INTRODUCTION TO SPECTROCOPY

ATOMIC AND MOLECULAR
SPECTROSCOPY
Presented by:
Tehseen Khalid
FATIMA JINNAH WOMEN
UNIVERSITY RAWALPINDI

- General Introduction
- What is spectroscopy and spectrophotomer?
- Electromagnetic Radiation and its parameters
- UV/VISIBLE Spectroscopy and its principle
- Beer Lambert Law and Conjugation
- IR Spectroscopy
- Applications of IR/UV/VISIBLE In Astronomy
Outline

Spectroscopy?
• The study of interaction of electromagnetic radiation with
matter.
• The energy of the radiation absorbed or emitted by the
system and the intensity of the spectral lines.
• Magnetic fields
• Using a spectrophotometer

• An instrument that measures the amount of photons
absorbed after passing through the sample solution
Spectrophotometer?

• Js
•
Spectrophotometric cells:

Electromagnetic Radiation:
“Recognized as Transmission of energy”
- Wave like property and Photons
- Each ray of ER consists of 2 components:
1: Electric
2: Magnetic
- Both these are perpendicular to each other and in phase.

- Frequency:
“The Number of waves passing through a fixed point”
UNITS: Hertz or cycle per second (1HZ=1cps)
- Wavelength: λ (μm)
“The distance between two adjacent crests (or) troughs”
UNITS: Angstrom, Nanometer, micrometer, meter.
Different Parameters of ER:

Different Parameters of ER:
- Wavenumber: υ (cm-1)
“Number of waves per unit distance”
UNIT: Reciprocal centimeter.
- Energy:
“Energy possessed by each photon”
UNIT: Commonly used “JOULE”
Energy of photon= 1eV=1.6022x10^-19J

Electromagnetic Spectrum:
- Covers a wide range of wavelengths and the radiations of
different wavelengths.
- For example: radiation of wavelength 400nm is violet in
color and 800nm is red
- The whole spectrum is divided into different regions by four
parameters mentioned earlier.

Cont..
1.UV-Visible radiations---excitation of electrons----uv-visiblespectrum
2.IR-radiations—vibration changes in electrons---IR spectrum
3.Microwave radiations---spin resonance----E.S.R spectrum
4.Radio frequency---spin rotational changes---N.M.R spectrum

• Same basic principle and same type of molecular excitation
• UV 10-400nm
• VISIBLE 400-800nm
• Region below 200nm cannot be studied by conventional
spectrophotometer
• Oxygen absorption in this region
• Flushed out by Nitrogen
• Vacuum region
UltraVoilet/VISIBLE Spectroscopy:

Principle Behind UV/VISIBLE Spectroscopy
- Excitation of electrons
- HOMO ------------- LUMO
- “Frank Condon Principle”
- One electron excites , others unaffected
- Too rapid electronic transition
- Even vibrating atoms don’t change their position

• qa

• ff

Electronic Transitions
σ  σ* or n σ* Usually weak absorptions
• Compound Methanol λmax 177nm
π  π* transition takes place around 170 -190 nm.
• Compound 1,3-Butadiene λmax 217

n  π* around 280 nm
Electronic Transitions

Probability of transition
•The intensity of absorption depends
largely on probability of transition
•Zero to one
•This probability in turn depends on
relative symmetries of orbital in the
ground state and excited state

THE INTENSITY OF ABSORPTION IS EXPRESSED AS MOLAR
ABSORPTIVITY (ε) i.e. 10^5
- Value > 10^4 = High intensity absorption and ALLOWED
transition
- Value <10^3 = Low intensity absorption and FORBIDDEN
transition
Probability of transition

• The position and intensity of an absorption band may shift
if the spectrum was recorded in different solvents.
• Conjugated dienes and aromatic hydrocarbons experience
very less “solvent effect”.
DIFFERENT SOLVENTS HAVE DIFFERENT
ABOSRBING EFFECTS:

• Absorption characteristics of 4-Methyl-3Penten-2-one
Solvent π  π* n  π*
Hexane 230nm 329nm
Water 243nm 305nm
DIFFERENT SOLVENTS HAVE DIFFERENT
ABOSRBING EFFECTS:

Terminologies
1: Chromophore:
• A covalently unsaturated group responsible for electronic
absorption (e.g., C=C, C=O, esters, amides, -NO2 etc.)
2: Auxochrome:
• A saturated group with non-bonded electrons which, when
attached to a chromophore, alters both the wavelength and
the intensity of the absorption (e.g., -OH, -NH2, -NR2 -SH etc.)

