Basic Concepts of UV & IR Spectroscopy

BASIC CONCEPTS OF UV-VISIBLE AND
IR SPECTROSCOPY
DR. BASAVARAJAIAH S. M.
M. SC., PH.D.
COORDINATOR
PG DEPARTMENT OF CHEMISTRY
VIJAYA COLLEGE
BENGALURU-560004.
drsmbasu@gmail.com

Spectroscopy
Spectroscopy is a general term referring to the interactions
of various types of electromagnetic radiation with matter.
Exactly how the radiation interacts with matter is directly
dependent on the energy of the radiation.

THE ELECTROMAGNETIC SPECTRUM
Important: As the wavelength gets shorter, the energy of
the radiation increases.

Electromagnetic radiation displays the properties of both particles
and waves.
The particle component is called a photon.
The energy (E) component of a photon is proportional to the
frequency . Where h is Planck’s constant and n is the frequency in
Hertz (cycles per second).
E = hν
The term “photon” is implied to mean a small, massless particle
that contains a small wave-packet of EM radiation/light-we will use
this terminology in the course.

Spectroscopy
The higher energy ultraviolet and visible wavelengths affect the
energy levels of the outer electrons.
Radio waves are used in nuclear magnetic Resonance and affect
the spin of nuclei in a magnetic field.
Infrared radiation is absorbed by matter resulting in rotation
and/or vibration of molecules.

Ultraviolet radiation stimulates molecular vibrations and
electronic transitions.
Absorption spectroscopy from 160 nm to 780 nm.
Measurement absorption or transmittance.
Identification of inorganic and organic species.
UV-Vis Spectroscopy

UV/VIS SPECTROSCOPY
Ultraviolet (UV) (10 – 380 nanometers).
Visible (380-780 nanometers).
Below about 200 nm, air absorbs the UV light
and instruments must be operated under a vacuum

Electronic transitions
The absorption of UV or visible radiation corresponds to the
excitation of outer electrons. There are three types of electronic
transition which can be considered;
Transitions involving σ, π, and n electrons.
Transitions involving charge-transfer electrons.
Transitions involving d and f electrons.
CLASSIFICATION OF ELECTRONIC TRANSITIONS

Possible Electronic Transitions of π, σ, and n electrons are;

Bathochromic Shift or Red shift: A shift of an
absorption maximum towards longer wavelength (λ) or
lower energy (E).
Hypsochromic Shift or Blue Shift: A shift of an
absorption maximum towards shorter wavelength (λ)
or higher energy (E).
Hyperchromic Effect: An effect that results in increased
absorption intensity (ε).
Hypochromic Effect: An effect that results in decreased
absorption intensity (ε).
Terminology in UV:

Wavelengths Absorbed by Functional Groups
Again, demonstrates the moieties contributing to absorbance from
200-800 nm, because π electron functions and atoms having no
bonding valence shell electron pairs.

Influence of conjugation on UV absorption

UV spectra of
1,2,3,7,8,8a-Hexahydronaphthalene

UV Spectra of Benzene and Styrene

UV Spectra of Naphthalene, Anthracene and Tetracene

UV Spectra of Lycopene (Polyene)

Comparison of UV Spectra of Acetone and Methyl vinyl ketone

Which of the following alkenes would have the largest λmax?

Which molecule absorbs at the longest wavelength, 1,3-hexadiene
or 1,4-hexadiene?
Why the λmax for the diene (I) is observed at lower nm than (II).
(I) (II)

INTRODUCTION
Infra-red spectrum is an important record which gives sufficient
information about the structure of a compound.
IR electromagnetic radiation is just less energetic than visible light.
The infrared spectral regions are as follows.
The absorption of Infra-red radiations (quantized) causes the
various bands in a molecule to stretch and bend with respect to one
another.

