Organic Spectroscopy: A Basic and Effective Knowledge of Spectroscopy for Undergraduate and Postgraduate Students, Study notes of Chemistry

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Preface

Today spectroscopy is the most important tools for the structure elucidation of unknown compounds. This book ”Organic spectroscopy” is primarily written for those students who like to have a basic and effective knowledge of spectroscopy as an important subject at the undergraduate as well as postgraduate level. It will help them venture into the weird but fascinating, field of advanced spectroscopy in future.

The content of this book is such that students can gradually understood comprehensive knowledge of techniques. The book has a large number of illustrative diagram including spectrum as well as instrument diagram related to the techniques. Each chapter contains individually solved problems. The last chapter of this book contains combine application of all the spectroscopic techniques related solved problems.

The author had to seek permission from the relevant source for reproducing data and spectra of compounds.

I express my heartfelt thanks to Saurashtra university for giving me the opportunity to publish this book for providing financial Grant. I am particularly thankful to Prof. Pratapsinh Chauhan, Hon. Vice Chancellor of Saurashtra University, Prof. Girishbhai Bhimani, Director, IQAC of Saurashtra University, Prof. Alok Chakrwal, Co-coordinator, IQAC, Saurashtra University and other committee members of

Contents

• 1. Electromagnetic Spectrum ………………….
• 2. Ultraviolet Spectroscopy ………….........
• 3. Infrared Spectroscopy ……………….
• 4. NMR spectroscopy ……………….
• 5. Mass Spectrometry ………………1
• 6. Numerical ………………1
•  References ………………

Introduction

Spectroscopy is a branch of physical science which deals with the study of the interaction between the molecule and electromagnetic radiation. For determination of energy difference between the molecular energy level as well as energy emitted or absorbed by atoms or molecule are studied under the title of “spectroscopy”. The instrumental device used for this type of specific study is known as “spectrophotometer”.

The result of the above process obtain from the “spectrophotometer” is represented as a “continuous graph of electrical transmittance (or absorption) of the function of wavelength or wave number” which is known as “spectrum”.

Electro-Magnetic Radiation

Electromagnetic radiation is a form of energy that is transmitted through space. Electromagnetic radiation is a mixture of the electrical and magnetic component which are perpendicular To each other and also perpendicular to propagation line as a shown in the figure.

Theory of EMR

According to the quantum theory, light has a dual nature i.e. wave and particle. The consist of a stream of energy bunch is also important.

This concept was confirmed by physicist Max-plank means the concept of „Light quanta packet‟.

Particles are considered as an energy bundle called „photons‟ According to Einstein- Plank relation.

E= hν where h=6.63 ⨯ 10 -27^ erg.sec & h= plank constant (5)

∴ energy depends on frequency but not on the intensity of radiation.

∴ but and ̅ (6)

∴̅ (7)

Equation 5, 6, 7 told us about the relation of energy with frequency, wavelength and wave numbers.

Energy has a direct relation with frequency and wave number but the reciprocal relation with ‘wavelength’.

Different component of EMR & spectroscopy:

Molecule absorbs the radiation, it is energy is increase as per the photon relation with energy(eq. 5,6,7)

C osmic rays gamma rays X -rays far ultra violet ultraviolet

R adio frequency

microwave

Infrared

visible

10 - 10 - 10 - 10 -

10 -

102 104

0

W A V E L E N

G T H

E N E R G Y

Energy absorb by molecule is not continuous absorption throughout the whole range but absorb only those frequency which is enough for a particular physical change thus, when molecule interact with EMR, it may change their energy from E 1 to E 2 by absorption of radiation frequency ν

∴ n is integer (8)

After absorption of radiation energy is utilised for the internal energy of a molecule is raising, the study of above reaction is known as “absorption spectroscopy” or after absorption, the excited species come back to the ground state by emitting this energy as a radiation with a different frequency is known as “emission spectroscopy”.

 Type of molecular Energy [ Energy distribution in molecule]: Apart from translation energy, the internal energy of molecule may be regarded as the sum of rotational, vibrational and electronic energies. EInternal= Evib. + Eelec. + Erot. (9)

Chapter 2

Ultraviolet

Spectroscopy

 Introduction:

Ultraviolet and visible spectroscopy is also known as electronic spectroscopy. Generally, it can be used to determine the degree of unsaturation within the molecules.  Theory of Ultra-Violet absorption: Ultra-violet absorption associated with the measurement of energy by electrons is promoted to the excited level. When a continuous radiation passes through a transparent material some of the portion of the radiation absorbed by a molecule and the residual passes through the prism, yields a spectrum with gaps of absorption region, which is known as an absorption spectrum. The intensity of absorption depends on the nature of molecule as well as the wavelength of light. In the case of ultraviolet and visible spectroscopy, the transition that the result of absorption of EMR transition between electronic energy level. The most favourable transition of an electron from higher occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Most of the molecule; highest occupied with the formation of 6- band. The π-orbitals lie at somewhat higher as compare to 6- orbitals. The possible electronic transition is illustrated in fig.(2)

 The ultraviolet spectrum:

Generally, an electronic transition occurs due to the absorption of EMR in the range of 10 nm to 780 nm. So this region is known as a ultra-violet visible region which is further divided into the following region.

(1) Far (vacuum) ultraviolet region(10-200 nm) The absorption below 200 nm (atomic absorption) appeared in this region. To study the specific system in this region, a vacuum is necessary, so this region is known as „vacuum- UV‟ region. (2) Near UV region (200-380): The atmosphere is transparent in this region. The absorption due to p & d electronic transition appears in the range of the ultraviolet spectrum. (3) Visible region (380-780): If enough no. of π bond in conjugation, absorption takes place in this region.  Type of transition: The important transition occur in ultraviolet absorption spectrum are classified below (A) Transition between bonding and anti-bonding orbitals. The transition between bonding to antibonding orbitals are illustrated in fig.(2) they are of two types: (i) σσ^ ( bonding σ to anti-bonding σ) (ii) ^ ( bonding  to anti-bonding )

 This band designated as “1/k band” with  value  10,000.  The benzenoid band also appears due to ^ transition at 180 nm with max  2000.  Ethylenic band also appears due to ^ transition at 200 nm.

(B) Transition between non-bonding atomic orbitals and anti-bonding orbitals.

This type of transition also illustrated in fig.(2) which is further classified in two groups

(1) n→π^ (non-bonding atomic orbital to anti bonding π) (2) n→σ^ (non-bonding atomic orbitals to anti bonding σ)

(1) n→π* → This type of transition occurs on those compound which possesses double bond with a hetero atom. → Non-bonding electrons are held more loosely than σ bonding electrons and consequently undergo transition at higher wavelength → This type of transition is An important characteristic of aldehyde and ketone and also sensitive toward chromophoric substitution. → This transition designated as “R-Band”.

(2) n → σ* → The excitation of a unsaired electron on oxygen, Nitrogen or any hetero Atom to an anti-bonding σ orbital is called n→π*^ transition.

→ This transition requires less energy as compared to σ→σ*, so absorption takes place at higher wavelength & shows less value of molar absorptivity.

 The absorption laws and measurement of absorption intensity. The absorption of light by matter (molecule) is explained by the following laws: (a) Beer’s law: This law state that absorption of light is proportional to the number of absorbing molecules in the light path and will increase with respect to concentration. K ∝c (13) K=∈c ∈= molar absorptivity constant, c= concentration (b) Lambert’s law: It states the absorption of incident light by the homogeneous medium is independent of the intensity of incident light but decreasing rate depends on the thickness of absorbing medium. I= Intensity of incident light (14) t = thickness of medium