3: Bathochromic Shift:
The shift of absorption to a longer wavelength (also known
as “red shift”).
4: Hypsochromic Shift:
The shift of absorption to a shorter wavelength (also known
as “blue shift”).
5: Hyperchromic Effect: An increase in absorption intensity.
6: Hypochromic Effect: A decrease in absorption intensity.
Terminologies

THE CONJUGATED SYSTEM; CONJUGATED DIENE
CORRELATIONS
• Conjugation?
• Tomatoes carrots etc.
• Presence, nature and extent of conjugation is determined
by UV/VIS spectroscopy
• Longer conjugation leads to higher intensity

CONJUGATED DIENE CORRELATIONS
• Homoannular Diene:- Cyclic diene having conjugated double
bonds in same ring
• Heteroannular Diene:- Cyclic diene having conjugated double
bonds in different rings
• Endocyclic double bond:- Double bond present in a ring.
• Exocyclic double bond: - Double bond in which one of the double
bonded atoms is a part of a ring system.

• Woodward (1941) predicted λmax values only for the lowest energy
transition, 1959 Fieser modifications.
• Calculates the position of max wavelength By relating the position
and degree of substitution of chromophore
WOODWARD-FIESER RULE:

-- BASE VALUES FOR :
WOODWARD-FIESER RULE FOR
CONJUGATED DIENE ABSORPTION :
Acyclic diene 217
Heteroannular diene 214
Homoannular diene 253

• Examples:
WOODWARD-FIESER RULE; Examples

WOODWARD-FIESER RULE; Examples

• BASE VALUES FOR:
WOODWARD-FIESER RULE FOR THE ABSORPTION OF
α,β-UNSATURATED CARBONYL COMPOUNDS
Acyclic and six membered
cyclic ketones
215
Five membered cyclic
ketones
202
Aldehydes 207
Carboxylic Acid and Esters 195

• d
WOODWARD-FIESER RULE FOR THE ABSORPTION OF
α,β-UNSATURATED CARBONYL COMPOUNDS

• Vibrational
• Frequency 4000 cm-1  400 cm-1
• Fundamental Absorption
• Overtones
• Combination bands
• Difference bands
• Fermi Resonance
• “BOND PROPERTIES ALONG ABSORPTION”
INFRARED SPECTROSCOPY

M1 Force constant, k M2
Ball and spring representation of 2
atom of molecule vibrating in the
direction of bond
Vibrational frequency2
Factors influencing absorption frequency2
Masses of attached atoms. As masses increase, wave number decreases.
Strength of chemical bond. As bond strength
increases, wave number increases.
Hybridization. Bonds are stronger in the order
sp > sp2 > sp3.
 Resonance. Conjugation lowers the energy
to vibrate bond.
2. B.K. SHARMA," Infrared spectroscopy” ,spectroscopy ,20th edition, Goel publications, Delhi, 2007. print.
40

1. Selection rules
2. Types of vibrations
3. Number of possible vibrational modes
4. Vibrational frequency
5. Factors influencing vibrational modes
Matching of
Frequency
Dipole
moment
Translational
motion
Rotational motion Vibrational motion
A. Phase and solvents
used
B. Coupled
interactions
C. Hydrogen bonding
D. Fermi resonance
E. Electronic effects
INFRARED SPECTROSCOPY;Conditions for absorption:

► 3600—3000cm-1
---OH, --NH2 , >NH, C-H.
► 3200—3000cm-1
C-H, Ar— C-H.
►3000—2500 cm-1
--C—H of methyl/methelene
asymmetric stre. --C—H, --COOH
►2300—2100 cm-1
Alkynes 2210---2100
Cyanides 2260—2200
Isocyanides 2280—2250
►1900—1650 cm-1
strong bands--- >c=o---1725—1760
anhydrides ----- 1850---1740
Imides ------ two broad band at 1700
Functional [11,13]
group region
11.Dudles H,Williams,Ian Flemming ,”infrared spectroscopy”, Spectroscopy Methods In Organic Chemistry, 5thedition,Tata mecGrawHill.Education.
Newyork, Singapore, Sydney, page no. 45-60. 2004 . Print.
13.Harold F.Walton,Jorge Reyes, "infrared spectroscopy", Modern Chemical Analysis And Instrumentation,IMBD, Mumbai, Reprint 2001page no 201-203.
Print.
43
General guidelines for IR [11,13]