The frequency of IR radiation is commonly expressed in wave
numbers.
Wavenumber (ῡ): The number of waves per centimeter, cm-1 (read
reciprocal centimeters).
Expressed in wavenumbers, the vibrational IR extends from 4000
cm-1 to 670 cm -1
.
Convert a wavenumber (ῡ) to a frequency (υ) by multiplying it by
the speed of light.
The main reason chemists prefer to use wave numbers as units is
that they are directly proportional to energy.
Wavenumber (ῡ) = 1/ Frequency (υ) = c/

USES OF THE INFRARED SPECTRUM
Fingerprint region:
The 1500-600 cm-1 range of an infrared spectrum, called the fingerprint
region because (like a human fingerprint) this region of the spectrum is
almost unique for any given compound.
Functional group region:
The functional group region runs from 4000 cm-1 to 1500 cm-1.

To locate a point in three-dimensional space requires three coordinates.
To locate a molecule containing N atoms in three dimensions, 3N
coordinates are required. The molecule is said to have 3N degrees of
freedom.
To describe the motion of such a molecule, translational, rotational, and
vibrational motions must be considered.
In a nonlinear molecule:
3 of these degrees are rotational and 3 are translational and the
remaining correspond to fundamental vibrations;
In a linear molecule: (Linear molecules cannot rotate about the bond axis)
2 degrees are rotational and 3 are translational.
The net number of fundamental vibrations:
THEORETICAL VIBRATIONAL NORMAL MODES

Ethane, C2H6 has eight atoms (N=8) and is a nonlinear molecule so
of the 3N=24 degrees of freedom, three are translational and three
are rotational. The remaining 18 degrees of freedom are internal
(vibrational). This is consistent with:
3N−6=3(8)−6=18
Carbon Dioxide, CO2 has three atoms (N=3) and is a linear
molecule so of the 3N=9 degrees of freedom, three are translational
and two are rotational. The remaining 4 degrees of freedom are
vibrational. This is consistent with:
3N−5=3(3)−5=4

THE MODES OF VIBRATION: STRETCHING AND BENDING
The simplest types, or modes, of vibrational motion in a molecule
that are infrared active-those, that give rise to absorptions-are the
stretching and bending modes.
Stretching vibration involves a continuous change in the inter-
atomic distance along the axis of the bond between two atoms. These
are two types; Symmetric and Asymmetric Stretching.
Bending vibrations are characterized by a change in the angle
between two bonds and are of four types: Scissoring, Rocking,
Wagging and Twisting.

Stretching frequencies are higher than corresponding bending
frequencies.
Symmetric and Asymmetric Stretching Vibrations
Symmetrical stretching: The atoms of a molecule either move away
or towards the central atom, but in the same direction.
Asymmetric stretching: One atom approach towards the central
while other departs from it.

BENDING VIBRATIONS
Scissoring is the movement of two atoms toward and away from
each other.
Rocking is like the motion of a pendulum on a clock, but an atom is
the pendulum and there are two instead of one.
Wagging is like the motion in which you make a "V" sign with your
fingers and bend them back and forth from your wrist.
Twisting is a motion as if the atoms were walking on a treadmill.

VIBRATIONAL MODES OF H2O (3 ATOMS –NON LINEAR)
 Vibrational modes (degrees of freedom) = 3 x 3 - 6= 3
 These normal modes of vibration:
 are a symmetric stretch, and asymmetric stretch, and a scissoring
 (bending) mode.

 Fundamental Vibrational modes = 3 x 3-5 = 4.
FUNDAMENTAL VIBRATIONAL MODES OF CO2 (3 ATOMS –LINEAR)

Let us now consider how bond strength and the masses of the
bonded atoms affect the infrared absorption frequency.
The natural frequency of vibration of a bond is given by the
equation (Hooke’s law).
VIBRATIONAL FREQUENCY

A new expression is obtained by inserting the actual values of π and
c:
Examples:
Note: Vibrational frequency is directly proportional to force constant
(K) (Bond strength) and inversely proportional to reduced mass (μ).

