Download IR Spectroscopy

Infrared Spectroscopy
Chapter 16
Dr. Nizam M. El-Ashgar
Chemistry Department
Islamic University of Gaza
1
IR Spectroscopy
I.
Introduction
A. Spectroscopy is the study of the interaction of matter with the
electromagnetic spectrum
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 n is the frequency in Hertz (cycles per
second)
E = hn
4.
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
2
Electromagnetic Radiation
• Electromagnetic radiation: light and other forms of
radiant energy
• Wavelength (): the distance between consecutive
identical points on a wave
• Frequency (n): the number of full cycles of a wave
that pass a point in a second
• Hertz (Hz): the unit in which radiation frequency is
reported; s-1 (read “per second”).
• Molecular spectroscopy: the study of which frequencies of
electromagnetic radiation are absorbed or emitted by substances
and the correlation between these frequencies and specific types
of molecular structure
3
• Wavelength
 (wavelength)
Unit
meter (m)
millimeter (mm)
micrometer (m)
nanometer (n m)
Angstrom (Å )
Relation
to Meter
---1 mm = 10-3 m
1 m = 10-6 m
1 nm = 10-9 m
1 Å = 10-10 m
4
IR radiation does not have enough energy to induce
electronic transitions as seen with UV.
Absorption of IR is restricted to compounds with small
energy differences in the possible vibrational and
rotational states.
For a molecule to absorb IR, the radiation must interact
with the electric field caused by changing dipole moment
5
What is Infrared?
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.
6
IR in Everyday Life
Night Vision Goggles
7
• Infrared radiation lies between the visible and microwave
portions of the electromagnetic spectrum.
• Infrared waves have wavelengths longer than visible and
shorter than microwaves, and have frequencies which are
lower than visible and higher than microwaves.
•
The Infrared region is divided into: near, mid and farinfrared.
– 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.
8
• The primary source of infrared radiation is thermal
radiation. (heat)
• It is the radiation produced by the motion of atoms and
molecules in an object. The higher the temperature, the
more the atoms and molecules move and the more
infrared radiation they produce.
• Any object radiates in the infrared. Even an ice cube,
emits infrared.
• Useful range of IR is from about 2.5 m to 15, 25 or 50 m
• Infrared used to determine the major functional groups
present.
• Quantitative measurements possible but subject to large
amount of error.
• Atoms or groups of atoms in molecules are in continuous
motion with different modes of vibration relative to each
other.
• Absorption of radiation changes amplitude of vibration but
not frequency.
9
• The vibrational IR extends from 2.5 x 10-6 m to 2.5 x 10-5 m
(2.5 m -25 m ).
– the frequency of IR radiation is commonly expressed in
wavenumbers
– wavenumber: the number of waves per centimeter, cm-1 (read
reciprocal centimeters)
– expressed in wavenumbers, the vibrational IR extends from
4000 cm-1 to 400 cm -1
Wavenumber = 1/
-2
-1
n   10 m•cm
2.5 x 10-6 m
= 4000 cm -1
n =
10-2 m•cm
-1
2.5 x 10-5 m
= 400 cm -1
Wave Numbers can be converted to a frequency (n) by multiplying them
by the speed of light (c) in cm/sec
n (Hz) = n c = c /  (cm /sec /cm = 1/sec)
Recall: E = h c / 
Thus, wavenumbers are directly proportional to energy
10
IR Spectroscopy
I.
Introduction
5. Because the speed of light, c, is constant, the frequency, n, (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:
n=
c
___

