INTERPRETATION OF 1H NMR : INTERPRETATION OF 1H NMR B.LAKSHMI PRASANNA
CHALAPATHI INSTITUTE OF PHARMACEUTICAL SCIENCES INTRODUCTION:- : INTRODUCTION:- NUCLEAR MAGNETIC RESONANCE
It is concerned with the magnetic properties of certain atomic nuclei.
Involves change in the spin state at the nuclear level.
proton acts as a tiny spinning
bar magnet and possesses both
electrical charge and mechanical spin. 2 SHELDING & DESHELDING : 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. 3 CHEMICAL SHIFT : 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 4 SCALE OF NMR SPECTRUM : SCALE OF NMR SPECTRUM 5 δ τ Highly shielded signals up field Deshielded signals downfield FACTORS INFLUENCING CHEMICAL SHIFT: : 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.
Methine group appear at greater chemical shift than methylene and methyl
Methine > methylene > methyl.
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. 6 SPIN-SPIN COUPLING & SPIN SPIN SPLITTING : 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. 7 PASCAL’S TRIANGLE: : 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. 8 (n+1) Rule: : (n+1) Rule: If a signal is split by non equivalent protons, it split in to n+1 peaks. 9 COUPLING CONSTANT(J) : 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). 10 NMR SPECTRUM : NMR SPECTRUM A spectrum of absorption of radiation Vs applied magnetic strength is called as NMR spectrum.
1. Number of signals
2. Their intensity (as measured by area under peak)
3. Splitting pattern (multiplicity) 11 NUMBER OF SIGNALS : 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. 12 1 x H environments so 1 peaks in NMR spectrum. Slide 13: 13 4 x H environments so 4 peaks in NMR spectrum. INTENSITY OF PEAKS : 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. 14 SPLITTING PATTERN(MULTIPLICITY) : 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) 15 Slide 16: CH3 Funct. Grp + + = Peak is from Splitting indicates Chemical shift Final structure
a CH2 adjacent CH3 indicates adjacent
functional group Slide 17: 17 1H NMR Absorptions by type ofcompound: : 1H NMR Absorptions by type ofcompound: Alkanes:
Alkanes can have three different types of hydrogens (methyl, methylene, and methyne)
Methyl > methylene > methyne. 18 Octane: : Octane: 19 CH3 CH2 Alkenes: : Alkenes: 20 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). 2-butene: : 2-butene: 21 CH3 CH Aromatic compounds: : Aromatic compounds: 22 Aromatic compounds have two type of hydrogens: aromatic ring hydrogens (benzene ring hydrogen) and benzylic hydrogens(those attached to an adjacent carbon atom) Benzene: : Benzene: 23 Alkynes: : Alkynes: 24 Pentyne: Alkyl halides: : Alkyl halides: F > Cl > Br > I 25 Methyl bromide : Methyl bromide 26 Alcohols: : Alcohols: Alcohols have two type of hydrogen's: hydroxyl proton and α hydrogen's. 27 Ethanol: : Ethanol: 28 CH2 OH CH3 Ethers: : Ethers: The hydrogen's on the carbons attached to the oxygen are deshielded due to the electro negativity of the oxygen. 29 Propyl ether: : Propyl ether: 30 Amines: : 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) 31 Propyl ether: : Propyl ether: 32 Nitriles: : Nitriles: The hydrogen's are slightly deshielded by the cyano group. 33 Acetonitrile: : Acetonitrile: 34 Aldehydes: : Aldehydes: 35 Acetaldehyde: Ketones: : Ketones: 36 Acetone: CH3 Esters: : Esters: Two types of hydrogens are found I esters:- alcohol part & acid part.
2.1-2.5 ppm 3.5-4.8 ppm
ETHYL ACETATE: 37 - 2HC-C-O-CH2 - O Carboxylic acids: : Carboxylic acids: These have acidic protons, and the α hydrogen. 38 Acetic acid: REFERENCES : REFERENCES Introduction To Spectroscopy, Pavia, 4th Edition, pg.no:127-161.
Elementary Organic Spectroscopy, Y.R.Sharma,pg.no:189-192.
Organic Spectroscopy, William Kemp, 3rd Edition,Pg.no:102-110.
Spectroscopy Of Organic Compounds , P.S. Kalsi, 6th Edition,pg.no:193-196.
http//:youtube.com/watch1H NMR spectrum IB Chemistry 39 Slide 40: 40