Kalsi p s spectroscopy of organic compounds 8 e

About the Book:
In the new eighth edition the text in almost all the chapters has been updated by adding new materials and
deleting the old ones. New problems with their solutions have been added. By working through these
examples,thestudentsarelikelytodeveloptheirskillsinanalyzingspectraldatatosolveproblems.
More new problems with their solutions have been added in chapter eight while the old ones have been
deleted. The major aim has been to develop strategy in solving problems from spectral data.Anew chapter
(ninth) has been added with twenty six solved problems. These are aimed to develop skill to solve
spectroscopicproblems.
TheaboveupdationandrevisionwouldgreatlyhelpstudentsincompetitiveexaminationslikeNET, etc.
About theAuthor:
P S Kalsi obtained his PhD degree from Pune University, Pune under the guidance of Professor
S C Bhattacharya at National Chemical Laboratory, Pune in 1964. He has published over 150 research papers in
national and international journals of repute in the area of chemistry of natural products. Prof. Kalsi was
honoured by the PunjabAgricultural University in 1969 in recognition of his merit as a teacher. Prof. Kalsi was
invited by the Swedish Royal Academy of Sciences to submit proposals for the award of the Nobel Prize for
Chemistry, 1985. Indian Chemical Society in 2003 conferred on him S C Ameta medal for his outstanding
research contributions. In 2011, Indian Chemical Society conferred on him LifetimeAchievementAward for his
outstanding contributions to chemical education on the eve of InternationalYear of Chemistry. He was honoured
as the best teacher of chemistry in India at 28th Gujarat Science Congress held at North Gujarat University,
Patan on 22nd–23rd February, 2014 deliberating on Excellence in Science Education in India–A challenge
ahead. He is actively involved in teaching in different universities/postgraduate colleges and serves as a UGC
resource person to deliver lectures in refresher courses. In an academic year he visits about 12 different
universities anddelivers about200lectures.
SPECTROSCOPY OF
ORGANIC COMPOUNDS
ISBN : 978 81 943696 8 4
Price : 450.00
Pub Date : 2020
Format : Paperback
Extent : 652 Pages
Click Here to Buy the Book Online
P S KALSI
Professor of Eminence, Shoolini University, Solan,
Visiting Professor, Gujarat Forensic Sciences University, Gandhinagar
Visiting Professor, Kanoria PG Mahila Mahavidyalaya, Jaipur
Former Visiting Professor of Chemistry, Indira Gandhi National Open University (IGNOU), New Delhi
Former Dean of Colleges, Punjab Technical University, Jalandhar
Former Professor and Head, Department of Chemistry, College of Basic Sciences & Humanities
Punjab Agricultural University, Ludhiana
Readership and Market Potential: Undergraduate/postgraduate students and faculties of Chemistry • University and college libraries • Biochemists, Medical and
Pharmacologystudentsandotherrelatedprofessionals.
•
LONDON • NEW DELHI • NAIROBI
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERSCommitted to Educate the Nation
IN INDIA
EIGHTH EDITION
Contents:
Ÿ Energy–The
Electromagnetic
Spectrum and the
Absorption Spectrum
Ÿ Ultraviolet (UV) and
Visible Spectroscopy
Ÿ Infrared Spectroscopy
(IR)
Ÿ Proton Nuclear Magnetic
Resonance
Spectroscopy–¹H NMR
Ÿ Carbon–13 NMR
Spectroscopy (¹³C
NMR)
Ÿ Two–Dimensional
Correlated
Spectroscopy (2D
COSY)
Ÿ Mass Spectrometry
Ÿ Spectroscopic Problems
for NET Eligibility
Ÿ More Typical
Spectroscopic Problems
for NET Eligibility

