Nmr spectroscopy

basic desciption about NMR spectroscopy by praveen kumar litoriya, department of physics, Dr. hari singh gour university, sagar

1. Dr. Hari Singh Gour University, Sagar Department of Physics Submitted By: Praveen Kumar Litoriya Registration no.- Y-16273028 1 Nuclear Magnetic Resonance Spectroscopy

2. NMR is abbreviation of N uclear M agnetic R esonance

3. 3 NMR History 1937 Rabi predicts and observes nuclear magnetic resonance 1946 Bloch, Purcell first nuclear magnetic resonance of bulk sample 1953 Overhauser NOE (nuclear Overhauser effect) 1966 Ernst, Anderson Fourier transform NMR 1975 Jeener , Ernst 2D NMR 1985 Wüthrich first solution structure of a small protein (BPTI) from NOE derived distance restraints 1987 3D NMR + 13C, 15N isotope labeling of recombinant proteins (resolution) 1990 pulsed field gradients (artifact suppression) 1996/7 new long range structural parameters: - residual dipolar couplings from partial alignment in liquid crystalline media Nobel prizes 1944 Physics Rabi (Columbia) 1952 Physics Bloch (Stanford), Purcell (Harvard) 1991 Chemistry Ernst (ETH) 2002 Chemistry Wüthrich (ETH) 2003 Medicine Lauterbur (University of Illinois in Urbana ), Mansfield (University of Nottingham)

4. NMR spectroscopy is a form of absorption spectrometry. NMR Spectroscopy Most absorption techniques ( e.g. – Ultraviolet-Visible and Infrared) involve the electrons… in the case of NMR, it is the nucleus of the atom which determines the response. An applied (magnetic) field is necessary to “develop” the energy states (produce a separation of the energy states) necessary for the absorption to occur.

5. Introduction The most important parts of an NMR instrument are: The magnet, The RF generator, The sample chamber or probe, (which not only houses the sample but also the RF transmission and detection coils). In addition, the instrument requires: A pulse generator An RF receiver, Lots of electronics, and a computer for data processing 5

6. NMR Schematic Diagram 6

7. 7 Magnet Legs Probe Sample Bore Cryogens Magnet Coil NMR Magnet Cross-Section

8. Primitive NMR Spectrometer

9. 9 When an external magnetic field is applied, hydrogen nuclei can align with the external field or against it ∆ E radio waves External magnetic field Nucleus aligned with magnetic field – low-energy state. Nucleus aligned opposed to magnetic field – high-energy state. As nuclei relax back to the low-energy alignment, energy in the radio wave frequency is released. This energy is detected and recorded as peaks on a spectrum. Energy Differentiation

10. B o = 0 B o > 0 Randomly oriented Highly oriented B o Ensemble of Nuclear Spins N S Each nucleus behaves like a bar magnet.

11. B o = 0 B o > 0 E D E Allowed Energy States for a Spin 1/2 System antiparallel parallel D E = g h B o = h n -1/2 +1/2 Therefore, the nuclei will absorb light with energy D E resulting in a change of the spin states.

12. Preparing a sample 12 To obtain the 1 H NMR spectrum of a sample it is usually necessary to dissolve the sample in a solvent. Solvents must not contain protons that will interfere with the sample being measured. A solvent must: contain no hydrogen atoms, eg tetrachloromethane , CCl 4 or have the hydrogen atoms replaced with deuterium ( 2 H), eg CDCl 3 or CD 3 OD.

13. Explaining spectra 13 The scale runs from right to left and is called the chemical shift . It is measured in parts per million ( ppm ). Peaks to the left of TMS peak are said to be downfield of TMS. TMS ( tetramethylsilane ) is added to the solvent and provides a reference peak. The protons in TMS are assigned the value ppm and the rest of the spectrum is calibrated relative to this.

14. 14 Example: How many proton environments are there in ethanol? The three CH 3 protons are in one environment. The two CH 2 protons are in a second environment. The OH proton is in a third environment. Determining proton environments

15. 15 Example 1 Methyl ethanoate

16. 16 n + 1 rule The number of peaks in a multiplet can give additional information about the structure. The splitting of peaks is caused by the neighbouring carbon’s hydrogen atoms. Protons in the same environment are said to be equivalent and as such behave as one proton. This follows the n + 1 rule. n is the number of hydrogen atoms attached to the next-door carbon n + 1 is how many peaks will be seen in the cluster.

17. 17 Example 2 – Assign the peaks and suggest a structure Molecular formula C 3 H 7 Br CH 3 A triplet due to CH 2 group adjacent. CH 2 This is not a simple quartet. There are extra splittings due to CH 3 and CH 2 neighbouring groups. CH 2 A triplet due to CH 2 group adjacent.

18. 18 Application MRI is the best medical application for imaging, it is working on same principal as Nuclear Magnetic Resonance.

19. 19 Solid State NMR for studying Nuclear waste glass Vitrification of high level nuclear wastes (HLW) is an important step for storing nuclear waste Vast experiences gathered from these investigations have identified sodium borosilicate glasses as one of the most promising inert host matrices

20. 20 Thank you