Nuclear magnetic resonanace spectroscopy

nuclear magnetic resonance spectrometer

1. IN 504 Analytical Instruments Module 4 1 Presented by; Anju Sunny CUSAT Reference Text: R S Khandpur “Handbook of Analytical Instrumentation”

2. Nuclear Magnetic Resonance (NMR) Spectroscopy 2

3. Introduction Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules. Two common types of NMR spectroscopy are used to characterize organic structure: 1 H NMR is used to determine the type & number of H atoms in a molecule; 13 C NMR is used to determine the type & number of carbon atoms in the molecule.

4. Principle The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency. When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1 H and 13 C. When a charged particle such as a proton spins on its axis, it creates a magnetic field . Thus, the nucleus can be considered as a tiny bar magnet.

5. Principle These tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B , they are oriented with or against this applied field.

6. Principle In a magnetic field, there are two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B , and a higher energy state in which the nucleus aligned against B . When an external energy source ( hν ) that matches the energy difference ( Δ E) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.

7. Principle The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum. Thus, two variables characterize NMR : an applied magnetic field, B , the strength of which is measured in tesla (T), and the frequency, ν of radiation used for resonance, measured in hertz (Hz),

8. Principle The frequency needed for resonance and the applied magnetic field strength are proportionally related; ν α Bo

9. Two general types of NMR 1) Continuous Wave (CW NMR) 2) Pulsed or Fourier Transform (FT NMR)

10. CW-NMR or NMR Instrumentation

11. Extra to read… Fermions : Odd mass nuclei with an odd number of nucleons have fractional spins. I = 1/2 ( 1 H, 13 C, 19 F, 31 P ), I = 3/2 ( 11 B, 33 S ) & I = 5/2 ( 17 O ) Bosons : Even mass nuclei with odd numbers of protons and neutrons have integral spins. I = 1 ( 2 H, 14 N ) Even mass nuclei composed of even numbers of protons and neutrons have zero spin I = 0 ( 12 C, and 16 O, 32 S) Spin of Nuclei

12. Extra to read… Chemical shift In difference chemical environments, Nuclei in a molecule, absorb slightly different frequencies. The position of the signals in the spectrum is called the “chemical shift”

13. Electron Spin Resonance (ESR) Spectroscopy 13

14. Introduction The ESR is a physical method of observing resonance absorption of microwave power by unpaired electron spin in a magnetic field. Electron Spin Resonance (ESR) is similar to NMR, rather than analyze the nuclear spins, ESR analyzes the electron spins of a sample.

15. Principles

16. Principles Species having one unpaired electron has two electron spin energy levels: E = g m B B o M s ==> D E = g m B B o   where, g: proportionality constant independent of field direction m B : Bohr magnetron (9.274 x 10 -24 J T -1 for electron) M S : electron spin quantum number (+1/2 or –1/2) Bo: external magnetic field commonly 0.34 – 1.24 T ∆ E ==> corresponding frequency 9.5 (X-band) – 35 (Q-band)GHz

17. Hyperfine splitting Electron interacts with a neighboring nuclear magnetic dipole, the energy levels become:   E = gm B B o M S + am B M S m I m I : nuclear spin quantum number for the neighboring nucleus a : hyperfine coupling constant Energy levels and transitions for a single unpaired electron in an external magnetic field with no coupling coupling to one nucleus with spin 1/2

18. The Basic ESR Equipment

19. The Basic ESR Equipment Absolute requirements: 1 or more unpaired electrons microwave region of spectrum: (9.5 GHz ) Magnetic fields: (0.35 Tesla)

20. Types of magnets and probes Refer…