NEW AGE
SPECTROSCOPY
OF
ORGANIC
COMPOUNDS
TH
8Edition
(In two colours)
P S KALSI

SPECTROSCOPY
OF
ORGANIC
COMPOUNDS

EIGHTH EDITION
(In two colours)
SPECTROSCOPY
OF
ORGANIC
COMPOUNDS
P S Kalsi
Professor of Eminence
Shoolini University, Solan
Visiting Professor
Gujarat Forensic Sciences University, Gandhinagar
Visiting Professor
Kanoria PG Mahila Mahavidyalaya, Jaipur
Former Visiting Professor of Chemistry
Indira Gandhi National Open University (IGNOU), New Delhi
Former Dean of Colleges
Punjab Technical University, Jalandhar
Former Professor and Head
Department of Chemistry
College of Basic Sciences & Humanities
Punjab Agricultural University, Ludhiana
LONDON • NEW DELHI • NAIROBI
NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS
•
v

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Preface to the Eighth Edition
Many times, after the first edition of the book appeared in 2002, I have been asked where students could
find more exercise problems to further help them with the study of this text. The demand was centered
on comprehensively stimulating problems covering the subject matter of the book to face the problems
of such competitive examinations like NET (UGC).
Keeping this in mind this new edition was prepared which contains almost 65 more pages than the
seventh edition. All the chapters have been updated by rewriting the material, adding new material and
deleting some old material.
NEW TO THE EIGHTH EDITION
• A new ninth chapter “Spectroscopic Problems for NET Eligibility’’ containing twenty six
problems has been added. The problems of this new chapter cover most of the areas of spectroscopy.
The answers, in addition, to providing solutions frequently include additional explanations of the
underlying principles.
The major focus is to prepare and stimulate a student to face successfully the problems in a NET
Test.
• Eighth chapter has been extensively modified by deleting some old routine problems and replacing
these with new ones.
• All the remaining chapters have been accordingly updated. The main aim of the author has been to
make the material in this new edition to improve the thinking and reasoning power of the student
for reaching the correct answers to the problems they are likely to face.
• No attempt has been made to arrange the problems in order of increasing difficulty. Often a
difficult problem is followed by a much simpler one, to help stimulate and maintain the interest in
the student for further study.
A lot can now be learnt from reading the text of the book itself which has been revised and updated
in this new edition. More involvement is however, needed not only to gain a real understanding of
spectroscopy, but to expand the knowledge to solve the problems which the student may face in a
competitive examination. The new solved problems throughout this new edition is a positive move by
the author in this direction.
Ludhiana P.S. KALSI
xi

Foreword (vii-ix)
Preface to the Eighth Edition (xi)
Preface to the First Edition (xii)
Acknowledgements (xiii)
Spectra of Some Compounds (xx)
1. Energy—The Electromagnetic Spectrum and the Absorption Spectrum 1–7
1.1 Introduction—What is Molecular Spectroscopy?............................................................1
1.2 Absorption Spectroscopy and Electromagnetic Spectrum...............................................2
1.3 Symbols..............................................................................................................................5
1.4 Absorption of Electromagnetic Radiation by Organic Molecules..................................5
1.5 A Spectrophotometer—An Absorption Spectrum and Units...........................................6
1.6 Energy Levels and Absorption Bands/Absorption Lines................................................7
2. Ultraviolet (UV) and Visible Spectroscopy 8–73
2.1 Absorption of UV-vis Electromagnetic Radiation...........................................................8
2.2 Electronic Transitions and Energy Levels........................................................................8
2.3 Isolated Chromophores C=C and C=O...................................................................... 11
2.4 The Beer-Lambert Law...................................................................................................12
2.5 Effect of Conjugation on UV Spectra (lmax)................................................................13
2.6 Terminology of UV Spectroscopy..................................................................................15
2.7 Selection Rules—Allowed and Forbidden Transitions..................................................16
2.8 Solvents Used in Ultraviolet Spectroscopy....................................................................17
2.9 General Applications of Ultraviolet Spectroscopy—Effect of
Geometrical and Constitutional Factors.........................................................................20
2.10 Woodward-Fieser Rules (Calculation of UV Absorption Wavelength
in Conjugated Dienes and Trienes)................................................................................27
2.11 Polyenes...........................................................................................................................31
2.12 Carbonyl Compounds......................................................................................................33
2.13 Woodward Fieser Rules for a, b-Unsaturated Ketones and
Aldehydes for p → p* Transition..................................................................................35
Contents
xv

