PROTON NMRSPECTROSCOPY

Presented By:Bhushan. L. C.

Guided By:

Prof. Dr. J. S. Biradar

DEPARTMENT OF POSTTGRADUATE AND

RESEARCH IN CHEMISTRY,GUK

Contents :1) Light2) Introduction to

Spectroscopy3) Types of

Spectroscopy4) Introdruction to

NMR5) Proton NMR6) Principle7) Instrumentation

8) Chemical Shift9)Internal

Standard10)Spin-Spin

Splitting11) Deuterium

Exchange 12) n+1 Rule13) Applications

LIGHT Light is electromagnetic radiation within a

certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).Often, infrared and ultraviolet are also called light.

The term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles

Electromagnetic Spectrum & Visible Light Generally, EM radiation, or EMR (the designation

'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region that we perceive as light, ultraviolet, X-rays and gamma rays.

Introduction to Spectroscopy Spectroscopy may be defined as the

“Technique whereby we measure the amount of radiation absorbed by a substance at various wavelengths”.

Absorption of photons by the molecule may change its internal energy (electronic, vibrational or rotational energy) or may cause transition between different spn orientations of nuclei in a magnetic field

From this useful information,to ascertain the structure of an unknown organic compound whether obtained by synthetic process or form a natural sources is a fundamental operation in chemistry.

TYPES OF SPECTROSCOPY The study of spectroscopy can be studied mainly

two types as followings,1) Atomic Spectroscopy2) Molecular Spectroscopy

TYPES OF MOLECULAR SPECTROSCOPY1. Infrared Spectroscopy2. Pure Rotation (Microwave ) Spectroscopy3. Ultra violet & Visible Spectroscopy4. Nuclear Quadruole Resonance (NQR) & Nuclear

magnetic resonance (NMR) Spectroscopy5. Electron Spin Resonanace (ESR) Spectroscopy6. Mass spectroscopy

INTRODUCTION TO NMR

The study of absorption of radiofrequency radiation by nuclei in a magnetic field is called Nuclear Magnetic Resonance.

• Nuclear magnetic resonance spectroscopy is basically another form of absorption spectrometry. It involve change of the spin state of a nucleus, when the nucleus absorb electromagnetic radiation in a strong magnetic field.

• The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency.

• When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements having spin state 1/2, including 1H and 13C.

1H NMR The Nucleus of a hydrogen atom(proton) behaves

as a spinning bar magnet because it possesses both electric field and magnetic spin. Like any other spinning charged body, the nucleus of hydrogen atom generates a magnetic field.

Nuclear Magnetic Resonance involves the interaction between an oscillating magnetic field of electromagnetic radiation and the magnetic energy of the hydrogen nucleus or some other type of nuclei when these are placed in an external static magnetic field.

Principle Of Proton NMR• When a charged particle such as a proton

spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet.

• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy.

•

All nuclei carry a charge. In same nuclei charge spins on the nuclear axis and this circular, charge generates a magnetic dipole along the axis. The angular momentum of the spinning charge can be described in terms of quntum number I, “ I = 0, ½, 1, 3/2 5/2…….. and so on. If spin I = 0 no spin and hence no 1H NMR phenomenon

Spin active nuclei have permanent magnetic moments and quantized nuclear spin states. The number of spin states for a given nucleus is given by the expression (2I +1) where I is the overall nuclear spin.

In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B0, and a higher energy state in which the nucleus aligned against B0

Alignment with the field (Lower energy state or parallel to the field I = +1/2)

Alignment against the field (Higher energy state or antiparallel to the field. I = -1/2)

As shown in figure. Two energy levels one is lower energy states i.e, aligned or parallel I = +1/2 whose population is N other is higher energy state I = -1/2, antiparallel whose population is N then N > N in accordance with the boltzman distribution.

Therefore,

, .

since

Where , h Planck’s constant. Frequency of oscillator B o Applied external magnetic field strength. Gyromagnetic ratio ( being ratio between the nuclear

magnetic moment)

Intrumentations : Continuous Wave (CW) Spectrometer :

In the CW Spectrometers the spectra can be recorded either with field sweep or frequency sweep.. Keeping the frequency constant, while the magnetic field is varied (swept) is technically easier than holding the magnetic field constant and varying the frequency.