► 1650--1000cm-1
confirms ---
esters, alcohol, ethers. Nitro
► 1000—800 cm-1
C— Cl, C-Br
► 800—710cm-1
meta substituted benzene
► 770—730cm-1
strong mono substituted benzene.
► 710—665cm-1
ortho, Para, benzene.
Finger print
region[11,13]
11.Dudles H,Williams,Ian Flemming ,”infrared spectroscopy”, Spectroscopy Methods In Organic Chemistry, 5thedition,Tata mecGrawHill.Education.
Newyork, Singapore, Sydney, page no. 45-60. 2004 . Print.
13.Harold F.Walton,Jorge Reyes, "infrared spectroscopy", Modern Chemical Analysis And Instrumentation,IMBD, Mumbai, Reprint 2001page no 201-203.
Print.
44
General guidelines for IR interpretation [11,13]

O—H
N—H
C—H
C—C
HO-C=O
C=_N
C=O C=N C=C C=S N=O S=O C—N C—O
benzene
%T
Graphical interpretation of functional groups in IR [2,10]
45
2. B.K. SHARMA," Infrared spectroscopy” ,spectroscopy ,20th edition, Goel publications, Delhi, 2007. print.
10. Jag Mohan ,”infrared spectroscopy”, Organic Spectroscopy, Principles And Applications, 2ndedition,Narosa,Newdelhi, Chennai 2005. Print.
OH, --NH2 , >NH, C-H
C-H, Ar— C-H
C—H, --COOH
esters, alcohol, ethers, Nitro groups

Alkanes
C–H stretch from 3000–2850 cm-1
C–H bend or scissoring from 1470-1450 cm-1
C–H rock, methyl from 1370-1350 cm-1
C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
Wave number cm-1
90
0
C-H stretch
2971 2963
4000 2000 1000 500
1470 728
1383
C-H rock
C-H
scissoring
Long chain
CH2 stretch
Octane spectrum
11.Dudles H,Williams,Ian Flemming ,”infrared spectroscopy”, Spectroscopy Methods In Organic Chemistry, 5thedition,Tata mecGrawHill.Education. Newyork,
Singapore, Sydney, page no. 45-60. 2004 . Print.
46

Alkenes :-
C=C stretch from 1680-1640 cm-1
=C–H stretch from 3100-3000 cm-1
=C–H bend from 1000-650 cm-1
90%transmittance
Wave number cm-1
1 4
5
2 3
6
7
1. 3083- =C-H stretch
2. 2966- C-H stretch
3. 2863 –C-H stretch
4. 1644- C=C str
5. 1455 C-H sis
6. 1378 C-H rock
7. 1004 =C-H bond
1- Octene spectrum
4000 2000 1000 500
47
General guidelines for IR interpretation[1011]

Alkynes :-
–C≡C– stretch from 2260-2100 cm-1
–C≡C–H: C–H stretch from 3330-3270 cm-1
–C≡C–H: C–H bend from 700-610 cm-1
90
0
C-H stretch
3324
2971
4000 2000 1000 500
1470
636
1383
C-H rock
C-H
scissoring
C-H scissoring
CC-
H
CC-
2126
2679
1- hexyne spectrum
% transmittance
Wavelength cm-1
48

Alkyl halides :-
C–H wag (-CH2X) from 1300-1150 cm-1
C–X stretches (general) from 850-515 cm-1
C– Cl stretch 850-550 cm-1
C–Br stretch 690-515 cm-1
90
0
C-H stretch
2976 2940
4000 2000 1000 500
1470 651
1291
C-H wag
C-H
scissoring
Long chain,
C-Br stretch
1- bromo propane spectrum
49

