds like O-H, N-H, and C-H occur due to their lighter atomic masses
and higher energy absorption. The Triple Bond region (2700–2000 cm
peaks for C C and C N bonds, which exhibit higher energy than double and single bonds. The
Double Bond region (2000–1600 cm
indicating the presence of key functional groups like carbonyls and imines. The Fingerprint region
(1600–600 cm
like C-C, C-N, and C-O are found. This region aids in distinguishing subtle differences between
similar molecular structures
-1
), often complex but highly specific to individual molecules, where single bonds
-1
), where bonds such as C=O, C=N, and C=C resonate,
-1
), featuring absorption
-1
), where
unknown compounds, as shown in Fig. 4.9. However, it is essential to recognize that
the wavenumber ranges mentioned earlier are approximate and may vary depending
on the specific molecular environment and bonding context.
4.12 Calculation of Vibrational Frequencies
Hooke’s law provides a simplified model for calculating vibrational frequencies in
molecules, which is essential for understanding molecular vibrations in techniques
like IR spectroscopy. By treating chemical bonds as springs, it helps predict how
molecules interact with IR radiation, facilitating the identification of various functional groups based on their characteristic vibrations. Hooke’s law describes the
behavior of springs and elastic materials, stating that the force required to extend or
compress a spring is proportional to the displacement from its equilibrium position,
as long as the material remains within its elastic limit. Mathematically, it is
expressed as:
4.12 Calculation of Vibrational Frequencies 185
F =-kx
Where:
• F is the restoring force,
• k is the spring constant (measure of the stiffness of the spring),
• x is the displacement from the equilibrium position.
This law assumes that the material behaves elastically, and the deformation is
proportional to the force applied.
Application to Molecular Vibrations
In molecular systems, atoms bonded together act like masses connected by springs
(chem
ical bonds). Hooke’s law can be used as a simple model to describe the
vibrations of these atoms. The atoms in a molecule vibrate about their equilibrium
positions, much like the masses at the ends of a spring in a spring-mass system.
• Vibrational frequency calculation: The vibrational frequency of a diatomic mol-
ecule
can be derived using Hooke’s law and is given by the following equation:
1
v =
π k μ
2
Where:
• V is the vibrational frequency,
• k is the force constant (analogous to the spring const
ant, representing bond
stiffness),
• μ is the reduced mass of the two atoms involved, calculated as:
1m2
m
Where m1 and m2 are the
• Interpretation in
infrared (IR) spectroscopy: The vibrational frequencies calcu-
μ =
masses of the two atoms.
þ
m1 m2
lated using Hooke’s law are crucial for understanding IR spectroscopy. Molecules
absorb IR radiation when the frequency of the radiation matches the vibrational
frequency of the bond. By measuring the IR absorption frequencies, the force
constants and bond strengths of molecular bonds can be determined.
– Example
: In a diatomic molecule like HCl, Hooke’s law can estimate the
vibrational frequency by considering the bond between hydrogen (H) and
chlorine (Cl) as a spring. Using the mass of H and Cl atoms and the bond
force constant, the vibrational frequency can be calculated, which correlates
with its IR absorption.
186 4 Comprehensive Insights into Infrared Spectroscopy
4.13 Factors Affecting Vibrational Frequency
Vibrational frequencies in molecules are influenced by several factors beyond just
bond strength and mass, such as vibrational coupling and hydrogen bonding. Here is
a more in-depth discussion of these factors:
1. Vibrational coupling
• Interaction between vibrational modes: When two vibrational modes occur in
proximity (similar frequencies), their motions can couple. This means
close
the energy of one vibration affects the other, leading to shifts in observed
vibrational frequencies. Coupling is common in molecules where atoms are
interconnected through multiple bonds (e.g., CH
• Fermi resonance: A specific type of vibrational coupling, Fermi resonance
occurs
when two vibrational modes (typically a fundamental vibration and an
overtone or combination band) are close in energy. This leads to a mixing of
their characteristics, often shifting their positions in the IR spectrum and
affecting intensity.
