Understanding Polarization/Population Transfer in NMR Experiments

1. 核磁共振光譜與影像導論 Introduction to NMR Spectroscopy and ImagingLecture04Polarization/Population Transfer Experiments(Spring Term, 2011)(Fall Term, 2011)(Spring Term, 2014)(Spring Term, 2015)(Spring Term, 2016)(Spring Term, 2017)Department of ChemistryNational Sun Yat-sen University

2. Polarization/Population Transfer Experiments Population, polarization and coherence Population means the number of spins of one nuclear species at certain energy level. Polarization is the difference between the numbers of the spins of one nuclear species at two energy levels. A Coherence is a coherent transition of the spins (from one or more nuclear species) between two energy levels.

3. The power of NMR lies to a great extent in the fact that the population, polarization and coherence can be manipulated in a myriad of ways. The most important one is to facilitate the transfer of population between different energy levels, transfer of polarization between different spins and transfer of coherences.

4. Density Matrix View population transfer polarization transfer coherence transfer Many 1D NMR experiments involve polarization/population transfer to perform spectrum editing, whereas most 2D experiments make use of coherent transfer to realize correlation.

5. Population Transfer In population manipulations, the most commonly used technique is selective population transfer (SPT):

6. Inversion

8. Heteronuclear Experiments Normal SPT: SPI:

9. Selective Polarization Transfer Population transfer can be realized between different spin species, leading to polarization transfer.

10. B. Using hard pulses

11. Non-Selective Polarization Transfer-INEPT As shown above, the polarization can be transferred from one nucleus to another. Therefore, PT can be used for increasing the NMR signal of insensitive nuclei such as dilute spins. PT has been widely in sensitivity enhancement and spectral editing. INEPT: Insensitive Nuclei Enhancement with Polarization Transfer:

12. y

15. The density matrix analysis of INEPT is similar to the polarization transfer with hard pulses in previous section and is left as an exercise. The result is given as follows The schematic spectra of N-15 spectrum of 15NH4 in water. (a) Without PT, (b) INEPT spectrum Clearly, above INEPT pulse sequence cannot be used in proton decoupling experiments because the decoupling during acquisition would cause Iz change rapidly from –Iz to Iz, leaving zero S signal enhancement.

16. Refocused INEPT The proton decoupling problem can be solved with the following modification: Vanish with proton decoupling In this way, the final density matrix becomes Sx and it is not affected by proton decoupling during acquisition time.

17. Distortionless Enhancement by PT (DEPT) This pulse sequence is particularly useful for differentiating CHn (n=0,1,2,3) in organic compounds. The subspectra are obtained in following way: CH: S(π/2) CH2: S(π/4)- S(3π/4) CH3: S(π/4)+S(3π/4) -√2S(π/2)

18. θ/π (DEPT) ΔJ (INEPT)

19. CH: S(π/2) CH2: S(π/4)- S(3π/4) CH3: S(π/4)+S(3π/4) -√2S(π/2)

21. DEPT Example

22. DEPT Example CH3: S(π/4)+S(3π/4) -√2S(π/2) CH2: S(π/4)- S(3π/4) CH: S(π/2)

24. Normal Spectrum HMC13CDEPT90 HMC13CDEPT135

25. Try them B,D: Only two methyls, C: no methylene, E: CO chemical shift > 150 ppm C,D,E: Only 1,1,0 methyl, A: wrong methylene chemical shift.

27. Attached proton test (APT)

30. Coherence Transfer • Future lectures • 2D/3D/4D experiments