ACHARYA NAGARJUNA UNIVERSITY COLLEGE OF PHARMACEUTICAL SCIENCES

PRESENTED BY O.SASIVARDHAN

Roll.No:Y15MPH326 PHARMACEUTICAL ANALYSIS

INTERPRETATION OF NMR SPECTROSCOPY

Contents1. How NMR Works2. Information Obtained From NMR

Spectrum3. Interpretation of NMR Spectrum4. Conclusion5. References

How NMR Works

BO

Nuclear spins random Nuclear spins will aligned or oppose the field

Oppose with BO (α- spin state)

Aligned the BO (β-spin state)

Energy

β

EE R

Flipping Relaxation

01.02.03.04.05.06.07.08.09.010.0

Chemical shift (d, ppm)

CCH2OCH3N

OCH3

NCCH2O

Information Obtain From Proton NMR Spectrum

1. # of signals indicate the no of different types of hydrogen's (chemical equivalence)

2. Integration or peak area indicates how many hydrogen are in each signal. It is given in ratio

3. Chemical shifts are given in δ (delta) values, the Chemical shifts values indicate the electronic environment of the hydrogen's (shielded or de-shielded

4. Splitting patterns indicate the # of neighbouring hydrogen's. The magnitude f the coupling constants (J values) depend upon the spatial relationship of the two atoms.

Interpretation Of Proton NMR Spectra

Hydrogen atoms in different environments respond differently to the field

Each different environment of protons produce signal in a different positions

Protons can classified as 1. Equivalent Protons2. Non-Equivalent protons

Equivalent protons will shows single signal Non – equivalent protons will shows more than

one signal.

1.Number Of Signals In Proton NMR

• Equivalent ProtonsEx. Methane

• Non-Equivalent protons

Ex.acetaldehydeH

H

H

HC H

H

H

HCC

o

H3CCH2CH3chemically equivalent

CH3CH2CH2ClClCH2CH2CH3

Chemically equivalent protons

Replacing protons at C-1 and C-3 gives same compound (1-chloropropane)C-1 and C-3 protons are chemically equivalent and have the same chemical shift

REPLACEMENT TESTChemically equivalent protons

Replacement by some arbitrary test group generates Diastereoisomers

Diastereotropic protons can have differentchemical shifts

Diastereotropic protons

C C

Br

H3C

H

H

d 5.3 ppm

d 5.5 ppm

Chemically Non Equivalent Protons

Enantiotropic protons

Are in mirror-image environmentsReplacement by some arbitrary test

group generates enantiomersEnantiotropic protons have the same

chemical shift

C CH2OH

H3C

HH

Enantiotropicprotons

C CH2OH

H3C

ClH

C CH2OH

H3C

HCl

R S

2. 1H NMR— PEAK AREA

Peak area proportional to hydrogen are in each signal.

It is given in ratio.

Methyl a,a-Dimethylpropionate

METHYL ACETATE

3. 1H NMR— CHEMICAL SHIFTSThe position Of the signals in the

spectrum helps to know the nature of protons viz . aromatic, aliphatic. Acetylinic, vinylinic, adjacent to some electron attracting or electron releasing group.

Each of these types of protons will have different electronic environments and thus, they absorb at different applied field strengths.

When a molecule is placed in a magnetic field, its electrons are caused to circulate and thus, they produce secondary magnetic fields i.e. induced magnetic field.

Rotation of electrons (specially pie electrons) about the nearby nuclei generates a field that can either oppose or reinforce the applied field at the proton.

If the induced field opposes the applied field, then proton is said to be Shielded*.

But, if the induced fielded reinforce (added strength) the applied, then proton feels a higher field and thus, such a proton is said to be Deshielded*.

Such shifts (compared with a standard reference) in the positions of NMR absorption which arise due to shielding or deshielding of protons by the electrons are called Chemical shifts*.

The degree of shielding depends on the density of the circulating electron.

Primary RCH3 0.9

Secondary R2CH2 1.3

Tertiary R3CH 1.5

Vinylic C=C-H 4.6-5.9

Allylic C=C-CH3 1.7

Chemical shifts for various types of protons with TMS as standard reference

Aromaric Ar-H 6-8.5

Alcohols HC-OH 3.4-4Ethers HC-OR 3.3-4Esters HCOOR 2-2.2Aldehydes R-CHO 9-10Amino R-NH2 1-5Carboxylic R-COOH 10.5-12Phenolic Ar-OH 4-12Hydroxylic R-OH 1-5.5Fluorides HC-F 4-4.5Chlorides HC-Cl 3-4Bromides HC-Br 2.5-4

Methyl Acetate

CO

R O

H3C C O

Base Chemical Shift = 0.87 ppm one = 2.88 ppm TOTAL = 3.75 ppmO

CH3

CO

R

Base Chemical Shift = 0.87 ppm one = 1.23 ppm TOTAL = 2.10 ppm

2,3-Dimethyl-2-Butene

(Hydrogen under consideration)Base Chemical Shift = 0.87 ppm one (CH3) = 0.78 ppm TOTAL = 1.65 ppm

H2C CH

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• Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons.

4. 1H NMR—Spin-Spin Splitting

Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs:• When placed in an applied field, (B0), the adjacent proton

(CHBr2) can be aligned with () or against () B0. The likelihood of either case is about 50% (i.e., 1,000,006 vs 1,000,000).

• Thus, the absorbing CH2 protons feel two slightly different magnetic fields—one slightly larger than B0, and one slightly smaller than B0.

• Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet, where the two peaks of the doublet have equal intensity.

Spin-Spin Splitting in 1H NMR Spectra• Peaks are often split into multiple peaks due to magnetic

interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting

• The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule”

• The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle

• The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5=pentet, 6=hextet, 7=heptet…..)

1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 11 6 15 20 15 6 1

singletdoublettripletquartetpentethextetheptet

Rules for Spin-Spin Splitting• Equivalent protons do not split each other

• Protons that are forther than two carbon atoms apart do not split each other

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Splitting is not generally observed between protons separated by more than three bonds.

If Ha and Hb are not equivalent, splitting is observed when:

CONCLUSIONNMR Spectrum is a qualitative tool widely used in

pharmaceutical ,chemical and fertilizer industry’s for structural elucidation of drugs, chemical and etc.

REFERENCES P.T.F. Williamson, M. Ernst, B.H. Meier: MAS Solid-State

NMR of Isotropically Enriched Biological Samples in BioNMR in Drug Research, Ed. O. Zerbe, Wiley 2003.

R. Tycko, Biomolecular Solid State NMR: Advances in Structural Methodology and Applications to Peptide and Protein Fibrils, Annu. Rev. Phys. Chem. (2001), 52, 575-606.

D.D. Laws, H.-M. Bitter, A. Jerschow, Solid-State Spectroscopic Methods in Chemistry, Angew. Chem. (2002), 41, 3096-3129.

S.J. Opella, C. Ma, F.M. Marassi, Nuclear Magnetic Resonance of MembraneAssociated Peptides and Proteins, Meth. Enzymol. (2001), 339, 285-313.

• Derome, A.E. Modern NMR Techniques for Chemistry Research, Pergamon: Oxford, 1987.

• Richards, S.A. Laboratory Guide to Proton NMR Spectroscopy, Blackwell: Oxford, 1988.

• Keeler, J. Chem. Soc. Rev., 1990, 19, 381.

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