• Geometry and symmetry: In symmetric molecules, coupling between vibra-
modes is more pronounced. In contrast, in highly asymmetric molecules,
tional
vibrational coupling may be minimal.
2. Hydrogen bonding
• Effect on bond strength: Hydrogen bonding has a significant effect on vibra-
tional
frequencies, particularly those involving O-H, N-H, or F-H groups.
When a hydrogen bond forms, the bond participating in the interaction
becomes weaker, leading to a lower vibrational frequency (red shift). For
example, the O-H stretching frequency in water or alcohols shifts to lower
wavenumbers when hydrogen bonds are present.
• Strength and environment: The degree of hydrogen bonding depends on the
environment (e.g., solvents, molecular arrangement). Strong hydrogen bonds
can cause a considerable shift in vibrational frequency, whereas weak hydrogen bonds might only produce minor changes.
3. Bond strength and atomic mass
• Bond strength: Stronger bonds (like triple bonds in C C) vibrate at higher
frequen
cies compared to single or double bonds (like C-C or C=C). The force
constant κ in Hooke’s law is higher for stronger bonds, leading to a higher
vibrational frequency.
• Atomic mas s: According
to Hooke’s law, vibrational frequency is inversely
proportional to the square root of the reduced mass of the bonded atoms.
Lighter atoms (such as H) vibrate at higher frequencies than heavier atoms
(such as Cl), which is why C-H stretching vibrations occur at higher
frequencies than C-Cl stretching.
4. Bond order: Mu
ltiple bonds, such as double and triple bonds, have higher
vibrational frequencies than single bonds due to stronger interactions between
atoms. For example, C C bonds vibrate at higher frequencies than C=C bonds.
groups or ring structures).
2
4.14 Interpretations of IR Spectrum 187
5. Hybridization and bond order
• Hybridization: The hybridization state of the atoms involved in bonding
influences the vibrational frequency.
Bonds involving
sp-hybridized carbons
(as in alkynes) have higher vibrational frequencies than those involving sp2
(as in alkenes) or sp3 (as in alkanes) hybridized carbons.
• Bond order: Multiple bonds (double, triple) exhibit higher vibrational
frequencies than single bonds. For instance, the stretching frequency for a
C C bond is higher than that of a C=C bond due to the increased bond
strength.
6. Electron delocalization: In conjugated systems or aromatic rings, electron delocalization lowers the bond strength, causing a decrease in the vibrational frequency. This effect is often seen in the C=C stretching vibrations of aromatic
compounds compared to isolated double bonds.
7. Solvent effects: Polar solvents can interact with the solute, altering bond strength
and shifting vibrational frequencies. Hydrogen bonding between solvent and
solute can further impact these frequencies.
8. Solvent effects
• Polar vs. nonpolar solvents: Solvent polarity can shift vibrational frequencies.
In polar solvents, interactions between solvent molecules and polar functional
groups can alter bond strengths, often resulting in frequency shifts. Nonpolar
solvents generally have less effect on vibrational frequencies.
• Solvent–solute interactions: For example, hydrogen bonding between a solute
(e.g., alcohol) and a polar solvent can further shift the O-H stretching frequency due to additional stabilization of the hydrogen bond.
4.14 Interpretations of IR Spectrum
Interpreting an IR spectrum involves identifyin g the various absorption bands or
peaks in the spectrum and assigning them to specific types of molecular vibrations or
chemical bonds. Here are some common interpretations of IR spectra:
• Functional groups: One of
the fundam
is identifying the functional groups present in the compound. Different functional
groups, such as alcohols, amines, carbonyls, and alkenes, have characteristic
absorption bands. For example, the presence of a broad peak around
3200–3700 cm
• Bond types:
-1
suggests the presence of an -OH group.
IR spectroscop
y is sensitive to the types of chemical bonds in a
molecule. Stretching and bending vibrations of bonds are observed as absorption
peaks. For example, C-H stretching vibrations typically appear around
2800–3000 cm
• Peak positions:
information. For example, C=O groups typically absorb in the range of
1650–1750 cm
-1
.