Contents vii
2.14 Effect of Structure–Strain-Steric Effect in Biphenyls and
Chromophore Distortion..................................................................................................38
2.15 Aromatic Systems—Benzene and its Substituted Derivatives......................................40
2.16 Polycyclic–Aromatic Hydrocarbons...............................................................................46
2.17 Heteroaromatic Compounds............................................................................................47
2.18 Analytical Uses of UV Spectroscopy.............................................................................48
2.19 Study of Charge-Transfer Complexes............................................................................49
2.20 Use of Sunscreens and Dyes..........................................................................................49
2.21 Linearly Polarized Light.................................................................................................50
2.22 Circularly Polarized Light...............................................................................................51
2.23 The Terms (ORD) and (CD)-Chiroptical Properties.....................................................52
2.24 UV-vis and Chiroptical Spectroscopy............................................................................52
2.25 The Octant Rule..............................................................................................................55
2.26 Application of (ORD CD) and Octant Rule for Ketones.............................................57
• Problems and Exercises..............................................................................................62
• Answers........................................................................................................................67
• Summary......................................................................................................................71
3. Infrared Spectroscopy (IR) 74–168
3.1 Introduction......................................................................................................................74
3.2 Absorption in the Infrared Region—The Chart Paper
—Presentation of the IR Spectra....................................................................................76
3.3 Molecular Vibrations—Complexity and Simplicity of IR Spectra...............................77
3.4 What to Look for in an IR Spectrum. Detection of Some Important
Functional Groups...........................................................................................................83
3.5 Factors which Complicate the IR Spectra and Affect Group Frequencies..................86
3.6 Factors which Simplify the Infrared Spectra—Symmetry and Infrared Spectra ........94
3.7 Instrumentation................................................................................................................96
3.8 Sample Handling.............................................................................................................98
3.9 Applications of Infrared Spectroscopy...........................................................................98
3.10 Preparing for the Interpretation of Infrared Spectra
(High Frequency Region)..............................................................................................102
3.11 Systematic Interpretation of Infrared Spectra of Organic Compounds via
Specific Spectral Regions.............................................................................................. 111
3.12 Particularly Low and High C==O Stretchings............................................................138
3.13 The C==C Stretch of Alkenes......................................................................................150
3.14 Fingerprint Region (1000 – 1200 cm–1)......................................................................151

viii Spectroscopy of Organic Compounds
3.15 Interpretation of IR Spectra—Some Practice Problems..............................................156
3.16 Some Typical Cases—Compound Containing Nitrogen
Sulphur and Halogens...................................................................................................160
3.17 IR Radiation and Greenhouse Effect............................................................................163
3.18 Infrared Radiation and Automobile Pollutants.............................................................163
3.19 The Breath Analyzer Tool.............................................................................................164
• Problems and Exercises............................................................................................164
• Answers......................................................................................................................165
• Summary....................................................................................................................167
4. Proton Nuclear Magnetic Resonance Spectroscopy 1H NMR 169–327
4.1 Introduction–Useful Information can be Obtained about Structure............................169
4.2 Theory of 1H NMR Spectroscopy, a and b Spin States............................................174
4.3 The 1H NMR Spectrum................................................................................................178
4.4 Working of NMR Spectrometers...................................................................................182
4.5 Measurement of the Position of an 1HMR Peak (Chemical Shift).
Need for a Reference Compound like TMS................................................................184
4.6 The Position of an NMR Signal (Chemical Shift) Depends on the
Environment of the Nucleus.........................................................................................187
4.7 Factors Influencing Chemical Shift—An Introduction................................................188
4.8 Some Examples of Interpretation of 1H NMR Spectra–Analysis of
Olefinic Hydrogens and Hydrogens on Aromatic Rings.............................................194
4.9 Pi—Electron-Induced Fields—Diamagnetic Anisotropic Effects.................................199
4.10 Structure from Chemical Shift Positions for Protons
—An Introduction.........................................................................................................207
4.11 Chemically Equivalent Protons are Chemical Shift Equivalent.................................214
4.12 Symmetry and 1H NMR Spectra..................................................................................227
4.13 Magnetic Equivalence...................................................................................................230
4.14 Theory of Spin-Spin Splitting......................................................................................237
4.15 Interpretation of Spectra of Some Simple Organic Compounds................................241
4.16 The Coupling Constant and Appearance of 1H NMR Spectra
at Low and High Field Strengths.................................................................................248
4.17 Spin Decoupling............................................................................................................259
4.18 More Complex 1H NMR (Second Order Spectra) – Distortion of Multiplets..........261
4.19 Rotations about Single Bonds – Variable Temperature Spectra
— Geminal Non-Equivalence.......................................................................................279
4.20 Proton Exchange Reactions and Hydrogen Bonding Influences
—Hydroxyl and Amino Groups...................................................................................283