 E = hn =
hc
___

c = 3 x 1010 cm/s
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 (though opposite in how much) 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
11
IR Spectroscopy
8.
The entire electromagnetic spectrum is used by chemists:
Frequency, n in Hz
~1019
~1017
~1015
~1013
~1010
~105
0.01 cm
100 m
~10-4
~10-6
Wavelength, 
~.0001 nm
~0.01 nm
10 nm
1000 nm
Energy (kcal/mol)
> 300
g-rays
nuclear
excitation
(PET)
X-rays
300-30
300-30
UV
IR
Microwave
core electronic molecular
electron excitation vibration
excitation
(p to p*)
(X-ray
cryst.)
Visible
molecular
rotation
Radio
Nuclear
Magnetic
Resonance
NMR (MRI)
12
ABSORPTION OF IR RADIATION BY MOLECULES
• Molecules with covalent bonds may absorb IR radiation.
• This absorption is quantized, so only certain frequencies of
IR radiation are absorbed.
• When radiation, (i.e., energy) is absorbed, the molecule
moves to a higher energy state.
• The energy associated with IR radiation is sufficient to cause
molecules to rotate (if possible) and to vibrate.
• If the IR wavelengths are longer than 100 m, absorption will
cause excitation to higher rotational states in the gas phase.
• If the wavelengths absorbed are between 1 and 100 m, the
molecule will be excited to higher vibrational states.
• Because the energy required to cause a change in rotational
level is small compared to the energy required to cause a
vibrational level change, each vibrational change has multiple
rotational changes associated with it.
13
Gas phase IR spectra:
Consist of a series of discrete lines (a narrow line spectrum) because of
Free rotation)
Condensed phases.
The IR absorption spectrum for a liquid or solid is composed of broad
vibrational absorption bands.
Molecules absorb radiation when a bond in the molecule vibrates at the
same frequency as the incident radiant energy.
After absorbing radiation, the molecules have more energy and vibrate
at increased amplitude.
The frequency absorbed depends on:
• The masses of the atoms in the bond.
• The geometry of the molecule.
• The strength of the bond,
• and several other factors.
Not all molecules can absorb IR radiation.
The molecule must have a change in dipole moment during vibration
in order to absorb IR radiation.
14
The types of vibrations available to a molecule
are determined by the:
• Number of atoms
• Types of Atoms
• Type of bonding between the atoms
As a result, IR absorption spectroscopy is a
powerful tool in characterizing pure organic
and inorganic compounds.
15
Requirements for the absorption of IR radiation by
molecules:
1. The natural frequency of vibration of the molecule
must equal the frequency of the incident radiation.
2. The frequency of the radiation must satisfy E = hn,
where E is the energy difference between the
vibrational states involved.
3. The vibration must cause a change in the dipole
moment of the molecule.
4. The amount of radiation absorbed is proportional to
the square of the rate of change of the dipole during
the vibration.
5. The energy difference between the vibrational energy
levels is modified by coupling to rotational energy
levels and coupling between vibrations.
16
– Atoms joined by covalent bonds undergo continual vibrations
relative to each other
– The energies associated with these vibrations are quantized;
within a molecule, only specific vibrational energy levels are
allowed
– The energies associated with transitions between vibrational
energy levels for most covalent bonds are from 2 to 10 kcal/mol
(8.4 to 42 kJ/mol)
– These energies correspond to frequencies in the infrared region
between 4000 to 200 cm-1
17
Bond Dipole Moments
• In case of polar molecules to differences in electronegativity.
• Dipole moment depend on the amount of charge and distance of separation.
• Unit in debyes,
•
 = 4.8 x  (electron charge) x d(angstroms)
18
Molecular Dipole Moments
• Depend on bond polarity and bond angles.
• Vector sum of the bond dipole moments.
• Lone pairs of electrons contribute to the dipole
moment.
19
Dipole moment change
• Molecule must have change in dipole moment due to vibration or
rotation to absorb IR radiation.
• Absorption causes increase in vibration amplitude/rotation
frequency
• Molecules with permanent dipole moments (µ) are IR active
20
21
Electric field interaction: e.g. HCl
The alternating electric field interacts with the changing
dipole moment.
If the electric vector polarity is positive, it will repel the
partial positive charge on the hydrogen atom because like
charges repel.
Consequently, the hydrogen atom will move away from the
electric vector and the H-Cl bond will shorten.
Once the polarity of the electric vector changes to being
negative, it will attract the hydrogen atom because
opposite charges attract.
Therefore, the H-Cl bond will get shorter and longer and
shorter and longer.
During this process, the molecule vibrates at the same
frequency as the electric vector, and the energy of the
light beam is transferred to the molecule.
22
Hence an infrared absorbance takes place.
The IR Spectroscopic Process
•
The quantum mechanical energy levels observed in IR spectroscopy are
those of molecular vibration
•
We perceive this vibration as heat
•
When we say a covalent bond between two atoms is of a certain
length, we are citing an average because the bond behaves as if it were
a vibrating spring connecting the two atoms
•
For a simple diatomic molecule, this model is easy to visualize:
23
The IR Spectroscopic Process
◦ As a covalent bond oscillates – due to the oscillation of the
dipole of the molecule – a varying electromagnetic field is
produced
◦ The greater the dipole moment change through the
vibration, the more intense the EM field that is generated
24
Infrared Spectroscopy
The IR Spectroscopic Process
8.
9.
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
The coupled wave now vibrates with twice the amplitude
IR beam from spectrometer
“coupled” wave