In general, triple bonds are stronger than double or single bonds
between the same two atoms and have higher frequencies of vibration
(Higher wavenumbers):
The C-H stretch occurs at about 3000 cm-1. As the atom bonded to
carbon increases in mass, the reduced mass (μ) increases, and the
frequency of vibration decreases (wavenumbers get smaller):

Bending motions occur at lower energy (lower frequency) than the
typical stretching motions because of the lower value for the bending
force constant K.
Hybridization affects the force constant K, also. Bonds are stronger in
the order sp>sp2>sp3, and the observed frequencies of C-H vibration
illustrate this nicely.

Factors affecting group frequencies
The value of vibrational frequency of a bond calculated by Hooke’s
Law is not always equal to their observed value.
The force constant is changed with the electronic and steric effects
caused by other groups present in the surroundings.
Following are some important factors affecting the vibrational
frequency of a bond.
Effect of Bond Order
Bond order affects the position of absorption bands. Higher the bond
order larger is the band frequency.
A C-C triple bond is stronger than a C=C bond, so a C-C triple bond
has higher stretching frequency than does a C=C bond.

Similarly, a C=O bond stretches at a higher frequency than does a C-O
bond and a C-N triple bond stretches at a higher frequency than does a
C=N bond which in turn stretches at a higher frequency than does a C-N
bond.

Electronic Effects:
Changes in the absorption frequencies for a particular group take place when the
substituent's in the neighbourhood of that particular group are changed.
The frequency shifts are due to the electronic effects which include Inductive
effect, Mesomeric effect, Field effects etc.
 Under the influence of these effects, the force constant or the bond strength
changes and its absorption frequency shifts from the normal value.
The introduction of alkyl group causes +I effect which results in the lengthening
or the weakening of the bond and hence the force constant is lowered and
wavenumber of absorption decreases.
Wavenumber of νC=O Formaldehyde (HCHO) 1750 cm-1
Acetaldehyde (CH3CHO) 1745 cm-1
Acetone (CH3COCH3) 1715 cm-1
Note: Aldehydes absorb at higher wavenumber than ketones

The introduction of an electronegative atom or group causes –I effect
which results in the bond order to increase.
Thus, the force constant increases and hence the wavenumber of
absorption rises.
Wavenumber of νC=O Acetone (CH3COCH3) 1715 cm-1
Chloroacetone (ClCH2COCH3) 1725 cm-1
Dichloroacetone (Cl2CHCOCH3) 1740 cm-1
Conjugation lowers the absorption frequency of C=O stretching whether
the conjugation due to α, β-unsaturation or due to an aromatic ring.
O
O
Methyl vinyl ketone Acetophenone
νC=O 1706 cm-1 1693 cm-1
Note: -I effect is dominated by mesomeric effect.

The electron pair on nitrogen atom in amide is more labile and participates more in
conjugation, hence the amide absorbs less frequency than the esters.
The lone pair of electrons participates more in conjugation in compound I as
compared to that compound III. Thus, in compound I, ν(C=O) absorption occurs at
lower wave number compared to that in compound III. In compounds II and IV,
inductive effect dominates over mesomeric effect and hence absorption takes place
at comparatively higher frequencies.

Hydrogen Bonding
The presence of hydrogen bonding changes the position and shape of
an infrared absorption band.
Frequencies of both stretching as well as bending vibrations are
changed because of hydrogen bonding.
The X-H stretching bands move to lower frequency usually with
increased intensity and band widening. The X-H bending vibration
usually shifts to higher frequencies.
Stronger the hydrogen bonding, greater is the absorption shift towards
lower wavenumber from the normal values.
The two types of hydrogen bonding (intramolecular and
intermolecular) can be differentiated by the use of infrared
spectroscopy.