Contents ix
4.21 Proton Nuclear Magnetic Resonance Spectra of Compounds Containing
Fluorine (19F) and other Halogens...............................................................................286
4.22 Protons on Nitrogen (14N)............................................................................................287
4.23 31P Nucleus....................................................................................................................288
4.24 Proton Nuclear Magnetic Resonance 1H NMR Spectra
of Carbocations..............................................................................................................290
4.25 The Nuclear Overhauser Effect (NOE): A Thorough Space Phenomenon................291
4.26 Simplification (Improvement) of Complex Spectra.....................................................293
4.27 Magnetic Resonance Imaging (MRI)...........................................................................297
4.28 Interpretation of Spectra of some Organic Compounds..............................................297
• Problems and Exercises............................................................................................ 311
• Answers......................................................................................................................317
• Summary....................................................................................................................323
5. Carbon—13 NMR Spectroscopy (13C NMR) 328–370
5.1 Introduction....................................................................................................................328
5.2 Carbon NMR Utilises an Isotope in Low Natural Abundance (13C).........................330
5.3 13C NMR Spectrum — A General Study....................................................................330
5.4 Operating Frequency.....................................................................................................333
5.5 Multiplicity – Broad Band (BB) Proton Decoupled Spectra – Hydrogen
Decoupling gives Single Lines — Complete Removal of 13C—H Coupling...........333
5.6 Off-Resonance Decoupling...........................................................................................334
5.7 DEPT 13C NMR............................................................................................................336
5.8 Chemical Shift Equivalence—Presence of Symmetry.................................................339
5.9 Chemical Shifts.............................................................................................................340
5.10 Carbon Chemical Shifts—A Review............................................................................362
• Answers......................................................................................................................367
• Summary....................................................................................................................369
6. Two-Dimensional Correlated Spectroscopy (2D COSY) 371–412
6.1 Introduction....................................................................................................................371
6.2 The 1H—1H COSY (H, H – COSY)...........................................................................374
6.3 13C—1H COSY (C, H COSY): HETCOR (HETCOR is Carbon
Detected Experiment)....................................................................................................382
6.4 Other Variations of 2D (C, H) Correlations as HETCOR and COLOC in
the form of Inverse Experiments as HMQC or HSQC and HMBC..........................386
6.5 The NOESY Spectrum..................................................................................................393

