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160 3 Comprehensive Insights into UV-VIS Spectrophotometry
Thomas O, Burgess C, editors. UV-visible spectrophotometry of water and wastewater.
Elsevier;
Waters C
Watson DG.
2017.
orporation. U
V-VIS spectroscopy. Accessed November 10, 2024.
Pharmaceutical analysis E-book: a textbook for pharmacy students and pharmaceutical
chemists. Elsevier Health Sciences; 2015.
Comprehensive Insights into Infrared Spectroscopy
Abstract
Infrared (IR) spectroscopy is a potent analytical method for probing how matter
interacts with infrared radiation. This comprehensive overview encompasses the
principles, techniques, applications, and future trends associated with IR spec-
troscopy. At its core, IR spectroscopy entails exposing a sample to infrared light
and measuring absorbed light intensity across wavelengths, resulting in a unique
spectrum. Notably, the IR spectrum is divided into regions, each revealing bond-
related information. Among these, the fingerprint region excels in compound
identification. Varieties of IR spectroscopy techniques were examined, including
dispersive IR spectroscopy, Fourier-transform IR (FT-IR) spectroscopy, and
near-IR spectroscopy, each bearing distinct merits and applications. For instance,
dispersive IR spectroscopy boasts high spectral resolution, while FT-IR spectros-
copy offers speed and sensitivity. Accurate interpretation of IR spectra
necessitates grasping various factors, from chemical functional groups and peak
positions to spectral intensity and shape, sample purity, and instrument settings.
These elements significantly influence spectral analysis and foster precise struc-
tural determination. The utility of IR spectroscopy spans diverse domains, featur-
ing roles in chemical analysis, pharmaceuticals, environmental monitoring, and
materials science. In the medical sphere, it plays a diagnostic role by leveraging
vibrational frequencies in biochemical substances like proteins, lipids, and
carbohydrates. Specialized techniques like atte nuated total reflection spectros-
copy, diffuse reflectance infrared Fourier-transform spectroscopy, and
two-dimensional infrared spectroscopy provide augmented capabilities for spe-
cific sample types and interactions. Advanced instrumentation has birthed mod-
ern IR spectrometers, marked by enhanced data analysis, enabling real-time
examination and imaging. The future indicates miniaturization, integration with
complementary techniques, refined data processing incorporating machine
learning and artificial intelligence, and a pivotal contribution to green chemistry
4
161
162 4 Comprehensive Insights into Infrared Spectroscopy
pursuits. The latter is fortified by IR spectroscopy’s nondestructive character and
applicability in real-time monitoring and management of chemical processes,
aligning with ecologically responsible practices.
Keywords
Infrared spectroscopy · FT-IR · Spectral interpretation · Functional group
analysis · Molecular vibrations · Sample preparation techniques
4.1 Introduction
Infrared spectroscopy is a technique used to measure the absorbance of a given
sample within the infrared (IR) region of the electromagnetic spectrum (EMS). IR
radiation falls between the microwave and visible regions of the EMS. The wavelength of IR radiation is longer than that of visible light but shorter than microwaves.
Conversely, the frequency of IR radiation is lower than that of visible light but higher
than microwaves. IR is a segment of the EMS characterized by wavelengths longer
than those of visible light yet shorter than radio waves. Different regions of the EMS
are illustrated in Fig.
4.1.
Fig. 4.1 The electromagnetic spectrum and its applications in molecular and nuclear excitations:
The electromagnetic spectrum covers a broad range of radiation types with distinct energies and
corresponding applications. Higher energy γ-rays (gamma rays) are used for nuclear excitation
processes such as positron emission tomography (PET), while X-rays are used for core electron
excitation, a key technique in X-ray crystallography. Ultraviolet (UV) radiation excites electrons,
particularly in molecular p to p* transitions, commonly applied in UV-visible spectroscopy.
