1H NMR

2. Introduction - NMR NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1H 13C 15N 19F 31P

3. Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field.

4. External Magnetic Field When placed in an external field, spinning protons act like bar magnets.

5. Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field.

6. Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. CH3 - CH2 - CH2 - Cl CH3- CH2- CH- -OH -NH

7. Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts.

8. The NMR Graph

9. NMR Signals The number of signals shows how many different kinds of protons are present. The location of the signals shows how shielded or deshielded the proton is.

11. NMR Signals The intensity of the signal shows the number of protons of that type.

12. NMR Signals Signal splitting shows the number of protons on adjacent atoms.

13. Standard TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal.

14. Chemical Shift Measured in parts per million. Ratio of shift downfield from TMS (Hz) to total spectrometer frequency (Hz). Same value for 60, 100, or 300 MHz machine.

15. Location of Signals More electronegative atoms deshield more and give larger shift values.

16. Location of Signals Effect decreases with distance. CH3-Cl 3.5 ppm CH3-CH2-Cl 3.0 ppm CH3-CH2-Cl 2.6 ppm CH3-CH2-CH2-Cl 2.0 ppm

17. Location of Signals Additional electronegative atoms cause increase in chemical shift. CH3-CO-Cl 2.8 ppm Cl-CH2-CO-Cl 4.1 ppm

18. Typical Values

19. Spin-Spin Splitting Non equivalent protons on adjacent carbons always interact each other. Equivalent protons do not not split each other. CH3 � CO - CH3 Do not split CH3 � CH2 - Cl Split each other

20. Spin-Spin Splitting

21. Spin-Spin Splitting

22. Spin-Spin Splitting

23. Spin-Spin Splitting If a signal is split by N equivalent protons, it is split into N + 1 peaks.

24. Spin-Spin Splitting Equivalent protons do not split each other. Protons bonded to the same carbon will split each other only if they are not equivalent. CH3-C-H

25. Spin-Spin Splitting Protons on adjacent carbons normally will couple. Protons separated by four or more bonds will not couple. CH3-CH2-CH2-CH2-Cl

26. Coupling Constants Distance between the peaks of multiplet measured in Hz called coupling constant. Not dependent on strength of the external field

27. Typical coupling constant

28. Spin Decoupling It�s a powerful tool for determining 1. The connectivity of the protons. 2. Assigning proton peaks

29. Spin Decoupling Irradiation of one proton in a spin coupled system removes its coupling effect on the neighboring protons to which it had coupled.