x Spectroscopy of Organic Compounds
6.6 The TOCSY Spectrum – Total Correlation Spectroscopy (TOCSY).........................394
6.7 A Combined Application of NMR Experiments DEPT 135, DEPT 90,
COSY, TOCSY, and HSQC, HMBC To Solve Structural Problems..........................396
6.8 13C—13C Correlations-Inadequate (Incredible Natural Abundance
Double Quantum Transfer Experiment).......................................................................410
• Summary.................................................................................................................... 411
7. Mass Spectrometry 413–519
7.1 Ionisation of a Molecule on Electron Impact (EI)–Presentation of
the Molecular Ion..........................................................................................................413
7.2 Molecular Weights of Organic Compounds—The Molecular Ion
and its Intensity.............................................................................................................416
7.3 The Base Peak — The Mass Spectrum — Ratio of Mass to Charge (m/z)
— Breakage of Molecular Ion.....................................................................................417
7.4 Detection of the Isotopes of the Elements and the Recognition of
Molecular Ion Peak — Use of Heavier Isotope Peaks...............................................418
7.5 Instrumentation — The Mass Spectrometer — The EI (Electron Impact)
Technique.......................................................................................................................423
7.6 Fragmentation from (EI) Method — A General View................................................425
7.7 Recognition of the Molecular Ion from (EI) Spectra.................................................434
7.8 The Nitrogen Rule.........................................................................................................439
7.9 The Index of Hydrogen Deficiency..............................................................................439
7.10 Molecular Weight–Molecular Formula–High Resolution Mass Spectrometry...........441
7.11 General Appearance of Mass Spectrum — Nature of the
Compound and Metastable Ions...................................................................................443
7.12 Fragmentation and Mass Spectra of Classes of Organic Compounds.......................453
7.13 Gas Chromatography—Mass Spectrometry (GC/MS)................................................. 511
• Answers......................................................................................................................514
• Summary....................................................................................................................518
8. Spectroscopic Problems for NET Eligibility 520–596
9. More Typical Spectroscopic Problems for NET Eligibility 597–617
• Index 619–630

THE NAMES of various forms of electromagnetic energy have now become familiar terms. The X-rays
are used in medicine, the ultraviolet rays lead to sunburns and the radio and radar waves are used in
communication and visible light are all different forms of the same phenomenon, i.e., electromagnetic
radiation.
1.1 INTRODUCTION—WHAT IS MOLECULAR SPECTROSCOPY?
Molecular spectroscopy is an experimental process to measure as to which frequencies of radiation are
absorbed or emitted by a particular substance and then attempt to correlate patterns of energy absorption
or emission with details of molecular structure.
In the quantum mechanical terms, electromagnetic radiations are considered to be propagated in
discrete packets of energy called photons. These photons have very specific energies and are quantized.
The energy of a photon is derived by the relationship (equation Scheme 1.1).
E = hν
Scheme 1.1
Recall the orbitals that an electron can occupy in an atom. The energies of these orbitals are quantized
(there is no orbital with an energy intermediate between that of a 1s orbital and that of a 2s orbital).
Moreover, all of the energy states of a molecule are quantized. It is this fact that makes spectroscopy
possible.
An atom or molecule can be made to undergo a transition from energy state E1
to a higher energy
state E2
by irradiating the atom or molecule with electromagnetic radiation corresponding to the energy
difference between states E1
and E2
(Scheme 1.2). When the atom or molecule returns from state E2
to
state E1
, an equivalent amount of energy is emitted. Thus, excitation of an organic molecule involves
absorption of specific quanta of electromagnetic radiation. The photon of electromagnetic radiation will
only be absorbed by a molecule if its energy corresponds exactly to an energy difference ∆E between
the two states of the molecule. The energy difference between e.g., the excited state (E2
) and the ground
CHAPTER
Energy—The Electromagnetic
Spectrum and the
Absorption Spectrum
1
1

