Nmr 2

nmr

1. Nuclear Magnetic Resonance -Roll no:-3

2. Introduction Nuclear Magnetic Resonance (NMR) is a spectroscopy technique which is based on the absorption of electron magnetic radiation in the radio frequency region by nuclei of atoms. The radio frequency radiation has the frequency range of 4-900 MHz corresponded to the wavelength region of 75-0.5 m. For example we can have NMR signals for the following atoms:- 1H and 13C.

3. Uses of NMR To record differences in the magnetic properties of various magnetic nuclei present. To study different kind of environment within the molecule. To study which atoms are present in neighbouring groups. To deduce the position of nuclei within the molecule.

4. Spinning Nucleus (Principle) Nucleus of the hydrogen atom(proton) behaves as a tiny spinning bar magnet as it possesses both electric charge and mechanical spin. Any spinning charged body , generate a magnetic field.

5. Effect of an external magnetic field Proton responds to the influence of an external magnetic field by aligning itself with that field. Proton can only adopt two orientations to the external magnetic field. Aligned with the field (lower energy state or parallel). Opposed to the field (higher energy state or antiparallel ).

6. Spin Quantum Number Spin Quantum no.(I) is related to the atomic and mass number of the nucleus. Elements with odd mass number or odd atomic number have nuclear spin. Mass N0. Atomic No. Spin No.(I) Eg . odd Odd/even ½,3/2,5/2..... 1H,11B even even 12C,16O even odd 1,2,3,... 2H,14N

7. Shielding and Deshielding Upfield :- The nucleus feels weaker magnetic field. Shielding:- A nucleus whose chemical shift has been decreased due to the addition of electron density , magnetic induction or other effects.

8. Deshielding Downfield:- The nucleus feels stronger magnetic feild . Deshielding :- A nucleus whose chemical shift has been increased due to removal of electron density, magnetic induction or other effects.

9. Chemical Shift Chemical shift is difference between the absorption position of the sample proton and absorption position of reference standard. Variation of position of NMR absorption due to the electronic shielding and deshielding .

10. Measurement of chemical Shift For protons and 13C ,the accepted reference is Tetramethylsilane , TMS. Low boiling point. Soluble in most organic solvents. Not soluble in water.

11. The chemical shift numbers (signals) of Hs depend on the surrounding (neighbours) atoms. The higher chemical shift are for most deshielded Hs and lower chemical shift are for most shielded Hs. In the above spectrum which is for ethylbenzene that Hs in the benzene ring are highly deshielded giving the NMR signal around 7.1 ppm . The Hs in CH2 are close to the benzene ring appearing at 2.6 ppm . The Hs in Ch3 are far away from the benzene ring , more shielded , giving NMR signal at 1.6 ppm .

12. 1H NMR chemical shift Protons in different environments experience different degrees of shielding and have different chemical shifts.

13. ethylbenzene

14. electronegativity The chemical shift simply increases as the electronegativity of the attached element increases. The greater the electronegativity of the substituents , the more deshielding of protons and hence greater is the chemical shift of those protons.

15. Interpretation of NMR spectra Numbers of signals- indicates how many different kind of protons presents. Position of signals-indicates magnetic environments of protons. Relative intensity of signals-proportional to numbers of protons present. Splitting of signals- indicates the number of nearby nuclei usually protons.

16. 1H NMR – number of signals The number of NMR signals equals the number of different types of protons in a compound. Protons in different environments give different NMR signals. Equivalent protons give the NMR signal.

17. CH3-O-CH3 ↑ ↑ H(a) H(a) All equivalent Hs 1 NMR signals. CH3-CH2-Cl ↑ ↑ H(a) H(b) 2 types of Hs 2 NMR signals.

18. CH3-O-CH2-CH3 ↑ ↑ ↑ H(a) H(b ) H(c) 3 types of Hs and 3 NMR signals

19. 1H NMR –Positions of signals CH3-CH2-Cl ↑ ↑ H1) H(2) The H(2) protons are deshielded because they are closer to the electronegative cl atom, so they absorb downfield from H(1). BrCH2CH2F ↑ ↑ H(1) H(2) Because F is more electronegative than Br, the H(2) protons are more deshielded than Ha protons . And absorb downfield.

20. ClCH2CHCl2 ↑ ↑ H(a) H(b) The larger number of electronegative Cl atoms deshields H(b) more than H(a), so it absorbs downfield from H(a).

21. 1H NMR- Intensity of signals The area under an NMR signal is proportional to the number of absorbing protons. An NMR spectrometer automatically integrates the area under the peaks , and prints out a stepped curve on the spectrum. Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units.

22. Example Multiplicity of NMR signals:- The NMR signals of Hs can have multiplicity depending on numbers of neighbour Hs. Number of multiplicity= n+1 n is the number of neighbour Hs. Multiplicity Intensity Ratio Singlet (S) 1 Doublet(D) 1:1 Triplet(t) 1:2:1 Quartet(Q) 1:3:3:1 Quintet 1:4:6:4:1 Sextet 1:5:10:10:5:1 septet 1:6:15:20:15:6:1

23. Spin- Spin Coupling Splitting in other words. Protons on nearby carbons will interact and split (division) each others resonance into multiple peaks (multiples) n+1 rule:- Equivalent protons that have n equivalent protons on the adjacent carbon will be split into n+1 peaks.

24. Limitations of NMR It is limited to the measurement of nuclei with magnetic moments. It may be less sensitive than other spectroscopic and chromatograhic methods of analyses. Requires computer modelling- time consuming and expensive.

25. Applications of NMR Material Science:- A powerful tool in research of polymer chemistry and physics. Chemical Analysis:- A matured technique for chemical identification of chemicals whether synthetic or natural. Hydrogen bonding  :-A unique technique for the DIRECT detection of hydrogen bonding interactions. Drug screening and design  :-Particularly useful for identifying drug leads and determining the conformations of the compounds bound to enzymes, receptors, and other proteins. Protein hydration  :-A power tool for the detection of interior water and its interaction with biomacromolecules .

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