Aromatics:-
C–H stretch from 3100-3000 cm-1
overtones, weak, from 2000-1665 cm-1
C–C stretch (in-ring) from 1600-1585 cm-1
C–C stretch (in-ring) from 1500-1400 cm-1
C–H "loop" from 900-675 cm-1
C-H stretch aromatics
3068
% transmittance
90
0
C-H stretch alkyl
2925
1614
1505
C- H stretch In aromatic ring
Wavelength cm-1
1465
3032
3099
overtones
738
1035
1086
In-plane C-H bending
Aromatic C-H stretches are left to
3000, and aliphatic C-H stretches are
right to 3000
Spectrum of Toluene
50

Alcohol:-
O–H stretch, hydrogen bonded 3500-3200 cm-1
C–O stretch 1260-1050 cm-1 (s)
The spectrum of ethanol is shown below. Note the very broad, strong band of the
O–H stretch (3391) and the C–O stretches (1102, 1055).
O-H stretch
3391
Wave number cm-1
% transmittance
90
0
C-H stretch
2961
1102
1105
C-O stretch
Spectrum of Ethanol
51
General guidelines for IR interpretation[10,11]

ketones
C=O stretch:
aliphatic ketones 1715 cm-1
α, β-unsaturated ketones 1685-1666 cm-1
The spectrum of 2-butanone is shown below. This is a saturated ketone, and the C=O band appears at 1715.
Note the C–H stretches (around 2991) of alkyl groups.
C-H stretch
2991
1715 C=O stretch
Wave number cm-1
% transmittance
90
0
2-butanone spectrum
4000 3000 2000 1500 1000 500
52

Aldehydes:
H–C=O stretch 2830-2695 cm-1
C=O stretch:
aliphatic Aldehydes 1740-1720 cm-1
alpha, beta-unsaturated aldehydes 1710-1685 cm-1
The spectra of benzaldehyde and butyraldehyde are shown below. Note that the O=C stretch of the
alpha, beta-unsaturated compound -- benzaldehyde -- is at a lower wave number than that of the
saturated butyraldehyde.
C-H
Stretch alkyl
3073
1696 C=O stretch
Wave number cm-1
% transmittance
90
0
28272725
C-H
aldehyde
Benzaldehyde spectrum
4000 3000 2000 1500 1000 500
53

Carboxylic acids :-
O–H stretch from 3300-2500 cm--1
C=O stretch from 1760-1690 cm-1
C–O stretch from 1320-1210 cm-1
O–H bend from 1440-1395 and 950-910 cm-1
The spectrum of hexanoic acid is shown below. Note the broad peak due to O–H stretch
superimposed on the sharp band due to C–H stretch. Note the C=O stretch (1721), C–O stretch
(1296), O–H bends (1419, 948), and C–O stretch (1296
O-H stretch and
C-H stretch
2971
1721
C=O stretch
Wave number cm-1
% transmittance
90
0
1419
O-H
band
1296
C-O
stretch
948
O-H
54

Esters :-
C=O stretch
aliphatic from 1750-1735 cm-1
α, β-unsaturated from 1730-1715 cm-1
C–O stretch from 1300-1000 cm-1
The spectra of ethyl acetate and ethyl benzoate are shown below. Note that the C=O stretch of ethyl
acetate (1752) is at a higher wavelength than that of the α, β-unsaturated ester ethyl benzoate (1726).
Also note the C–O stretches in the region 1300-1000 cm-1
.
90
90
%transmittance
Wave number cm-1
4000 3000 2000 1000 500
1 2 3
1
2 3 4
Ethyl acetate
1. 2981- C-H stretch
2. 1752- C=O ester
stretch
3. 1250- C-O stretch
4. 1055- C-O stretch
4
Ethyl benzoate
1. 3078- C-H aromatic
stretch
2. 2966- C-H alkyl
stretch
3. 1726-C=O stretch
4. 1266, 1117- C-O
stretch
55
General guidelines for IR interpretation[10,11]

Amines :-
N–H stretch 3400-3250 cm-1
1° amine: two bands from 3400-3300 and 3330-3250 cm-1
2° amine: one band from 3350-3310 cm-1
3° amine: no bands in this region
N–H bend (primary amines only) from 1650-1580 cm-1
C–N stretch (aromatic amines) from 1335-1250 cm-1
C–N stretch (aliphatic amines) from 1250–1020 cm-1
N–H wag (primary and secondary amines only) from 910-665 cm-1
56
General guidelines for IR interpretation