2 Spectroscopy of Organic Compounds
E2
E1
ENERGY
Atom or molecule
in energy state E2.
Atom or molecule
in energy state E1.
Absorption of energy
in the form of
electromagnetic
radiation.
Absorption of energy as
electromagnetic radiation
causes an atom or molecule
in energy state E to change
to a higher energy state E .
1
2
D - nE E E h= =2 1
Two energy states of a molecule.
Absorption of energy equal to E E2 1-
excites a molecule from its lower-energy
state to the next higher state.
Scheme 1.2
state (E1
) will correspond to a certain frequency (ν) or wavelength (λ) of electromagnetic radiation,
which depend on the type of transition (and thus the separation between energy levels). The relationship
between the energy of a transition and the frequency is given by (Scheme 1.3).
∆E = hν
and so:
∆E =
hc
λ
or ∆E = hcν
Scheme 1.3
The energy of a particular transition is, thus proportional to the frequency or wave number
( ν = 1/λ) and inversely proportional to the wavelength. In conclusion, therefore spectroscopy is the
study of interaction of molecules and electromagnetic radiation.
1.2 ABSORPTION SPECTROSCOPY AND ELECTROMAGNETIC SPECTRUM
Aphoton corresponding to any possible transition within the molecule might be absorbed. The majority of
the molecules exist initially in the unexcited ground state and the easiest and the most efficient, absorption
process involves the lowest energy input, elevating the molecule to its first excited state. The strongest
absorption of electromagnetic radiation, thus occurs at an energy corresponding to transitions from the
molecule from its ground state to the first excited state of the transition.
Visible light, infrared light, ultraviolet light, microwaves, and radio waves are examples of
electromagnetic radiation, and all of them travel at the speed of light (about 3 × 1010
cm/sec), but they
differ in frequency and wavelength. The frequency of a wave is the number of complete wave cycles that
pass a fixed point in a second. Frequency, represented by the Greek letter ν (nu) is given in hertz (Hz).

Energy—The Electromagnetic Spectrum and the Absorption Spectrum 3
CHAPTER1
The wavelength, represented by the Greek letter λ (lambda), is the distance between any two peaks (or
any two troughs) of the wave (Scheme 1.4).
l
Light wave
The wavelength and frequency are inversely proportional and are related by the equation (I)
νλ = c or λ =
c
ν
...(I)
where
c = Speed of light (3 × 1010
cm/sec)
ν = Frequency in hertz
λ = Wavelength in centimeters
Scheme 1.4 Electromagnetic waves travel as photons, which are massless packets of energy.
The relationship between the energy (E) and the frequency (ν) or wavelength (λ) of electromagnetic
radiation of a photon is described by the equation (II, Scheme 1.5).
E = hν =
hc
λ
...(II)
where h is Planck’s constant, 1.58 × 10−37
kcal-sec or 6.62 × 10−37
kJ-sec.
Scheme 1.5
When a molecule is struck by a photon and absorbs the photon’s energy, the molecule’s energy is
increased by an amount equal to the photon’s energy hν. Thus, one often represents the irradiation of a
reaction mixture by the symbol hν.
Another way to describe the frequency of electromagnetic radiation—and one most often used
in infrared spectroscopy—is wave number ( )v , the number of waves in 1 cm. Therefore, its units are
reciprocal centimeters (cm−1
). Wave number (in cm−1
) and wavelength (in µm) are related by the equation
(Scheme 1.6).
ν ( )cm−1
=
104
λ µ( )m
(because 1 µm = 10−4
cm)
Scheme 1.6
Thus, high frequencies, large wave numbers, and short wavelengths have high energy.
Wave numbers (in cm−1
) are used to specify IR absorption. The wave number is proportional to the
frequency (ν) of the wave, so it is also proportional to the energy of a photon of this frequency (E = hν).
It is important to remember that the energy of light is directly proportional to its frequency and
inversely proportional to its wavelength.

