NMR spectroscopy by G Shashikanth

NMR spectroscopy

1. CHEMICAL SHIFT AND SPIN SPIN COUPLING UNDER THE GUIDENCE OF : PRESENTED BY: M. ANURADHA GUNTI.SHASHIKANTH M. Pharm, ( Ph.D ) M.PHARMACY 1 ST YEAR ASSISTANT PROFESSOR PHARMACEUTICAL ANALYSIS DEPARTMENT OF PHARMACY UNIVERSITY COLLEGE OF TECHNOLOGY,OU

2. NUCLEAR MAGNETIC RESONANCE (H 1 ) The radio frequency radiation (RFR) is used for NMR spectra. The frequency is 10 7 Hz. The energy of RFR is expressed by E= hv ( planck constant). h= 6.624 × 10 −27 v= 10 7 HZ E= 6.624 × 10 −27 × 10 7 E= 6.624 × 10 −20 The energy is not sufficient to induce either of excitation, vibration , rotation (depends on own state). This energy is sufficient to effect the “nuclear spinning of an atom of a molecule” of sample. Or induce “flipping process” or effect the nuclear spinning either clock wise or anti clock wise.

3. We are not creating the spinning we can change the direction of spin that is called induce flipping process. A case analytical study ,in which the presence of external magnetic field (EMF) and the absorption of Radiofrequency radiation (RFR) is called nuclear magnetic resonance. The possibility of NMR spectra is based in on the 3 conditions . 1.RFR 2.EMF 3.Nuclear spinning .

4. To calculate Δ E value we need to have 2 0r more energy levels. In the sample protons & neutrons produce magnetic filed but very little /low so, energy not enough to RFR . EMF 50% energy+ sample have 50%...then net energy is 100%...it leads to formation of orbit those orbit is called as Precessional orbit. From the orbit release Precessional frequency . PF=RFR absorption takes place

5. Absence of external magnetic field , there is a average spin energy state hence RFR cannot absorbed in the presence of EMF , there is a clear difference between 2 spin energy state. Hence radio frequency radiation Can be absorbed . In the ground state , magnetic filed created by spinning nucleus is aligned with external magnetic field is called Precissional orbit with some energy is called pressional frequency. In the excited state, direction of spinning nuclei is anti –aligned with the direction of EMR .Hence it never produce pressional orbit . When the pressional frequency 100% is equal to the frequency of RFR(100%) then absorption takes Place.

6. s.no Mass no Atomic no Spin quantum no ( I ) spinning 1 even even I = 0 No spin 2 even odd I > 1 spin 3 odd odd / even I > 0 Spin

7. 2 nd condition always produce numerical value 1,2,3,4,5, etc.. Depends up on atomic weight. 3 rd condition always integral value 1/2 , 3/2, 5/2, 7/2, 9/2,etc..

8. NMR spectroscopy consists of 4-steps: 1.Number of signals : it tells us how many kind or type of protons are present in molecule. 2.Intensity of signals : it tells us the number number of protons in each kind (hight of the peak). 3.position of signal : it tells us the environment of a proton in each kind . 4.Spin –spin splitting of signals : it tell us the environment pf proton with other adjacent proton. 1.number of signals: A set of proton with identical environment are called equivalent protons. A set of protons with different environment are called non-equivalent protons.

9. Examples:1 Chemically equivalent protons are magnetically equivalent hence which are NMR signal on the spectra . example: Benzene it gives as a 1 signal. Example2 : Acetone gives 1 NMR signal Example 3: Ethanol gives 3 NMR signals

10. i . Alkene class: Always look for CIS and TRANS arrangement of hydrogen with respect to bulkier groups. Example: 1 Ethene gives 1NMR signal Example : 2 These particular Alkene class gives 4 NMR signals

11. ii. AROMATIC CLASS:- Always look for environment of a proton At Ortho , Meta , Para positions. EXAMPLE-1 Methyl benzene gives 4 NMR signals. EXAMPLE -2 Dimethyl benzene gives 2 NMR signals.

12. iii. Cycloalkanes:- Always look for the environment of proton towards above the plane and below the plane. Example 1 methyl cyclopropane gives 4NMR signals. Some Examples like : 1 ) methyl cyclobutene it gives 6 NMR signals. 2 ) dimethyl cyclopropane it gives 2 NMR signals etc..

