NMR Spectroscopy
- 1. Presented by Rahul Chakraborty Student of Sem-(IV) , Chemistry honours Department of Chemistry Raiganj Surendranath Mahavidyalaya 1 NMR SPECTROSCOPY
- 2. Introduction Theory&principleofNMR Chemicalshift Sheilding&desheildingofprotons Effectof Resonanceinchemical shift Effectof Hydrogenbondingin chemicalshift Spin-spin splitting Couplingconstant KarplusEquation& KarplusCurve ApplicationsofNMR 2 Content
- 3. Nuclear magnetic resonance (NMR) is a spectroscopic technique which is based on the absorption of electromagnetic radiation in the radio frequency region 4 to 900 MHz by nuclei of the atoms. NMR Spectroscopy is used to determine the structure of a compound. 3 Introduction
- 4. Spin quantum number “I” is related to the atomic number and atomic mass of the nucleus. Atomic number Atomic mass NMR (Active/ Inactive) Spin quantum number (I) Examples Odd/Even Odd Active I= ½,3/2,5/2,... 1H1, 13C6, 19F9 etc. Even Even Inactive I=0 12C6,16O8 etc. Odd Even Active I=1,2,3,4,… 2H1,14N7 etc. 4 Theory of NMR
- 5. If external magnetic field is applied, then total number of possible orientation calculated by (2I+1). Hydrogen has spin quantum number I=½ and possible orientation is (2×½+1=2) two +½ & -½. 5
- 6. Magnetic nuclei have two type of of motions; spin and precession. The Spining nucleus generates a magnetic field. 6 Principles of NMR
- 7. The theory behind NMR comes from the spin of a nucleus and it generates magnetic field. If an external magnetic field is applied, an energy transfer (∆E) is possible between ground state to excited state. 7
- 8. When the spin returns to its ground state level, the absorbed radiofrequency energy is emitted at the same frequency level. The emitted radiofrequency signal that give the NMR spectrum of the concerned nucleus. 8
- 9. The emitted radio frequency 𝛎 is directly proportional to the strength of the applied field. ν= B0=External magnetic field experienced by proton. 𝛾 = Gyromagnetic ratio (The ratio between the nuclear magnetic moment and angular moment). 9
- 11. The following solvents are normally used in which hydrogen replaced by deuterium. CCl4 – Carbon tetrachloride. CS2 – Carbon disulfide. CDCl3 – Deuteriochloroform. D2O - Deuterium oxide. 11 Solvents used in NMr
- 12. A chemical shift is defined as the difference in parts per million (ppm) between the resonance frequency of the observed proton and tetramethylsilane (TMS) hydrogen. δx = the chemical shift of proton Hx, in ppm ν X = the frequency of signal for proton x in Hz ν TMS= the frequency of signal for TMS in Hz and ν0= the operating frequency of the instrument in MHz x S 0 Chemical shift 12
- 13. TMS is the most common reference compound in NMR, it is set at δ=0 ppm. 13 Internal standard Si CH3 H3C CH3 CH3 Tetramethylsilane 1. TMS having 12 magnetically equivalent protons gives a sharp peak even at low concentration, 2. Inert towards most of reagents , 3. Soluble in most of the organic solvents.
- 14. High electron density around a nucleus shields the nucleus from the external magnetic magnetic field and the signals are upfield in the NMR spectrum. Lower electron density around a nucleus deshields the nucleus from the external magnetic field and the signals are downfield in the NMR spectrum. 14 Sheiding & Desheiding of protons
- 15. Effect of resonance on chemical shift 15 In case of nitrobenzene, due to –R effect of –NO2 group both the o-positions and p- position become electron deficient i.e, desheilded. The protons at o-positions(Hc) and p-position (Ha) resonate higher δ values compared to m-Hb .
