Nuclear Magnetic Resonance Spectrometry

1. Nuclear Magnetic Resonance Spectrometry

2. Identification of Compound

5. Nuclear Spins in Absence (a) and Presence (b) of External Magnetic Field

6. Methyl Ester of Fatty Acid

7. Nuclear Magnetic Resonance Principle

8. Principles of NMR

9. Magnetic Properties of Nuclei Nuclei of certain atoms posses a mechanical spin or angular momentum. The angular momentum depends on the nuclear spin or spin number. The spin number (I) is related to the mass number and atomic number. Magnetic nucleus may assume any of (2I+1) orientation with respect to the direction of the applied magnetic field

10. Theory of Nuclear Resonance A proton in a external magnetic field assumes only two orientations corresponding to +/- uH. It is possible to induce transitions between two orientations. The frequency (v) of electromagnetic radiation necessary for such transition is given by v=2uH/h where H is the strength of the external magnetic field. The precession frequency of the spinning is exactly equal to the frequency of electromagnetic radiation necessary to induce a transition from one nuclear spin state to another.

11. Theory of Nuclear Resonance There is slightly excess of nuclei in the low spin state compared to high spin state. Boltzman�s distribution (low spin state / high spin state is 1.00001. This very small excess of lower energy state gives rise to net absorption of energy in the radio frequency. Without this small excess, there would be no NMR. Spin-spin relaxation and spin lattice relaxation Lattice is the frame work of molecules containing the precessing nuclei

12. Types of Bonds

13. Magnetic Properties of Nuclei

14. NMR Equation and Magnetic Field Strength

16. Relationship between Radio Frequency and Magnetic Field Strength for Proton

17. Energy Difference between Spin States as a Function of Magnetic Fields Strength

20. Chemical Shift

23. Chemical Shift If the d(Ha) and d (Hb) differs by 1ppm, the amount in 600 MHz instrument correspond to an energy difference of 600MHz or 6x10-8 cal/mole. To measure the small difference between Ha and Hb as separate states, they would have lifetimes in each conformation of at least ?t ~ 1 / 2 ? ?v = ?t ~ 1 / (2 ? ?E) =1 / (2 ? x 600 )=0.00027 sec. The average lifetime of a given conformation is only 10-11 sec in a energy barrier of only 3kcal/mole between one conformer to another.

24. Chemical Shift The combination of Heisenberg uncertainty principle and the small energy change characteristic of NMR spectroscopy is that two hydrogen states are convertible. If separate lifetimes > 1sec, NMR can be seen as two sharp peaks, < 1 msec as a combined single sharp peak. The two hydrogen states are magnetically equivalent. If the lifetimes are in an intermediate region, a broad peak results.

25. Chemical Exchange (Proton Transfer) Chemical Exchange describes the fact that in a given period of time, a single -OH proton may attached to a number of different ethyl alcohol molecules. The rate of chemical exchange (proton transfer) in pure alcohol ethyl alcohol is slow, this rate is very markedly increased in acidic or basic impurities. If the rate of chemical exchange is very slow, the expected multiplicity of hydroxyl group is observed. If the rate of chemical exchange is rapid, a single sharp signal is observed. An intermediate rates of proton transfer, the observation my occur as a broad peak

26. Chemical Exchange (Proton Transfer) The rapid chemical exchange causes spin decoupling (no multiplicity) Heisenberg uncertainty principle quantum mechanics: ?v ?t ~ 1 / 2 ? where ?v and ?t are the uncertainties in energy and time in units of Hertz and seconds. That is, we can not know precisely both the energy and the life time of a given state. The longer time the state, the more precisely can its energy content be evaluated.

27. Shielding Mechanism Ordinary proton magnetic resonance absorption frequencies are spread over 7000 cps at 600MHz NMR. The magnitude of the separation of the position of absorption of a proton from the reference is called the chemical shift. The shielding that a proton experiences is a combination of at least three types of electronic circulations: Local diamagnetic effects Diamagnetic and paramagnetic effects from neighboring atoms Effects from inter atomic currents. When the nucleus experiences a smaller magnetic field than that applied externally, It is said to be shielded.

28. Shielding Mechanism Diamagnetic shielding always reduces the apparent magnetic field at the proton, and consequently is a source of positive shielding.

29. Shielding Mechanism Paramagnetic shielding arises from electronic circulation within the molecule when they are specifically oriented with respect to the magnetic field. The orientation of the protons relative to the induced magnetic currents are called anisotropic effects. Aromatic nuclei contain large closed loops of p electrons in which strong magnetic currents are induced by the magnetic field. This effects results in a paramagnetic shielding at the aromatic proton and is called ring current effects.

30. Reference : TetraMethylSilane (TMS)

31. Absorbance Frequency

32. General Regions of Chemical Shifts

33. Spin-Spin Coupling (Spin-Spin Splitting)

34. Spin-Spin Coupling (Spin-Spin Splitting)

35. Spin-Spin Splitting

38. Information from NMR Spectrum

39. Summary As external applied magnetic filed increases Spinning proton magnetic dipole moment increases spinning proton angular momentum increases proton precession frequency increases the energy difference between high energy spin state and low energy spin state increases

41. Summary