Low-Energy Light High-Energy Light
Low frequency High frequency
Long wavelength Short wavelength
The electromagnetic spectrum is divided into a number of regions. The names of these regions
and the associated atomic/molecular transitions, together with the corresponding energies (wavelengths)
are shown in (Scheme 1.7). The regions of most interest to an organic chemist for spectral work
are highlighted. It is seen that nuclear magnetic resonance (NMR) transitions which correspond to
Wavelength ( )
m
l Molecular effectsEnergy
kcal/mol
10
–10
10
–8
10
–6
10
–4
10
–2
10 (1)
0
10
2
Gamma rays
X rays
Vacuum UV
Near UV
Visible
Infrared
(IR)
Microwave
Radio
(NMR)
10
4
10
6
10
2
10
1
10
–2
10
–4
10
–6
Ionization
Electronic transitions
Molecular vibrations
Rotational motion
Nuclear spin transitions
The electromagnetic spectrum
ENERGY
Higher frequency
shorter wavelength
Lower frequency
longer wavelength
Scheme 1.7
wavelengths in the radiowave region of the spectrum, have the smallest gap between the energy levels,
while electronic transitions in the ultraviolet-visible (UV-Vis) region have the largest energy gap between
transition levels. The following points may be noted:
• γ-Rays and X-rays have such high energy that they simply cause electrons to be ejected from the
molecule resulting in bond rupture, so their absorption provides little detailed structural information.
X-ray diffraction, of course, is a very powerful tool for structural analysis and provides a map of
the molecule in a crystal.
• Microwave (rotational) spectra are very complex even for diatomic molecules and therefore,
give little information on organic molecules. The broadness of infrared (IR) bands is often due to
rotational transitions, since every vibrational transition has several rotational transitions associated
with it.
• Increasing energies of electromagnetic radiation may cause rotational, vibrational and electronic
transitions to occur within a molecule and some of these frequencies of the electromagnetic
spectrum are of greater utility to organic chemists than others for spectroscopic analysis.

CHAPTER1
• Absorption spectra provide two kinds of information. The absorption wavelength or frequency
gives the energy associated with a particular excitation which can be related to the functional
group responsible for absorption. The absorption intensity shows both the ease of the transition
(information about the functionality undergoing excitation) and the concentration of the absorbing
species.
• Organic chemists calculate the absorption intensity in ultraviolet spectroscopy, but largely ignore
it in infrared spectra but for the description of absorptions as ‘strong’, ‘weak’ or ‘medium’. In
proton nuclear magnetic resonance spectroscopy however, signal intensity provides key piece of
information regarding to the relative number of protons in the molecule responsible for the signal.
1.3 SYMBOLS
Some of the common symbols used in spectroscopy are given in Table 1.1.
Table 1.1: Symbols used in spectroscopy
Symbol Definition
ν Frequency in Hz (cycles per second).
λ Wavelength.
µm Micrometer, same as micron (µ), 10−6
m.
nm Nanometer, same as millimicron (mµ), 10−9
m.
Å Angstrom, 10−10
m or 10−1
nm.
cm−1
Wave number: frequency in reciprocal cm, or 1/λ.
The different regions of the electromagnetic spectrum from the region X-rays to those of micro and
radiowaves have been used to determine structures of atoms and molecules (Table 1.2).
Table 1.2: Summary of spectroscopic techniques in organic chemistry
Spectroscopy Radiation absorbed Effect on the molecule
Ultraviolet (UV)-visible UV-visible
(λ = 200–750 nm)
Changes in electronic energy levels within
the molecule.
Infrared (IR) Infrared
(λ = 2.5–16 µm)
Changes in the vibrational and rotational
movements of the molecule.
Nuclear magnetic
resonance (NMR)
Radio
ν, 60–500 MHz
Induces changes in the magnetic properties of
certain atomic nuclei.
1.4 ABSORPTION OF ELECTROMAGNETIC RADIATION BY ORGANIC
MOLECULES
Quanta of defined energy bring about specific excitations
Molecules absorb electromagnetic radiation in discrete “packets”, of energy or quanta, which are
measurable by spectroscopy. Absorption occurs only when the radiation supplying exactly the
right packet interacts with the compound (∆E = hv). In UV/vis spectroscopy, e.g., absorption of
light results in the promotion of an electron from the HOMO to the LUMO.

