9. m. pharm interpretation of 1 h nmr jntu pharmacy

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NMR Spectroscopy

BYDr. Suman PattanayakAssociate ProfessorDepartment of Pharma Analysis & QA.

Vijaya Institute of Pharmaceutical Sciences for Women

M. Pharm/ I SemPharmaceutical Analysis

INTRODUCTION:-NUCLEAR MAGNETIC RESONANCE (NMR Spectroscopy) It is concerned with the magnetic properties of certain atomic

nuclei.

Involves change in the spin state at the nuclear level.

SPINNIG NUCLEUS: proton acts as a tiny spinning bar magnet and possesses both electrical charge and mechanical spin.

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SHELDING & DESHELDING The proton is shielded when the induced field opposes the applied

field.

Higher the shielding of protons lower will be the value of δ.

The proton is deshielded when the induced field reinforces the

applied field.

Greater the deshielding higher will be the value of δ.

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CHEMICAL SHIFT Each different hydrogen environment will appear in a different

position in the NMR spectrum. This is called Chemical Shift.

The chemical shift is generally expressed in parts per million

(ppm).

It is measured by δ scale, τ scale.

τ = 10 – δ

Greater the deshielding of protons, larger will be the value of δ

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SCALE OF NMR SPECTRUM

01.02.03.04.05.06.07.08.09.010.0

δ

10.09.08.07.06.05.04.03.02.01.00τ

Highly shielded signals up field

Deshielded signals downfield

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FACTORS INFLUENCING CHEMICAL SHIFT:Electro negativity effect:

Chemical shift increases as the electro negativity of the attached element increases.

Multiple substituent's have a stronger effect than a single substituent.

• Hybridization effect:Methine group appear at greater chemical shift than methylene

and methyl Methine > methylene > methyl.

• Hydrogen bonding: Acid hydrogens: least shielded protons are those attached to

carboxylic acids.Hydrogen bonding and exchangeable hydrogens: they are usually

found attached to a hetero atom.The more hydrogen bonding that takes place the more deshielded

a proton becomes.

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SPIN-SPIN COUPLING & SPIN SPIN SPLITTING

• The coupling interaction between two or more protons, most often

through the bonds,result in splitting of the spectral lines. this is

called spin-spin coupling.

It means that an absorbing peak is split by more than one

“neighbour” proton.

The spitting is a very essential part to obtain exact information

about the number of the neighbouring protons.

Chemical equivalent protons do not result in spin-spin splitting.

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PASCAL’S TRIANGLE:

The intensity ratios of multiplets derived from n+1 rule follows the entries in the mathematical device called pascal’s triangle.

Each entry in the triangle is the sum of entries above it and to it immediate left and right.

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(n+1) Rule: If a signal is split by non equivalent protons, it split in to n+1 peaks.

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COUPLING CONSTANT(J) The distance between the center of the two adjacent peaks in a

multiplet is usually constant and is called the coupling constant.

It is independent of the external field.

It is measured in Hertz(Hz) or in cps (cycles per second).

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NMR SPECTRUM

A spectrum of absorption of radiation Vs applied magnetic

strength is called as NMR spectrum.

It includes:-

1. Number of signals

2. Their intensity (as measured by area under peak)

3. Splitting pattern (multiplicity)

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NUMBER OF SIGNALS It shows how many different kinds of protons are present in the

molecule.

COUNTING HYDROGEN ENVIRONMENTS

One molecule can contain many hydrogen environments.

1 x H environments so 1 peaks in NMR spectrum.

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C C

CH3

H

H

H3C


H2C

CH2

OHH3C


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INTENSITY OF PEAKS The size or the area of each peak tells the number of protons in

each set number of protons each set.

Area under an NMR signal α number of protons giving rise to

signal.

Greater the number of protons → Greater will be the energy

absorbed & greater is the area under the absorption peak.

