Introduction To Proton NMR and Interpretation

In this slide, we'll discuss proton MR and how to interpret the 1H spectra.

1. PROTON NMR AND 1 H SPECTRA INTERPRETATION Presented by Aamir Malik Khalid Lateef M.PHARM FIRST YEAR(Sem. II) Department of Pharmaceutical Chemistry

2. Contents Introduction 1 H NMR (PROTON NMR) Principle Instrumentation Relaxation process Chemical shift Splitting of signals NMR spectra and interpretaion References 2

3. Introduction to NMR It is 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. 3

4. Proton NMR It is a technique which is based on the absorption of electromagnetic radiation in the radio frequency region 4 to 900 MHz by nuclei of the atoms. It is used to study a wide variety of nuclei: 1 H , 15 N, 19 F, 13 C, 31 P. The most common form of NMR is based on the hydrogen-1 ( 1 H), nucleus or proton. Shows how many kinds of nonequivalent hydrogen’s are in a compound. Equivalent H’s have the same signal while nonequivalent are “different” and as such may cause additional splitting (diastereotopic effect). 4

5. Principle Protons in different environments absorb at slightly different frequencies, so they are distinguishable by NMR. The frequency at which a particular proton absorbs is determined by its electronic environment. The size of the magnetic field generated by the electrons around a proton determines where it absorbs. Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of frequencies is applied to achieve the resonance of all protons. Only nuclei that contain odd mass numbers (such as 1 H, 13 C, 19 F and 31 P) or odd atomic numbers (such as 2H and 14N) give rise to NMR signals. 5

6. - spin states-spin states absorb Release ∆E Signals detected by NMR 6

7. Instrumentation A magne t to separate the nuclear spin energy state. Two RF channels , one for the field/frequency stabilization and one to supply RF irradiating energy. A sample probe , containing coils for coupling the sample with the RF field it consists of Sample holder, RF oscillator, Sweep generator and RF receiver. A detector , to process the NMR signals. A recorder , to display the spectrum. Basically NMR instrumentation involves the following units. 7

8. Schematic diagram of NMR spectrophotometer 8

9. Relaxation Process Two kinds of relaxation processes are: Spin-spin relaxation: This takes place by transferring energy to neighboring nucleus. A nucleus in the upper energy state can transfer its energy to a neighboring nucleus by mutual exchange of spin. Spin- Lattice Relaxation: It involves the transfer of energy from the nucleus in its higher energy state to the molecular lattice, the energy is transferred to the components of lattice as the additional translational, vibrational & rotational energy, and this process keeps the excess of nuclei in the lower energy state which is necessary for NMR Phenomenon. 9

10. Chemical Shift “Chemical shift is the difference between the absorption position of the sample proton and the absorption position of reference standard.” The position in NMR spectrum where signal occurs is called chemical shift. It indicates how far the signal is from the TMS reference peak. Chemical Shift, ppm (d )= Shift from TMS in Hz X 10 6 Spectrometer frequency( Mhz ) 10

11. Splitting of signals Each signal in an NMR spectrum represents one kind or one set of protons in a molecule. It is found that in certain molecules, a single peak (singlet) is not observed, but instead, a multiplet (groups of peaks) is observed. E.g. A molecule of CH3CH2Br, ethyl bromide. 11

12. Pascal’s Triangle n(no of neighboring H) Name Signal Row Singlet 1 1 Doublet 1 1 2 Triplet 1 2 1 3 Quartet 1 3 3 1 4 Quintet 1 4 6 4 1 5 Sextet 1 5 10 10 5 1 6 Septet 1 6 15 20 15 6 1 7 Octet 1 7 21 35 35 21 7 1 12

13. 1 H NMR Spectra 1 H NMR spectra consist of signals originated from hydrogens or protons present in molecule. Only those hydrogens will give signals which are having different environment. Hydrogens having same chemical environment will only appear as a single signal. 13

14. Steps in 1H spectra interpretation 1.Number of signals Indicate how many different kinds of protons present. 2.Position of signals Indicate something about (chemical shift), magnetic (electronic) environment of protons. 3.Relative intensity of signals Proportional to number of protons present 4.Splitting of signals (spin spin coupling) Indicate the number of near by nuclei usually protons. 14

15. Chemical Shift table for different groups 15

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17. Example of NMR spectra CH 3 CH 2 Br, ethyl bromide. Triplet = δ - 1.7,3H (CH 3 ) Quartet= δ -3.4, 2H (CH 2 ) 17

18. a b c CH 3 COCH 2 CH 3 , 2-Butanone c a b ( a)Singlet at 1.7 (b)Quartet at 2.5 (c)Triplet at 1.1 18

19. Toluene, a b ( a)Singlet at 2.3 (b)Multiplet at 7.2 19

20. a b a Octane , CH 3 (CH 2 ) 6 CH 3 (a)Triplet at 0.88 (b)Singlet at 1.26 a b 20

21. CH 3 Alkene, CH 3 CH 2 CH 2 C=CH 2 a b d c e (a)Triplet at 0.9 (b) Sixtet at 1.5 (c)Singlet at 1.8 (d)Triplet at 2 (e)Singlet at 4.6 21

22. Ester, CH 3 C-O-CH 2 -CH O CH 3 CH 3 22

23. Ethanol , CH 3 CH 2 OH a c b ( a)Triplet at 1.3 (b)Singlet at 2.5 (c)Quartet at 3.8 23

24. Propanoic acid, CH 3 CH 2 COOH a b c ( a)Triplet at 1.2 ( b)Quartet at 2.3 ( c)Singlet at 11.2 a b c 24

25. References Doglas A Skoog, F. James Holler,Timothy A. Nieman, Principles of Instrumental Analysis - 5th edition, Eastern press, Bangalore, 1998,pp no.551. Donald L.Pavia, Gary M.Lampman and George S.Kriz, “Introduction to Spectroscopy”, 3rd edition, Thomsan Learning 60 Albert street, Asia complex, Singapore.pp.102-111 and 139-150 Brian S. Furnis, Antony J.Hannaford, Peter W.G. Smith, Austin R. Tatchell; Vogel’s textbook of practical organic chemistry, 5 th edition , PEARSON publications. https:// en.m.wikipedia.org/wiki/Proton_nuclear_magnetic_resonance 25

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