13. 2.Position of signals: The absorption signals are measured by chemical shift values. Chemicals shift is the difference between reference standard and sample NMR signal the chemical shift scale is range 0-10 delta (PPM). Example: chloroethane(CH3-CH2-Cl).

14. change in the chemical shift value is due to shielding effect 2 . desheilding effect Sheilding effect (up field effect): The influence of EMF on to a nucleus is less due to more number of circulating electrons(higher secoundary induced magnetic field)hence it requires excess strength of EMF to create pressional orbit thus results in position Of NMR signal more towards up – field and the chemical shift values are lower. 2. Deshielding effect(Down field effect) The influence of EMF on to a nucleus is more due to less no of circulating electrons (low secondary magnetic Field). Hence it the normal strength of EMF is sufficient to produce pressional orbit. Thus result in position of NMR Signal more towards downfield and downfield and the chemical shift values are higher.

15. Factors effecting chemical shift value: 1.Inductive effect : by introducing electron withdrawing groups(F ,Cl , Br , I) it gives desheilding effect . F > Cl > Br > I When proton is attached with any electronegative group atom then proton will desheilded and delta values will be increased . Example : CH3 F (4.26) > CH3 Cl (3.05) > CH3 Br (2.68) > CH3 I (2.16) > CH4 (0.23) delta values. 2.Hydrogen bonding: causes desheilding effect. 3.Space effect( inisotropic effect ): Exhibit either shielding or desheilding based on special arrangement of H – atoms in multiple bonded molecule. e.g.: alkynes (acetylene) alkenes (ethene) Acetylene : it is linear molecule Because of linearity nature whatever the proton around the hydrogen atom the direction is completely to opposite to the direction of EMF because of opposite direction it cause shielding effect & that shielding gives lower chemical Shift value (2-3 delta ).

16. Diamagnetic protons : whatever the shielding proton is opposite direction to EMF that is called Diamagnetic protons( diamagnetic protons are shielding protons).

17. Alkene :

18. This is not linear there by whatever the 2 0 _ IMF electrons around the protons is aligned with same direction with magnetic field created by EMF so it exhibit desheilding effect ( delta value is 4-6 ) such a protons called paramagnetic protons. chemical shift values: R – CH3 (methyl) 0.9 R - CH2 1.2 R - CH 1.8 CH2 2-3

19. 3.Intensity of signal More no of protons will have high intense NMR signal, less number of protons will have low intense NMR signals. CH3 - CH2 – Cl ( chloro ethane )

20. 4.Spin – spin splitting Interaction of set of protons with other adjacent non equivalent protons leads to split within a signal is called spin-spin splitting or spin spin coupling. Example: 1 CH3 – CH2 – Cl ( cholro ethane ). CH3- CH-Cl , Cl – CH2- CH –Cl .

21. Example 2: CH3- CH- Cl2 (chloroethane)

22. Spin –spin splitting possible in non –equivalent protons Rules: Rule1: equivalent protons do not undergo spin spin splitting process. Ex: benzene, acetone. Rule2: even through non equivalent protons are present there is no splitting. No splitting because adjacent protons are not present so ultimately no splitting. Rule3 : N=n+1 N= number of peaks N= number of adjacent protons( non- equivalent). n=1 doublet (1:1) n=2 triplet (1:2:1) n=3 quartet (1:3:3:1)

23. Pascals rule :

24. Coupling constant (J): Measure the distance existing between centres of 2 adjacent split within a multiplet. Coupling scale consist of 0 to 20 hz (j=0-20 hz). Coupling constant independent on the EMF but it depends on spatial arrangement of atoms based on the shape of molecule .

25. For geminal protons the coupling constant value (2-18 hz). For vicinal protons the coupling constant value (0 -10 hz) . Based on the spatial arrangement of H atom j Gem > j vicinal

26. References: 1.Organic spectroscopy by William kemp ( third edition). 2.Introduction to spectroscopy by Pavia ( 5th edition ). 3.Instrumental method of chemical analysis by G.R Chatwal. 4.Text book of pharmaceutical analysis by Dr . S. Ravi Sankar. 5.A text book of pharmaceutical analysis by G. Vidyasagar volume -1.