Nuclear magnetic Resonance(NMR) spectroscopy

Theory of NMR, nuclear magnetic resonance, instrumentation, solvents, chemical shift, photon NMR, spin coupling, coupling constant and applications.
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1. GAUTAM BUDDHA UNIVERSITY PRESENTED BY, Preeti Choudhary (17/MAP/016)

2. NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (NMR)

3. O utline theory of NMR Nuclear Magnetic Resonance Instrumentation Solvents Chemical shift Proton NMR Proton NMR spectra of Ethanol 1H NMR spectra of benzene Interpretation of 1H NMR spectra Spin-spin coupling (splitting ) Coupling constant Applications

4. Theory of NMR A spectroscopy technique that gives us information about the number and types of atoms in a molecule. Nuclear magnetic resonance spectroscopy is a powerful technique used to characterize organic molecule by identifying carbon-hydrogen frameworks within molecules. Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR is used to determine the types and number of H atoms in a molecule; atoms 13C NMR is used to determine the types and number of C atoms in molecule. The source of energy in NMR is radio wave 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 certain atoms in presence of strong magnetic fields, including 1H and 13C. All the atoms contains nuclei and all nuclei contains protons (+ Ve ) charges in which some charge nuclei posses “spin” on their own axis.

5. Spin nuclei are those which contains either Odd atoms number or odd atomic mass number or both e.g. 1H, 2H, 13C, 14N, 17O, 35Cl etc. are useful for NMR. Those nuclei contains Even number of atomic and mass number are not useful for NMR e.g. 12C, 16o ect . The nuclei posses spin, they spin on their nuclear axis leads to generate magnetic dipole ‘ so the angular momentum of this spinning charge is quantified and described by Quantum Spin Number ‘Í’ .  

6. The individual protons have spin quantum number +1/2 or -1/2. two spin states allow for proton (H) these spin states have equal amount of energy in the absence of magnetic field. But when magnetic field is applied, the proton (H) posess spin and their own magnetic field align themselves either or opposite to magnetic field.

7. Nuclear Magnetic Resonance d

8. instrumentation The NMR spectroscopy consists of the following components; A magnet Sample and sample holder Radiofrequency generator Detector Recorder In NMR spectroscopy, the sample is dissolved in and placed in magnetic field. Then radiofrequency generator irradiates the sample with the short pulse of radiation causing resonance. When nuclei fall back to their low energy state, detector measures the energy released and spectrum is recorded.  

9. A line diagram of NMR spectroscopy along with its components are as follows;

10. Magnet:- A strong magnet provides stable and homogenous field. The magnet size is 15 inches in diameter and capable of producing strong fields up to 23,500 gauss for 100MHz. Sample and sample holder:- A 1-30 mg sample is used in the form of dilute solution (2-10%) and solvent does not contain hydrogen of its own ions. The sample holder is glass tube about 5mm in diameter and 15-20 cm in length. Radio frequency oscillator:- The RF oscillator is installed perpendicular to magnetic field and transmits radio waves of some mixed frequency such as 60, 100, 220, 300MHz. A sweep generator is installed to supply dc current to magnet. RF detector or Reciver :- it is installed perpendicular to both magnetic field and the oscillator coil and is tuned to the same frequency as transmitter. When precession frequency is match with RF the nuclei induces (EMF) in detector coil and this signal is amplified and sent to recorder. Recorder:- The recorder gives a spectrum as a plot of strength resonance signal on Y axis and strength of resonance signal is directly proportional to the number of nuclei resonating at that particular field strength.

11. S olvents

12. Chemical shift (position of signals) The utility of NMR is that all protons do not show resonance at same frequency ,it is surrounded by particular no. of valance electrons which vary from atom to atom so, they exist in slightly different electronic environment from one another. Position of signals in spectrum helps us to know nature of protons i.e. aromatic alphabetic, acetylinic , vinylic , adjacent to electron releasing . Thus they absorb at different field strength. When molecule placed in magnetic field so its surrounding electron circulate & generates counter field which opposes the applied magnetic field on proton so that, field feels by proton is reduced and that proton called as the shielded proton

13. Shielding of protons:- high electron density around a nucleus shields the nucleus from the external magnetic field and the signals are upfield in the NMR spectrum Deshielding of protons :- lower electron density around a nucleus Deshields the nucleus from the external magnetic field and he signals are downfield in the NMR spectrum

14. Proton NMR

15. Proton NMR spectra of Ethanol

16. 1H NMR spectra of benzene

17. Interpretation of 1H NMR spectra

18. Spin-spin coupling (splitting) The interaction between the spins of neighbouring nuclei in a molecule may cause the splitting of NMR spectrum. This is known as spin-spin coupling or splitting . The splitting pattern is related to the number of equivalent H-atom at the nearby nuclei.

19. Coupling constant The distance between the peaks in a given multiplet is a measure of the splitting effect known as coupling constant. It is denoted by symbol J, expressed in Hz. Coupling constant area measure of the effectiveness of spin-spin coupling and very useful in 1H NMR of complex structures.

20. APPLICATIONS OF NMR SPECTROSCOPY Identification of structural isomers Detection of hydrogen bonding Detection of aromaticity Distinction between Cis - Trans isomers and confirmers Detection of electronegative atoms or group Detection of some double bond and character due to resonance

21. THANKYOU