Instrumentations :

Pulse Technique & Fourier Tranform NMR Spectroscopy (FTNMR):

The modern Fourier Transform (FT) spectrometer operates with pulse technique when an rf pulse of short duration excites all the nuclei simultaneously and all the signals are collected at the same time with a computer. The individual method allows several hundred runs to be collected within seconds. The data are mathematically converted (a fourier transform ) to a spectrum.

Advantages of FTNMR: It is much faster and More sensitive. It can obtained with less than 5mg of compound.

Chemical Shift : The difference in the absorption of position

of a particular proton from the absorption position of a reference proton is called Chemical shift.

Chemical shifts position are normally expressed in (delta) units, which are defined as proportional differences, in parts per million(ppm), from an appropriate reference standard (TMS in case of proton NMR).

Internal Standards Reference materials which is commonly

employed as internal standard is the universally accepted reference is Tetramethyl Silane (TMS).

Characterstic Features of TMS

Si

CH3

CH3

CH3

H3C

•It is chemically inert and miscible with a large range of solvent•It is highly violatile and can be easily removed to get back the sample•TMS give only one peak•TMS gives an intense sharp peak at low concentrations.•It has a low boiling point 26.5 0C

Spin -Spin coupling or spin - spin splitting : Splitting of the spectral lines arises because of

coupling interaction between neighbour protons and is related to the number of possible spin orientation that these neighbours can adapt. The phenomenon is called either spin-spin spilitting or spin-spin coupling.

Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field.

This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.

Nonequivalent protons on adjacent carbons

1,1,2-Tribromoethane

Triplet: 2 Adjacent Protons

Doublet: 1 Adjacent Proton

Deuterium Exchange : If deuterium oxide, D2O, is used as solvent for

NMR work, the D2O exchanges with labile protons such as OH, NH and SH. In effect, because of the rapidity of the exchange, R-OH becomes ROD, RCOOH becomes RCOOD, RCONH2 becomes RCOND2 Etc.

ROH + D--O--D ROD + H--O--D This technique of detraction is widely used to

detect the presence of –OH, -NH & SH groups and is easily carried out,. The NMR spectrum can be carried out.

The method can be extended to detect reactive methylene groups, such as those flanked by carbonyl.

n+1 Rule : The number of peaks in a multiplet can give

additional information about the structure The splitting of peaks is caused by the

neighbouring carbon’s hydrogen atoms Protons in the same environment are said to

be equivalent and as such behave as one proton. This follows the n + 1 rule. n is the number of hydrogen atoms attached to

the next-door carbon n + 1 is how many peaks will be seen in the

cluster

Applications Of NMR :1) Distinction between cis-trans isomers and conformers. The cis and trans isomers of a compound can

be easily distinguished as the concerned protons have different values of the chemical shift as well as coupling constants.

Ex: High resolution PMR spectroscopy has been used to distinguish between cis and trans decline.

2) Determination of the molecular weight : Proton NMR spectroscopy has been used to determine

the molecular weight of an unknown compound by comparing the intergrated instensities of an added standard and a recongnisabel peak or group of peaks of the unknown in a solution containing a known weight of a each substance.

3) Determination of the relative amounts of tautomers in a tautomeric mixture:

Proton NMR spectroscopy has been used to Study the keto enol equilibrium β – diketones and β –

ketoesters Ex: Acetyl acetone

Enol protons: keto protons = 8:2 or 4:1 Thus we say that 80% of enolic form and 20% of

ketonic form are present in the equilibrium mixture of acetyl- acetone.

The nmr spectrum of pure acetylacetone shows signal for the following kinds of equivalent protons.

NMR has potential application in the fields of agriculture. The distribution of water, oil the nature of binding of water, the germination process can all be investigated. The 1H – MRI, in a specific variety of green chilli, for example, clearly demonstrantes there are different distribution of water, and oil in a specific variety of ground nut seed has been investigated

4) Clinical and agricultural application: The MRI is able to defect motion of the blood flow because the oxygenated and deoxygenated blood carry distinct NMR signals. While most MRI and in-vivo studies have been performed using PMR, phospours is another NMR abundance finds numerous applications in metabolic and other physiological studies.

General Applications of NMR Spectroscopy :

NMR is used in biology to study the biofuids, cells, perfused organs and bio macromolecules such as nucleic acids (DNA. RNA) carbohydrates proteins and peptides, And also labeling studies is biochemistry.

NMR is used physics and physical chemistry to study high pressure diffusion, Liquid crystals liquid crystal solutions, menbranes, rigid solids.

NMR is used in pharmaceutical science to study pharmaceutical and drug metabolism.

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