Infrared (IR) radiation causes molecular vibrations, forming the basis for infrared spectroscopy
used in chemical analysis. Microwave radiation induces molecular rotation and is applied in
microwave spectroscopy. At the lower energy end, radio waves are essential in nuclear magnetic
resonance (NMR) spectroscopy and magnetic resonance imaging (MRI), both used for structural
analysis in chemistry and medical imaging. The visible spectrum lies between UV and IR, although
it does not excite molecular bonds but allows for visual observation of materials
4.3 Principle 163
4.2 Regions of IR
Three types of IR regions are outlined in Table 4.1. The unit commonly employed is
wavenumbers (cm
-1
), typically within the range of 4000–400 cm
-1
. The
wavenumber is directly proportional to energy (E) and frequency (ν) but inversely
proportional to wavelength. These regions are defined based on the energy levels and
the types of molecular vibrations that are observed in each region. Infrared spectroscopy is a valuable analytical technique for identifying and characterizing compo unds
based on their unique infrared absorption patterns within these regions.
4.3 Principle
Molecules consist of atoms that are connected by various chemical bonds. The
motion of atoms and their associated chemical bonds can be likened to a spring
and ball system. This characteristic motion is referred to as the natural frequency of
vibration. When an IR frequency is applied, it matches the natural frequency of
vibration. As a result of this vibrational motion, there is a change in the dipole
moment of the molecule. The oscillation of covalent bonds leads to an oscillation of
the molecule’s dipole, generating an electromagnetic (EM) field. The greater the
change in dipole moment due to vibrational motion, the more intense the electromagnetic field that is produced. In essence, IR spectroscopy involves the measurement of the reflection, absorption, and emission of the IR spectrum.
The dipole moment of a molecule is a measure of the separation of positive and
negative charges within the molecule. The distinction between linear and nonlinear
molecules in terms of dipole moment arises from their molecular geometry.
Table 4.1 Regions of IR and their ranges
IR regions
Near-IR
(NIR)
region
Mid-IR
(MIR)
region
Far-IR
(FIR)
region
Wavenumber
range
12,500–
-1
4000 cm
4000–
-1
400 cm
667–100 cm
1
Wavelength
range
0.78–2.5 μm This region is closest to the visible spectrum and
2.5–25 μm This region is commonly used in organic
-
25–1000 μm This region is used for studying lattice vibrations
Characteristics
is often used for applications like chemical
analysis, material identification, and
pharmaceutical analysis
chemistry for analyzing the vibrational modes of
organic compounds. It provides information
about functional groups and molecular structures
in solids, intermolecular interactions, and
low-energy molecular vibrations. It is less
commonly employed in routine analytical
chemistry
164 4 Comprehensive Insights into Infrared Spectroscopy
• Linear molecules: In linear molecules, the atoms are arranged in a straight line. If
the bonds between the atoms are polar but symmetrical (e.g., carbon dioxide,
CO₂), the dipole moments of the individual bonds can cancel each other out. This
results in a net dipole moment of zero. In such cases, even though the molecule
has polar bonds, the overall molecule may be nonpolar because the individual
bond dipoles balance out.
– Example: Carbon dioxide (CO₂) is a linear molecule where the dipole
moments of the two C=O bonds cancel, resulting in no net dipole moment.
• Nonlinear (bent or angular) molecules: In nonlinear molecules, the atoms are not
arranged
in a straight line. The bond angles create an asymmetrical distribution of
charge, preventing the dipole moments of individual bonds from canceling out.
As a result, the molecule typically has a net dipole mom ent.
– Example: Water (H₂O) is a nonlinear molecule with a bent geometry. The
e moments of the two O-H bonds do not cancel out due to the bent shape,
dipol
giving water a net dipole moment.