Organic molecules absorb radiation, in discrete “packets” of ∆E = hν, which are also called quanta
of energy (Scheme 1.2). The spectra of a compound, i.e., the response of a substance subjected to radiation
of various wavelengths, are among the most important physical properties of an organic compound.
Spectroscopy, therefore, is a powerful tool in the hands of an organic chemist for structure determination.
When, one considers a molecule, it is found that it is associated with several different types of motion.
The molecule as a whole rotates, the bonds undergo vibrations and even the electrons move. Each of
these kinds of motion is quantised, i.e., the molecule can exist only in distinct states which correspond
to discrete energy contents. Thus absorption occurs only when radiation supplying exactly the right
“packet” of energy impinges on the compounds under study. Each state is characterised by one or more
quantum numbers and the energy difference between two such states, ∆E, is related to a light frequency
ν and Planck’s constant h (Schemes 1.2 and 1.3).
Spectroscopy is essentially a technical procedure by which the energy differences between the
allowed states of a system are measured (mapped and recorded) by determining the frequencies of the
corresponding light absorbed.
1.5 A SPECTROPHOTOMETER—AN ABSORPTION SPECTRUM AND UNITS
An infrared or ultraviolet spectrophotometer, for example, allows light of a given frequency to pass
through a sample and detects the amount of transmitted light (i.e., not absorbed). The instrument compares
the intensity of the transmitted light with that of the incident light. Automatic instruments gradually
and continuously change the frequency, and an automatic recorder plots a graph of absorption versus
frequency or wavelength, i.e., a spectrum.
The spectrum of a compound thus represents a graph of either wavelength or frequency, continuously
changing over a small portion of the electromagnetic spectrum versus either per cent transmission
(% T) or absorbance (A). The per cent transmission is the per cent of the intensity of the original radiation
which passes through the sample.
% T =
Intensity
Original intensity
×100
When a compound does not absorb any radiation at a particular wavelength, the per cent transmission
is 100 at that wavelength. Absorption of radiation at a particular wavelength leads to a decrease in the
per cent transmission to appear in the spectrum as a dip, called a peak, or absorption band. Absorbance
is a measure of the absorption of radiation by a sample:
A = log
Original intensity
Intensity




In this case, an increase in absorption appears as an increase (not a decrease) of the signal.
Because of the great difference in the wavelength of different regions it is not convenient
to use the same units throughout to specify a particular position in the spectrum. In the uv
(200–400 nm) and visible (400–800 nm) regions, the wavelengths are expressed as nanometers
(nm, 1 nm = 10−9
m), in the infrared region (4000–600 cm−1
), the wavelengths are expressed in micrometers
(µm, 1 µm = 10−6
m) or as the reciprocal wavelength in centimeters, 1/λ is termed as the wave number
ν . In the radiofrequency region (NMR) absolute frequencies are used rather than the wave numbers.
Thus a wavelength of 5 meters corresponds to a frequency of c/λ or 6 × 107
Hz (Hz = Hertz, defined as
cycles per second) and can be written as 60 MHz.

CHAPTER1
1.6 ENERGY LEVELS AND ABSORPTION BANDS/ABSORPTION LINES
Aspectrum is simply a plot of the amount of light which is absorbed versus the frequency (or wavelength)
of the light. The spectrum yields information about the spacing of the energy levels of the molecule. Since
these energy levels depend on the structure of the molecule, this information therefore, (with practice)
is used to determine the structure of the compound.
In some types of spectroscopy there are only a few, well-separated energy levels. In these cases,
only a very narrow range of wavelengths is absorbed each time the molecule is excited from its lowest-
energy state to some higher-energy state, to give an absorption line for each of these transitions. The
spectrum consists of a number of these absorption lines. In many cases, however, there are a number
of energy sublevels in each energy state. In these cases, a number of closely spaced wavelengths are
absorbed. The lines are often so close together that they cannot be resolved and the absorption appears
as a broad peak or band (Scheme 1.8).
Amountoflightabsorbed
Absorption lines
With only a few energy
levels and when the
spacing between them
is large enough, a line
spectrum is obtained.
l
ENERGY
Spectrum
(I)
Lowest energy level
(II)
ENERGY
Lowest energy
When there are many,
closely spaced sublevels
in each energy level,
the absorptions display
themselves on broad
bands since the
individual lines are
not resolved.
l
Spectrum
Absorption bands
Amountoflightabsorbed
Scheme 1.8

Kalsi p s spectroscopy of organic compounds 8 e

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