EM oscillating wave
from bond vibration
25
Types of Molecular Vibrations (called modes of
vibration).
• Stretching
– change in bond length
– Symmetric / asymmetric
• bending
– change in bond angle
– symmetric scissoring
– asymmetric wagging
– rocking
– twisting/torsion
26
The IR Spectroscopic Process
5. There are two types of bond vibration:
•
Stretch – Vibration or oscillation along the line of the bond
H
H
C
C
H
H
asymmetric
symmetric
•
Bend – Vibration or oscillation not along the line of the bond
H
H
H
C
C
C
H
H
scissor
rock
in plane
H
H
twist
C
H
wag
out of plane
27
Molecular vibration of CO2
The asymmetric stretching frequency occurs at 2350 cm-1 and the bending
vibration occurs at 666 cm-1.
28
iii.) Types of Molecular
Vibrations
Bond Stretching
symmetric
asymmetric
29
Bond Bending
In-plane rocking
In-plane scissoring
Out-of-plane wagging
Out-of-plane twisting
30
In some texts: The + sign in the circle indicates movement above the plane
of the page toward the reader, while the  sign in the circle indicates
movement below the plane of the page away from the reader.
Bends are also called deformations and the term antisymmetric is used in
place of asymmetric in various texts.
31
Theoretical Vibrational Normal modes
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:
32
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.
33
34
Fundamental Vibrational modes of CO2 (3 atoms –Linear)
• Fundamental Vibrational modes (degrees of freedom) =
• 3x3–5=4
• These normal modes of vibration:
• The asymmetrical stretch of CO2 gives a strong band in the IR at 2350
cm –1 (may noticed in samples due to presence of CO2in the
atmosphere).
• The two scissoring or bending vibrations are equivalent and therefore,
have the same frequency and are said to be degenerate , appearing in an
IR spectrum at 666 cm-1.
• The difference in behavior of:
• Linear Triatomic molecules:Two absorption
peaks.
• Non linear triatomic molecules:Three
absorption peaks.
• So illustrates how infrared absorption
spectroscopy can sometimes used to
deduce molecular shapes.
36
37
Fewer and more experimental peaks than calculated
• Fewer peaks
– Symmetry of the molecule (inactive)
– degenracy
• 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
38
Fundamental Peaks and Overtones
• Fundamental transition:
• The excitation from the ground state V0 to the first excited state V1 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 V2
, V3 , and so on.
• These absorptions occur at approximately integral multiples of the
frequency of the fundamental absorption.
• If the fundamental absorption occurs at frequency n, the overtones will
appear at about 2n, 3n, and so on.
• Overtones are weak bands and may not be observed under real
experimental conditions.
39
• Coupling modes:
• Vibrating atoms may interact with each other. Two
vibrational frequencies may couple to produce a new
frequency n3 = n1 + n2 .
• The band at n3 is called a combination band.
• If two frequencies couple such that n3 = n1 - n2 , the
band is called a difference band.
• Not all possible combinations and differences occur.
40
Rules for Vibrational coupling
• Coupling of different vibrations shifts frequencies
• Energy of a vibration is influenced by coupling
• Coupling likely when
– common atom in stretching modes
– common bond in bending modes
– common bond in bending+stretching modes
– similar vibrational frequencies
• Coupling not likely when
– atoms separated by two or more bonds
– symmetry inappropriate
We will explain rules of coupling after interpretation IR vibrational
theory
41
Classical vibrational motion
The stretching frequency of a bond can be approximated
by Hooke’s Law.
Mechanical model
-Two masses
-A spring
- Simple harmonic motion for single mass
Where,
y: displacement of mass from
its equilibrium position.
K: is force constant depends
on stiffness of spring
-ve sign means that F is a
restoring force (that F is
opposite to direction of
displacemet)
42
Energy of the Hooke’s law (PE of a harmonic Oscillator)
At rest eq. position (PE) E = zero.
On stretching or compressing of the spring PE = work required to displace
the mass.
For example moving mass from y to y+dy
Work required and so change in PE (dE) = F times distance dy
Combined with previous eq.
Integration between y=0 and y
Gives PE curve for simple harmonic oscilation (A parabola)
43
Harmonic Ocsillator Potential
Potential high
When the spring is
compressed or stretched.
Parabola function
* E=(1/2)ky2
* Minimum at equilibrium
position
* Maximum at max amplitude
A.
44
Classical vibrational frequency
•
•
•
•
•
Motion of mass as a function of time t:
F = ma = m(d2y/dt2) (Newton’s Law)
F=-ky
m(d2y/dt2) = -Ky-----------------------1
Solution of differential equation (must be a periodic function; its
2nd derivative =to the original function times (-k/m)
• A suitable cosine relationship meets this requirement
Y = A cos (2pnmt)-------------------------------2
D2y/dt2 = -4p2n2 A cos (2pnmt)-------------------3
where: nm is the vibrational frequency and A the maximum amplitude of the
motion
Substitution of eq 2 and 3 into eq 1 gives
- Ky = A cos (2pnt) K =-4p2n2m m A cos (2pnmt)
A cos (2pnt) =(4p2n2m m/K) A cos (2pnmt)
1
nm 
2p
K
m
45
Where nm is the natural frequency depends on:
-Force constant of spring
-Mass of attached body
-Independent on the energy imparted to the system. Change in energy
gives a change in amplitude A of the vibration
Modification
E= hnclass
In the classical harmonic oscillator, y is the displacement, the energy or
frequency is dependent on how far one stretches or compresses the spring,
which can be any value. If this simple model were true, a molecule could absorb
energy of any wavelength.
So it dose not completely describe the behavior of quantized nature of
molecular vibrational E .
46
Quantum treatment of vibrations
n: is the vibrational QN takes +ve
integer including zero
E  hn clas 
h
2p
k

