Spectroscopy
 Electromagnetic
radiation have both
particle
properties
and
wave
characteristics..
characteristics
Spectroscopy (Continued/…)
 "Quantum
theory": Assumed that an
theory":
electromagnetic radiation consists of
particles called photons
photons..
E = h
Where E = energy of photon.
h = Blank's constant.
 = frequency
IR Spectroscopy
Compound with covalent bond either 
organic or inorganic absorbs various
frequencies of electromagnetic radiation in
IR region
region..
IR Spectroscopy (Continued/…)
 At
the same time it lies at wavelengths
shorter than those of microwaves (longer
than 1 mm).
IR Spectroscopy (Continued/…)
 The
IR region extends from 0.8-200 .

is inversely proportional to the frequency
 where
 = c/
c/ where c = speed of light.
 Also,
energy is directly proportional to the
frequency.
IR Spectroscopy (Continued/…)
chemist used wavenumber (') as the
unit used to refer to radiation in vibrational
IR region rather than wavelength.
 Most
' = 1/ (cm)
IR Spectroscopy (Continued/…)
 The
absorption of IR radiation causes the
excitation of the molecules from the
ground state to the higher vibrational
state.
Modes of Vibrations
1- Stretching vibrations:
The distance between two atoms increases or
decreases but the atoms remain in the same axis.
It is subclasses into:
a)Asymmetric: the stretching in opposite direct.
It occurs at higher frequencies than the
symmetric one.
b)Symmetric: the stretching in the same
direction.
Modes of Vibrations (Continued/…)
2- Bending vibrations:
It produces a change in the bond angle, the
position of the atoms changes relative to the
original bond axis.
It is also subclassed into:
a) In the plane:
i) Scissoring
ii) Rocking
Modes of Vibrations (Continued/…)
b)
Out the plane:
i)
Wagging (2 motions above or below
the plane).
ii) Twisting (one motion above and one
below the plane).
IR Spectrometer
It provides spectra in the common range of
4000 to 400 cm-1.
IR Spectrometer (Continued/…)
IR Chart:
• It is a plot of frequency or  versus % the light
transmitted (not absorbed). As the detector
records the ratio of the intensities of the 2
beams.
So % transmittance: Is / Ir  100
Where Is = Intensity of sample beam.
Ir = Intensity of reference beam.
Stretching Frequency of Bonds
In any group of 2 or more atoms, at
least 2 of which are identical, there will be 2
modes of stretching and/or bending. The
symmetric and asymmetric modes e.g.:
Stretching Frequency of Bonds (Continued/…)
Symmetric stretch
Asymmetric stretch
H
H
Methyl
C
C
H
H
H
2962
2872
Anhydride
H
O
O
O
O
C
C
C
C
O
O
1760
1800
H
Amino
N
H
N
H
H
3300
3400
O
O
Nitro
N
N
O
1350
O
1550
Stretching Frequency of Bonds (Continued/…)
Hooke's Law
' = ½c√k/
Where ' = Frequency in cm-1.
c = velocity of light.
k = force constant of bonds in dynes/cm.
 = reduced mass of the system
m1m2/m1 + m2
So ' = ½ck(m1 + m2)/m1m2
Stretching Frequency of Bonds (Continued/…)
• (K) for triple bonds are 3 times those of single
bonds. Also double bond (K) constant are twice
those of single bon
Stretching Frequency of Bonds (Continued/…)
• A new Hooke's law expression is obtained by
inserting the actual values of  and C to be:
' = 4.12√k/
Where  = reduced mass of the system
m1m2/m1+m2, m1, m2 are atomic weight.
Application of IR Spectra
 The
function of IR spectroscopy is to
identify the functional groups in the
molecules..
molecules
Use of IR Spectra
 It
gives structural information about a
molecule.. The absorptions of each type of
molecule
bond (N
(N--H, C-H, O-H, C-X, X=O, C-O, CC, C=C … etc
etc..) are regularly found only in
certain small portions of the vibrational IR
region..
region
Group Structure Analysis
Interpretation of spectra is not always
so simple
simple.. The absorption band can be
shifted according to a number of factors
factors::
1.
Inductive Effect
Effect::
Electron donating groups attached to
C=O decrease the C=O stretching
frequency, while electron withdrawing
groups increase the C=O frequency
frequency..
Group Structure Analysis (Continued/…)
2.
Mesomeric Effect
Effect::
This effect (conjugation) decrease the
stretching frequency of a group
group..
3.
Hydrogen Bonding
Bonding::
A hydrogen bond formation between
the proton of R-OH or R2OH and the oxygen
atom of a C=O group either intermolecular of
intramolecular decreases the frequency of
N-H or O-H and the C=O groups
groups..
Group Structure Analysis (Continued/…)
4.
Ring Strain
Strain::
In cyclic carbonyl compounds more
strained rings (bond angle is less than 120
120))
give higher wave number of carbonyl
carbonyl..