4.4 Modes of Molecular Vibrations
Any subtle alteration in the shape of a molecule, achieved through the bending of
bonds, stretching of bonds, or internal rotation around a single bond, is referred to as
molecular vibration (see Fig. 4.2). When electromagnetic radiation (EMR) waves
interact
stretching vibrations. Molecular vibrations are categorized into various types based
on the specific vibrations that manifest as bands in the spectra:
with matter, they can influence the characteristics of bending and/or
4.4.1 Stretching Vibration
Vibrations that occur along the axis of a chemical bond are referred to as stretching
vibrations. These vibrations take place in the radial direction and result in a change in
bond length, either by increasing or decreasing it. They also involve changes in the
Fig. 4.2 Schematic representation of modes of molecular vibrations
4.4 Modes of Molecular Vibrations 165
interatomic distance along the axis of the bond between the two atoms. Such
vibrations require higher energy, typically falling in the range of 4000–1250 cm
-1
and correspond to shorter wavelengths. Stretching vibrations can be further
categorized into two types:
4.4.1.1 Symmetrical Stretching Vibration
Symmetrical stretching vibration refers to a specific type of stretching vibration in
molecular spectroscopy. In this vibration, atoms on both sides of a bond move away
from or toward the central atom simultaneously, maintaining a symmetrical or
uniform motion. This results in an increase or decrease in the bond length, where
both bond lengths change by the same amount simultaneously (Fig. 4.3a). Symmet-
stretching vibrations are often observed in diatomic molecules or in molecules
rical
where the two atoms attached to the central atom are identical. These vibrations can
be detected and analyzed in the IR spectrum, providing valuable information about
the chemical bon ds and molecular structure.
,
Fig. 4.3 Vibrational modes of a simple molecule in IR spectroscopy: This figure illustrates the six
primary vibrational modes that are detected in IR spectroscopy for simple molecules, providing
insight into molecular motion and bonding interactions. In (a) the symmetrical stretching vibration
is depicted, where both hydrogen atoms move in unison, causing the bond lengths to increase and
decrease simultaneously. In (b) the asymmetrical stretching vibration shows one bond lengthening
while the other shortens, creating an unbalanced stretch. (c) highlights the scissoring vibration,
where both hydrogen atoms move toward and away from each other in the same plane, resulting in a
reduction of the bond angle. (d) represents the rocking vibration, in which the atoms swing back and
forth in the same direction without altering the bond angle. The wagging vibration is shown in (e)
with the hydrogen atoms moving out of the plane alternately in opposite directions. Finally, in (f)
the twisting vibration demonstrates the atoms rotating in opposite directions along the axis of the
carbon–hydrogen bond. Each of these distinct vibrational modes contributes to characteristic
absorption patterns in IR spectroscopy, making them fundamental to the identification of molecular
structures
166 4 Comprehensive Insights into Infrared Spectroscopy
4.4.1.2 Asymmetrical Stretching Vibration
In this vibration, the two atoms on either side of a bond move in opposite directions
relative to the central atom. As a result, one bond length incre ases while the other
decreases (Fig.
rical stretching vibrations are commonly observed in molecules where the two atoms
attached to the central atom are different. These vibrations also appear in the IR
spectrum, and their analysis provides essential information about the nature of the
chemical bonds and the molecular structure, especially when there is a lack of
symmetry in the molecule.
4.3b), creating an asymmetric or unsymmetrical motion. Asymmet-
4.4.2 Bending Vibrations
“Bending vibrations,” also known as “deformation vibrations” or “vibrations of
deformation,” are a type of molecular vibration observed in molecules. Unlike
stretching vibrations that involve changes in bond lengths, bending vibrations
involve changes in bond angles within a molecule. These vibrations can be classified
into two main categories.
4.4.2.1 In-Plane Bending Vibrations
In-plane bending vibrations involve a change in bond angle and they occur in the
same plane. These are of the following two types.
4.4.2.1.1 Scissoring Vibration
Scissoring vibration, often referred to as “scissoring mode,” is a specific type of
in-plane bending vibration observed in molecules. During scissoring vibration, the
atoms within a molecule move within the plane defined by the chemical bonds,
exhibiting a scissor-like motion (Fig.
bond angles without the molecule moving out of the plane. The scissoring vibration
is one of the fundamental molecular vibrational modes that can be detected in the IR
spectrum. It provides valuable information about the flexibility and structural
characteristics of a molecule, particularly regarding the nature of the chemical
bonds within the molecule.