47
E  hn clas 
•
•
•
•
•
•
Where n is the wave number of
absorption peak in cm-, K is force
constant for the bond in N/m, c velocity
of light in cm/s and the reduced mass 
in Kg.
K values :
For single bonds the range:3x102 to
8x102 with an average of 5x102.
For double bond : 1x103.
For triple bond : 1.5x103.
n n m 
cn 
1
2p

Used to estimate n of fundamental
absorptions of first excitations.
n 
1
k
2p c

h
2p
E  Eradiation  hn 
n 
c

1
2p
k

h
2p
k

k

 cn
k
 5.3 x10 12
k

Q: calculate the aprox. Wave number and wave length of the fundamental
absorption peak due to stretching vibration of a carbonyl group.
48
GROUP FREQUENCIES
Estimation of frequencies of vibration for various groups possible
when force constant known.
E.g.1 force constant of C=O bond is 1.23x103 N/m, determine
vibrational frequency of this C=O group.
12.0gC
1kgC
1molC

x1atomCx
= 1.99x 10 26 kgC
molC
6.02x 10 23 atomsC
1x 103 gC
mO = 2.66x1026 kgO
26
26
mc 
•
• reduced mass of
• Substituting:
• Units:

1.99x 10
 2.67x 10
(1.99 + 2.67)x 10 26
n = 5.3x 10-12 s / cm
1.23x 103 N / m
1.13x 10
-26 kg
kg = 1.13x 10 26 kg
= 1742 cm-1
N
kg  m  s 2
1
1
 s 2
kg  m
kg  m
49
50