Example for interpretation of IR for known
structure
HN
OH
C
O
CH3
Acetaminophen 14
(4-acetamido-Phenol)
A. N-H Amide----3360 cm -1 .
B. Phenolic—OH -- 3000 cm -1 --3500 cm -1
C. C—H Stretching---3000 cm-1 .
D. Aromatic overtone ----1840 cm-1 --1940 cm -1
E. >C=O Amide stretching -----1650 cm -1
F. Aromatic C=C stretching--- 1608 cm -1 .
G. N-H Amide bending ----1568 cm -1
H. Aromatic C=C stretching ----1510 cm -1 .
I. >C—H bending --------810 cm -1
A
B
C
D
E
F
G
H
I
9. Robert M.Silverstien Francis X.Webster ,”infrared spectroscopy”, spectroscopic identification of organic compounds, 6thedition, John Wiley, Chichester,
Singapore, Toronto, Brisbane page no. 3.5, 2005. Print.
14.David watson,”infrared spectroscopy”, pharmaceutical Analysis, A test book for pharmacy students & pharmaceutical chemists, 2nd edition, Elsevier
churchil,livingston. Edinburgh,london,newyork,oxford,sydney, and Toronto. Print
57

Tips for interpretation of IR for unknown structure
 Always place relines to negative information evidence i.e., absence of band at
1900 cm-1---1600 cm-1----absence of >C=O, >CHO
 Always starts from higher frequency end of the spectrum.
 Absence of band at 880 cm-1—650 cm-1 indicates absence of aromatic ring.
 For easy identification go for fingerprint and functional group region.
 Finger print region range is 1400 cm-1--900 cm-1. In this region if absorbance band is present the groups esters, alcohols,
ethers, nitro are Confirmed.
 Functional region range is 4000 cm-1---1400 cm-1.amines, alcohols, aromatic rings, carboxylic acids, alkynes, alkanes,
alkenes, anhydrides, imides, etc, may be confirmed.
 Stretching vibrations at 4000 cm-1----600 cm-1.
 Bending vibrations at 1500 cm-1-----500 cm-1.
58
14.David watson,”infrared spectroscopy”, pharmaceutical Analysis, A test book for pharmacy students & pharmaceutical chemists, 2nd edition, Elsevier
churchil,livingston. Edinburgh,london,newyork,oxford,sydney, and Toronto. Print

Spectroscopy can be carried out under
the following heads:
Atomic Spectroscopy which deals with the interaction of
electromagnetic radiation with atoms.
Molecular Spectroscopy which deals with the interaction of
electromagnetic radiation with molecules.

Atomic-absorption spectroscopy
• Atomic-absorption (AA) spectroscopy uses the absorption of
light to measure the concentration of metals in the sample

PRINCIPLE:
• The technique uses basically the principle that free atoms
(gas) generated in an atomizer can absorb radiation at
specific frequency.
• The atoms absorb ultraviolet or visible light and make
transitions to higher electronic energy levels.
• Concentration measurements are usually determined from
a working curve after calibrating the instrument with
standards of known concentration

Composition of Atomic Absorption
spectrometer
• Detector
• Monochromator
• Atomizer
• Nebulizer
• Hollow Cathode Lamp

NEBULIZER:
• Suck up liquid samples at controlled rate. It creates a fine
aerosol spray for introduction into flame.
• Mix the aerosol and fuel and oxidant thoroughly for
introduction into flame.

Atomizer
• Elements to be analyzed needs to be in atomic sate.
• Atomization is separation of particles into individual
molecules and breaking molecules into atoms.
• This is done by exposing it to high temperatures in a flame

Calibration Curve
• A calibration curve is used to determine the unknown
concentration of an element in a solution. The instrument is
calibrated using several solutions of known concentrations.
• The absorbance of each known solution is measured and
then a calibration curve of concentration vs absorbance is
plotted.
• The sample solution is fed into the instrument, and the
absorbance of the element in this solution is measured .The
unknown concentration of the element isthen calculated
from the calibration curve

APPLICATIONS:
•Determination of even small amounts of
metals (lead,mercury, calcium,
magnesium, etc)
•Environmental studies: drinking water,
ocean water, soil.
•Food industry.
•Pharmaceutical industry.

INTRODUCTION TO SPECTROCOPY

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