.2 R
4.4.2.1
During rocki
dicular to the plane defined by the chemical bonds. This mot ion resembles the
rocking of atoms or groups of atoms as they pivot around the bond axis
(Fig.
and they are often observed in the IR spectrum. These vibrations provide valuable
information about the flexibility and structural properties of a molecule, especially
with respect to the nature of the chemical bonds within the molecule.
ocking Vibration
ng vibrations, the atoms within a molecule move in a direction perpen-
4.3d). Rocking vibrations are a fundamental part of molecular spectroscopy,
4.3c) This motion involves a change in the
4.5 Reference Guide for IR Spectra of Functional Groups 167
4.4.2.2 Out-Plane Bending Vibrations
Out-of-plane bending vibrations, sometimes referred to as “bending out of plane,”
are a category of molecular vibrations in which the atoms within a molecule move in
a direction perpendicular to the plane defined by the chemical bonds. These
vibrations involve changes in the bond angles and can be classified into two main
types.
4.4.2.2.1 Wagging Vibration
In a wagging vibration, the atoms within a molecule move back and forth in a
um-like motion, also out of the plane defined by the chemical bonds
pendul
(Fig. 4.3e).
4.4.2.2.2 Twisting Vibration
Twisting vibration happens when two atoms move to opposite sides of the plane.
One
atom moves up from the plane and second moves down from the plane
(Fig. 4.3f).
4.5 Reference Guide for IR Spectra of Functional Groups
Here is a detailed reference guide for the IR spectra of various functional groups,
including C-H, N-H, aldehydes, and esters:
C-H Stretching
In R spectroscopy, C-H stretching vibrations are important for identifying various
types
of hydrocarbons, including alkanes, alkenes, alkynes, and aromatic
compounds. Alkanes, with their sp
strong and sharp absorption peaks in the wavenumber range of 2850– 2960 cm
3
hybridized carbon–hydrogen bonds, display
-1
This distinct feature allows for easy identification of alkane functional groups. In
contrast, alkenes exh ibit medium-intensity peaks in the wavenumber range of
3020–3100 cm
vibrations of alkynes occur around 3300 cm
-1
, corresponding to the sp2 hybridized C-H bonds. The stretching
-1
, producing strong and sharp peaks
due to the presence of sp hybridized C-H bonds, which are more energetic. For
aromatic compounds, the C-H stretching vibrations are found between 3000 and
3100 cm
-1
, typically characterized by weak to medium sharp peaks. These
variations in C-H stretching allow chemists to differentiate between types of
hydrocarbons and gain insight into the molecular structure of a compound.
N-H Stretchi
ng (Amines and Amides)
In infrared (IR) spectroscopy, N-H stretching vibrations are critical for identifying
amines and amides. Primary amines exhibit two medium-intensity peaks in the
wavenumber range of 3300–3500 cm
-1
. These peaks correspond to symmetric
and asymmetric stretching of the N-H bonds, allowing for clear identification.
Secondary amines, on the other hand, display only one sharp, medium-inten sity
peak between 3300 and 3400 cm
-1
, as they lack one N-H bond. Amides also show
.
168 4 Comprehensive Insights into Infrared Spectroscopy
N-H stretching vibrations in the range of 3200–3500 cm-1, typically characterized
by a medium and broad peak. This broadness is primarily due to hydrogen bonding,
a key feature of amides that distinguishes them from amines in IR spectra. These
characteristic N-H stretching patterns provide valuable insights into the structural
properties of amines and amides.
Aldehydes (C-H and C=O Stretching)
In IR spectroscopy, aldehydes are characterized by distinct C-H and C=O stretching
tions. The C-H stretching in the aldehyde group appears as two weak bands,
vibra
known as the “aldehyde C-H stretches” or the Fermi doublet, within the
wavenumber range of 2700–2900 cm
are still significant for identifying aldehydes. Additionally, the C= O stretching
vibration of the aldehyde carbonyl group occurs between 1725 and 1740 cm
-1
. These bands are weaker in intensity but
-1
This vibration produces a strong, sharp peak, which is highly characteristic of
aldehydic carbonyl groups, making it a key marker in identifying aldehydes in IR
spectra.
Esters (C=O and C-O Stretching)
In IR spectroscopy, esters exhibit distinct C=O and C-O stretching vibrations. The
=O stretching of the ester carbonyl group typically occurs within the wavenumber
C
range of 1735–1750 cm
ester functionality. Additionally, the C-O stretching vibration in ester groups appears
between 1050 and 1300 cm
-1
, producing a strong, sharp peak that is a clear indicator of
-1
. These vibrations are characterized by medium to
strong, broad peaks, which are essential for identifying the ester group in a
compound’s IR spectrum. These peaks serve as reliable markers for detecting ester
compounds.
