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Nuclear magnetic resonance
- 1. -Gokul Arora NUCLEAR MAGNETICRESONANCE 3/18/2021 1 (PRINCIPLES AND TECHNIQUES)
- 2. INTRODUCTION Nuclear magneticresonance, NMR, is a physical phenomenon of resonance transition between magnetic energy levels, happening when atomic nuclei are immersed in an external magnetic field and applied an electromagnetic radiation with specific frequency. By detecting the absorption signals, one can acquire NMR spectrum. NMR results from specific magnetic properties of certain atomic nuclei. 3/18/2021 2
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- 7. CHEMICAL SHIFT(𝛿) ChemicalShift is the difference in resonance frequencies of two nuclei due to differences in their local molecular environment. Internal standard-tetramethylsilane (TMS; (H3C)4Si). The chemical shift arises from the applied field inducing secondary fields of about 0.15-0.2 mT at the proton by interacting with the adjacent bonding electrons. 3/18/2021 7
- 8. The chemicalshift of any hydrogen proton depends on its neighbouring environment. If the induced field opposes the applied field, the latter will have to be at a slightly higher value for resonance to occur. (shielded) Alternatively, if the induced and applied fields are aligned, the latter is required to be at a lower value for resonance. (deshielded) 3/18/2021 8
- 9. 3/18/2021 9 • The typeof proton giving rise to a particular band may thus be identified by the resonance peak position, i.e. its chemical shift. • The area under each peak is proportional to the number of protons of that particular type.
- 10. PULSE-ACQUIRE AND FOURIER TRANSFORMMETHODS 3/18/2021 10
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- 12. 3/18/2021 12 The effects encounteredin the dipolar interaction are transmitted through space over a limited distance in the order of 0.5nm or less.
- 13. 13C NMR 3/18/2021 13 13C-13Ccouplings ( homonuclear couplings) do not arise. 1H-13C interactions (heteronuclear couplings) are possible. DEPT ( Distortionless enhancement by polarization transfer) experiments are used for distinguishing between a CH3 group (methyl), a CH2 group ( methylene), and a CH group( methyne). In DEPT, sequences of multiple pulses are used to excite nuclear spins at different angles, usually 45°, 90° or 135°. Although interactions have been decoupled, in this situation the resonances exhibit positive or negative signal intensities dependent on the number of protons bonded to the carbon.
- 14. Two-dimensional nuclear magneticresonance spectroscopy (2D NMR) 3/18/2021 14 2D NMR is a set of nuclear magnetic resonance spectroscopy (NMR) methods which give data plotted in a space defined by two frequency rather than one.
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