The positions (wavenumbers) of absorption peaks provide critical
-1
.
ental aspects of IR spectrum interpretation
188 4 Comprehensive Insights into Infrared Spectroscopy
• Intensity of peaks: The intensity of absorption peaks can reflect the concentration
of a specific functional group. More intense peaks correspond to a higher concentration of that group.
• Peak shapes: The shape of the peaks can provide information about the environ-
ment
in which a functional group exists. Sharp peaks indicate isolated functional
groups, while broad peaks may suggest hydrogen bonding or complex molecular
interactions.
• Fingerprint region: The fingerprint region of the IR spectrum (typically below
1500
-1
) contains unique patterns of peaks that are highly specific to
cm
particular compound. These patterns can be used for compound identification.
• Absence of bands: The absence of absorption bands can also be informative. If a
peak
that is expected for a specific functional group is absent, it may indicate the
absence or low concentration of that group.
• Sample purity: The presence of impurities or contaminants can introduce addi-
peaks in the spectrum. Careful examination of unexpected peaks can help
tional
identify impurities.
• Sample state: The state of the sample (solid, liquid, or gas) affects the appearance
of
the spectrum. Different states exhibit characteristic spectral features.
• Hydrogen bonding: The presence of hydrogen bonds can lead to shifts in absorp-
tion
bands. For example, O-H stretching vibrations may shift to lower
wavenumbers when involved in hydrogen bonding.
Interpreting an IR spectrum often requires expertise, experience, and access to
nce databases and spectral libraries for compound identification.
refere
Spectroscopists use the knowledge of these principles and patterns to deduce the
molecular structure and composition of a sample. A schematic representation of IR
spectra has been illustrated in Fig. 4.10. In the following subsections, IR spectra of
few
compounds have been interpreted accordingly.
a
4.14.1 IR Spectra of Alkanes
Consider the IR spectra of octane (Fig. 4.11). The interpretation of the IR spectra for
compounds like octane is as follows: C-C stretching and bending vibrations typically
occur between 1360 and 1470 cm
1450–1470 cm
1360 and 1390 cm
range of 2800–3000 cm
-1
. Vibrations for the CH2-CH3 bond are typically observed between
-1
, while vibrations for sp3 C-H bonds are typically found in the
-1
-1
. The CH2-CH2 bond exhibits vibrations around
. These wavenumber ranges may vary slightly based on
the specific molecular environment and bonding context.
4.14.2 IR Spectra of Alkenes
Consider the spectra of 1-octene (Fig. 4.12). The interpretation of the IR spectra for
compounds like 1-octene with vinyl C-H and C=C bonds is as follows: C =C
4.14 Interpretations of IR Spectrum 189
Fig. 4.10 Interpretation of IR spectrum for functional group analysis. This figure represents an IR
spectrum, displaying the relationship between transmittance (%T) and wavenumber (cm
-1
) to help
identify various functional groups present in a sample. Peaks observed in an IR spectrum correspond to the vibrational frequencies of specific bonds in a molecule, providing insight into the
molecular structure. This graphical analysis serves as a fundamental tool for determining molecular
structures based on their IR absorption patterns
Fig. 4.11 Infrared spectrum of octane showing key absorption bands of alkanes: The IR spectrum
of octane, a representative alkane, highlights characteristic absorption bands associated with C-H
stretching and bending vibrations. The broad absorption between 2850 and 2960 cm
in blue) corresponds to strong (s) and weak (w) C-H stretching vibrations of alkyl groups. The
medium (m) absorption around 1375–1470 cm
vibrations. This spectrum illustrates the typical IR pattern for saturated hydrocarbons, where alkane
chains display distinct absorptions in the 2800–3000 cm
confirming the presence of sp
3
hybridized C-H bonds
-1
(highlighted in red) is due to C-H bending
-1
and 1350–1500 cm
-1
(highlighted
-1
regions,