- 16. N O O N O O N O O etc. canonical forms showing position charge at o- and p-positions i.e, these positions are deshielded 16
- 17. 17 Similarly,the +R effect of –OCH3 group in anisole increases the electron density at both the ortho and para positions conseqently these positions become more sheilded.Thus the ortho-H and para-H resonate at lower ẟ values (upfield) compared to Hs of benzene (ẟ =7.37). OCH3 H H H H H
- 18. 18 canonical forms showing negative charge at o- and p-positions i.e, these positions are shielded OCH3 OCH3 etc. OCH3
- 19. H-bonding effect on chemical shift The chemical shift depends on how much hydrogen bonding is taking place (observed in high conc). Hydrogen bonding lengthens the O-H and reduces the valance electron density around proton. It is deshielded and shifted downfield in the NMR spectrum. Alcohols vary in chemical shift from 0.5 ppm (free OH) to about 5.0 ppm (lots of H-bonding). O H R O R H O R H 19
- 20. The interaction between the spins of neighboring nuclei in a molecule may cause the splitting of NMR spectrum.This is known as spin-spin coupling or splitting. The number of peaks in a PMR signals for a particular set of protons depends on the number of equivalent protons (n) in the neighbouring C-atoms. 20 Spin-spin splitting
- 21. The splitting of a signal can be predicted by (n+1) rule. Zero H atom as neighbour n+1=0+1=1(singlet) One H atom as neighbour n+1=1+1=2(doublet) Two H atom as neighbour n+1=2+1=3(triplet) 21
- 22. 22 C C H C no of peaks=(n+1)=(0+1) =1(singlet)
- 23. 23 C C H H no of peaks=(n+1)=(1+1) =2(doublet)
- 24. 24 C C H H H no of peaks=(n+1)=(2+1) =3(triplet)
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- 26. 26 C H H C H C H H A A chemically equivalent H's No of lines = (nA+1) = 4+1= 5 (quintet)
- 27. 27 C C C H H C R C H H H R H A B C Chemically non equivalent H's no of peaks=(nA+1)(nB+1)(nC+1) =(2+1)(1+1)(2+1) =18 (multiplet)
- 28. 28 Example Of Spin-spin splitting CH3 C H2 H3C O 2.09 2.49 1.06 0 1 2 PPM Singlet Quartet Triplet TMS 2-Butanone
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- 30. N+ O O- 4.41 1.95 0.96 0 1 2 3 4 PPM H3C H2 C H2 C NO2 a b c Ewg(-NO2) is present deshelding of proton Ha value is more. a b c Triplet Multiplet Triplet 1-Nitropropane TMS 30
- 31. The distance between the peaks of a doublet or triplet or quartet is called the Coupling constant (J). The coupling constant denoted by J . Coupling constant are measure of the effectiveness of spin-spin coupling and very useful in 1H NMR of a complex structures. 31 Coupling constant
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- 33. 33 Vicinal Coupling ConstantS: Karplus equation The values of vicinal coupling constant (3JH-C-C-H) varies from 0-16 Hz depending upon the dihedral angle (ϕ). The relationship between dihedral angle (ϕ) & vicinal coupling constants (3JH-C-C-H) is given by the Karplus equation. The coupling constants gives information about the molecular structure/geometry.
- 34. 3JHA-HB =8.5 cos²ϕ – 0.28 for 0°≦ϕ≦90° 3JHA-HB=9.5 cos²ϕ – 0.28 for 90°≦ϕ≦180° 34 3JHA-HB =J0 cos²ϕ – 0.28 3JHA-HB =J180 cos²ϕ – 0.28 Karplus curve 3 J HA-HB (Hz)→ Dihedral angle (ϕ)→
- 35. 35 cis coupling & trans coupling H3C CH3 CH3 H3C cis-2-butene trans-2-butene H H H H Doublet Doublet Trans Coupling Cis Coupling J=12-18 Hz J=5-12 Hz
- 36. H H Ortho coupling =7-10 Hz Ortho, meta & coupling H H H H ortho coupling =7-10 Hz meta coupling =2-2.5 Hz H H para coupling < 0.5 Hz 36
- 37. 37 Applications of nmr Detection of hydrogen bonding: intermolecular hydrogen bonding shifts the absorption for a concerned proton downfield. • Splitting pattern : how many neighbouring hydrogens. Major application in chemistry is to determine the structure of molecules. • Medical practitioners employ magnetic resonance imaging (MRI),a multidimensional NMR imaging technique , for diagnostic purposes.
- 38. 38 references Pavia, Introduction to Spectroscopy , (fifth edition) Organic Spectroscopy by William Kemp, (third edition) Organic Molecular Spectroscopy- Ajay Kumar Manna Elementary Organic Spectroscopy , Y.R.Sharma , S Chand
- 39. AKNOWLEDGEMENT 39 I would to like to express my gratitude and appreciatiation to all who gave me this opportunity to complete this seminar. Also, i would like to express my deep sense of gratitude to our chemistry teacher Dr. Sujit Ghosh under whose valuable guidance, this, seminar has been carried out. I would like to extend my special thanks to our HOD Dr. Kamala Bhattyacharya, Sumi Saha, Mini Ghosh, Subrata Basak & my classmates, without their support and coordination we would not have been able to complete this seminar.
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