.
Characteristics for Differentiation of IR Spectra
In IR spectroscopy, specific characteristics allow for differentiation between functional groups based on their vibrational frequencies. This spectral reference provides
a baseline for interpreting the IR spectra of these functional groups in different
compounds.
-1
C-H: Alkane C-H stretches appear below 3000 cm
, while alkene and alkyne C-H
stretches occur at higher wavenumbers, aiding in distinguishing these
hydrocarbons.
N-H: N-H stretches also vary, primary amines show two peaks (due to symmetric
and
asymmetric stretching), secondary amines exhibit one peak, and amides
produce broader peaks, often attributed to hydrogen bonding.
Aldehyde C=O: Aldehyde carbonyl (C=O) stretches occur at slightly higher
numbers compared to esters but are accompanied by unique C-H stretches
wave
(Fermi doublet) that help identify aldehydes.
Esters: Esters,
on the other hand, exhibit a strong C=O stretch near 1740 cm
-1
and
distinct C-O stretches that set them apart from aldehydes.
4.7 Differentiating Between Amide I, Amide II, and Amide III Bands 169
4.6 Characteristic Peaks for Amines
Amines are characterized by their N-H stretching vibrations, which vary depending
on whether they are primary, secondary, or tertiary amines (Fig.
4.4):
• Primary amines: Primary amines (R-NH₂)
vibrations in the 3300–3500 cm
asymmetric stretching. Additionally, they display an N-H bending (scissoring)
vibration around 1600–1650 cm
-1
-1
. The N-H stretching absorption is less sensi-
exhibit
region, corresponding to symmetric and
tive to hydrogen bonding compared to O-H absorptions. In the gas phase and in
dilute CCl₄ solutions, free N-H absorption occurs in the 3400–3500 cm
For primary aliphatic amines, the asymmetric stretch occurs at a higher frequency,
with peaks separated by 80–100 cm
absorptions are 40–70 cm
-1
higher. A smaller absorption near 3200 cm
-1
, while in aromatic amines, these
result from the interaction between an overtone of the 1600 cm
symmetric N-H stretching band. C-N stretching absorptions occur at
1200–1350 cm
• Secondary amines: Secondary amines (R₂-NH) produce only one N-H stretching
peak, typically in the 3300–3400 cm
-1
for aromatic amines and 1000–1250 cm
-1
region, as there is only one N-H bond. The
bending vibration for secondary amines appears around 1550–1600 cm
Hydrogen bonding in concentrated liquids shifts these absorptions to lower
frequencies by about 100 cm
-1
. This absorption may appear at a slightly higher
frequency when the nitrogen atom is bonded to an aromatic ring. The C-N
stretching absorptions are found in similar ranges: 1200–1350 cm
secondary amines and 1000–1250 cm
-1
for aliphatic secondary amines, as seen
two distinct N-H stretching
-1
range.
-1
-1
band and the
-1
for aliphatic amines.
-1
for aromatic
may
-1
with primary amines.
• Tertiary amines: Tertiary
amines
(R₃-N) lack N-H stretching vibrations since they
have no N-H bonds, and therefore, N-H stretches are not detectable in their IR
spectra. They may exhibit C-N stretching vibrations, but these are weaker and
occur in the lower region of 1000–1250 cm
-1
for both aliphatic and aromatic
amines, similar to prima ry amines. Aside from the C-N stretch, the IR spectra of
tertiary amines will primarily display features characteristic of their alkyl or aryl
substituents.
.
4.7 Differentiating Between Amide I, Amide II, and Amide
III Bands
Amides, derivatives of carboxylic acids where the hydroxyl group is replaced by an
amine, exhibit distinct vibrational bands in IR spectroscopy, known as amide I,
amide II, and amide III (Fig.
and peptides, as they correspond to specific molecular vibrations within the amide
group, providing insights into protein structure.
• Amide I band:
The Amide I band is primarily associated with the C=O stretching
vibration of the amide group, occurring in the wavenumber range of
4.5). These bands are essential for identifying proteins