The extent of inter-molecular hydrogen bonding depends upon the concentration of
the solution and hence the position and the shape of an absorption band also depend
on the concentration of the solution.
The more concentrated the solution, the more likely it is for the OH-containing
molecules to form intermolecular hydrogen bonds. It is easier to stretch an O-H bond
if it is hydrogen bonded, because the hydrogen is attracted to the oxygen of
neighbouring molecule.
Therefore, the O-H stretching of a concentrated (hydrogen bonded) solution of an
alcohol occurs at about 3550 cm-1, whereas the O-H stretching band of a dilute
solution (with little or no hydrogen bonding) appears at 3650 cm-1 .
Additionally, hydrogen-bonded OH groups also have broader absorption bands
whereas the absorption bands of non-hydrogen–bonded OH groups are sharper.

Field effect:
In ortho substitution, inductive effect, mesomeric effect along with steric
effect is considered. In ortho substituted compounds, the lone pairs of
electrons on two atoms influence each other through space interactions and
change the vibrational frequencies of both the groups. This effect is called
field effect.
The non-bonding electrons present on oxygen atom and halogen cause electrostatic
repulsions. This causes a change in the C=O hybridization and which in turn makes it
to go out of plane of the double bond.
Thus, the conjugation is diminished and absorption occurs at a higher wavenumber.
Thus, for such ortho substituted compounds, cis absorbs (field effect) at a higher
frequency as compared to the trans isomer.

Bond angles
Smaller ring requires the use of more p-character to make the
internal C-C bonds for the requisite small angles.
This gives more s character to the C=O sigma bond which causes the
strengthening and stiffening of the exocyclic double bond. The force
constant K is then increased and the absorption frequency increases.

Complementarity of IR and Raman spectroscopy
For the infra-red spectrum to occur, the molecule must show a
change in the dipole moment. For the Raman spectra, there must be
a polarstability of the molecule.
As these two requirements are somewhat different, lines may be
formed in one of the spectra or in both. The symmetrical stretching
of the molecule which are usually missing in the infra-red appear
prominently in Raman spectra.
On the other hand, asymmetric vibrations show opposite behavior.
Thus, we say that vibrational modes which are inactive in Infra-red
are somewhat active in Raman spectra.

For carbon dioxide, the bending and antisymmetric modes are infrared active, while
the symmetric stretch mode is Raman active. This behaviour is typical of all
centrosymmetric molecules. Modes that are infrared active are Raman inactive and
vice versa.
This is the Rule of Mutual Exclusion, which states that no normal mode can be both
infrared and Raman active in a molecule that possesses a centre of symmetry.
Rule of Mutual Exclusion:

n-pentane
CH3CH2CH2CH2CH3
3000 cm-1
1470 &1375 cm-1
2850-2960 cm-1
sat’d C-H

cyclohexane
no 1375 cm-1
no –CH3

1-decene
C=C 1640-1680
unsat’
d
C-H
3020-
3080
cm-1
910-920 &
990-1000
RCH=CH2

ethylbenzene
690-710, 730-
770 mono-
1500 & 1600
Benzene ring
3000-3100
cm-1
Unsat’d
C-H

styrene
no sat’d C-
H
910-920 &
990-1000
RCH=CH2
mono
1640
C=C

1-butanol
CH3CH2CH2CH2-OH
C-O 1o
3200-3640 (b) O-H

methyl n-propyl ether
no O--H
C-O
ether

Applications of IR spectroscopy
Identification of organic compounds
Structure determination
Qualitative analysis of functional group
Distinction between two types of hydrogen bonding
Quantitative analysis
Study of chemical reaction
Study of Keto-Enol tautomerism
Study of complex molecules
Detection of impurity in a compound.

Spectroscopy Learning Websites
1. http://www.rsc.org/learn-
chemistry/collections/spectroscopy
2. http://www.rsc.org/learn-
chemistry/resource/res00001041/spectroscopy-videos.
3. http://www.spectroscopyonline.com
4. https://www.khanacademy.org/science/organic-
chemistry/spectroscopy
5. http://chem.sci.ubu.ac.th/e-learning

Basic Concepts of UV & IR Spectroscopy

Basic Concepts of UV & IR Spectroscopy

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Basic Concepts of UV & IR Spectroscopy