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SPLITTING PATTERN(MULTIPLICITY) The splitting of a signal; is due to the different environments of the

absorbing proton i.e., with respect to the nearby proton(proton

attached to the adjacent carbon atom)

H | ─C─ | H

CH

3 Func

t. G

rp.

CH3

Func

t. G

rp

+ + = H | ─C─ | H

Peak is from Splitting indicates Chemical shift Final structure a CH2 adjacent CH3 indicates adjacent functional group

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1H NMR Absorptions by type ofcompound:Alkanes: Alkanes can have three different types of hydrogens (methyl,

methylene, and methyne) Methyl > methylene > methyne.

Type of proton Chemical shift

R-CH3 0.7–1.3 ppm

R-CH2-R 1.2-1.4 ppm

R3CH 1.4-1.7 ppm

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Octane:

CH3

CH2

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Alkenes: Alkenes have two types of hydrogens: vinyl (those attached directly

to the double bond) and allylic hydrogens (those attached to the a

carbon, the carbon atom attached to the double bond).


C=C-H 4.5-6.5 ppm

C=C-C-H3 1.6-2.6 ppm

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2-butene:

CH3

CH

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Aromatic compounds:

Aromatic compounds have two type of hydrogens: aromatic ring

hydrogens (benzene ring hydrogen) and benzylic hydrogens(those

attached to an adjacent carbon atom)

Type of protons Chemical shift

Ar-H 6.5-8

Ar-C-H 2.3-2.7

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Benzene:

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Alkynes:

Pentyne:


C≡C-H 1.7-2.7 ppm

C≡C-CH- 1.6-2.6

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Alkyl halides:

F > Cl > Br > I


H-C-F 4-4.5

H-C-Cl 3-4

H-C-Br 2.5-4

H-C-I 2-4

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Methyl bromide

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Alcohols:

Alcohols have two type of hydrogen's: hydroxyl proton and α

hydrogen's.


C-OH 0.5-5 ppm

H-C-OH 3.2-3.8 ppm

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Ethanol:

CH2

OH

CH3

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Ethers:

The hydrogen's on the carbons attached to the oxygen

are deshielded due to the electro negativity of the

oxygen.


H-C-OR 3.3-4

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Propyl ether:

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Amines: Two type of hydrogen's are found in amines: those attached to

nitrogen (the hydrogen's of the amino group) and those attached to

the α carbon( the same carbon to which the amino group is

attached)


R-N-H 0.5-4.0 ppm -CH-N- 2.2-2.9 ppm

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Propyl ether:

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Nitriles:

The hydrogen's are slightly deshielded by the cyano group.


-CH-C≡N 2.1-3.0 ppm

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Acetonitrile:

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Aldehydes:Type of proton Chemical shift

R-CHO 9.0-10.00 ppm

Acetaldehyde:

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Ketones:Type of Proton Chemical shift

R-CH-C=O R

2.1-2.4 ppm

Acetone:

CH3

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Esters: Two types of hydrogens are found I esters:- alcohol part & acid

part.

2.1-2.5 ppm 3.5-4.8 ppmETHYL ACETATE:

- 2HC-C-O-CH2 -

O

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Carboxylic acids: These have acidic protons, and the α hydrogen.


R-COOH 11-12 ppm

-HC-COOH 2.1-2.5 ppm

Acetic acid:

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REFERENCES1. Introduction To Spectroscopy, Pavia, 4th Edition, pg.no:127-161.

2. Elementary Organic Spectroscopy, Y.R.Sharma,pg.no:189-192.

3. Organic Spectroscopy, William Kemp, 3rd Edition,Pg.no:102-110.

4. Spectroscopy, B.K.Sharma,638-643.

5. Spectroscopy Of Organic Compounds , P.S. Kalsi,

6th Edition,pg.no:193-196.

6. http//:wwwchem.uwimona.edu.jm/spectra/nmrintro.html

7. www.ebyte.it/library/educards/nmr/EM360Spectra

8. http//:youtube.com/watch1H NMR spectrum IB Chemistry

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Education

9. m. pharm interpretation of 1 h nmr jntu pharmacy