核磁共振光譜與影像導論

1. 核磁共振光譜與影像導論 Introduction to NMR Spectroscopy and ImagingLecture05Basic Two-Dimensional Experiments(Spring Term, 2011)Department of ChemistryNational Sun Yat-sen University

2. Basic Two-Dimensional Experiments Why multi-dimensional? • Resolution • Multi-quantum coherences • Coherence transfer Types of multi-dimensional spectra: To Correlate To Resolve To Exchange (Diffusion)

3. t1 From 1D to 2D, 3D… t2

4. If you look at the same data matrix in a different way…. You also get a series of FIDs.

5. In either direction, you see FIDs You really don’t know which is “acquisition dimension”, Do you?

6. F2 A 2D Spectrum t2 FT2 FT1 F1 t1

7. 2D FT

9. Typical 2D Lineshapes

10. Resolution The resolution in the second dimension is determined the same way as in 1D NMR. The resolution in the first dimension is determined by the longest evolution time in the first dimension (maximum of t1). The total evolution time in the indirect dimension is equivalent to the total acquisition time in the detection dimension.

14. Acquisition and data processing: F2 F1

16. t2 t1 Presat p=+1 p=0 p=-1 COSY pulse sequence (top) with a presaturation pulse to suppress water signal. Also shown in the diagram are the coherence transfer pathway (middle) and phase cycling scheme (bottom).

19. t2 t1 Presat p=+1 p=0 p=-1 COSY pulse sequence (top) with a presaturation pulse to suppress water signal. Also shown in the diagram are the coherence transfer pathway (middle) and phase cycling scheme (bottom).

23. Recollecting Chapter 2

24. Four basic lineshapes Anti-phase absorptive FT Real part In-phase dispersive FT Real part Anti-phase dispersive FT Real part FT Real part In-phase absorptive

25. A 2D dispersive peak

26. A 2D absorptive peak

28. + - - + +- -+ -+ +- +- -+ -+ +- + - - +

30. Absorptive cross peaks Dispersive diagonal peaks Dispersive cross peaks Absorptive diagonal peaks

31. Absorptive cross peaks

32. Absorptive cross peaks

33. Dispersive diagonal peaks

34. F E D C B A

36. 3 6 7 8 10 9 4 5 2 1

37. 3 1 2 3 6 10 5 8 7 4 9 6 7 8 10 9 4 5 2 1

38. Absolute Value Display

39. The major drawback of COSY: cross peaks near the diagonal line may be obscured by diagonal peaks.

42. DQF COSY

43. 3 6 7 8 10 9 4 5 2 1

44. 3 6 7 8 10 9 4 5 2 1

47. The Advantages of DQF COSY Having the same phase for both the diagonal and cross peaks; Because the magnetization is detected in anti-phase, the multiplet structure is retained with opposite phase. In other words, the two lines of a doublet are 180° out of phase with respect to each other. While this can lead to cancellation in crowded regions of the spectrum, it also allows for the easy identification of multiplets (based on their square, or box, shape), and for measuring the size of the scalar coupling constant connecting the two spins. Another advantage of the DQF-COSY experiment is not so immediately obvious. In order for a transition to create multiple quantum levels, and to survive a multiple quantum filter, you need to have at least two spins or three spins for a 2Q or 3Q transition, respectively. Thus, singlets are drastically reduced in intensity in a DQF-COSY spectrum.

48. The Disadvantages of DQF COSY • There are also disadvantages to the DQF-COSY relative to COSY. First, the sensitivity of DQF is about a factor of two lower than regular COSY. Second, MQ relaxation in 1Hs is very slow, so the experiments require a relatively long d1 relaxation delay. • Thus, while a good COSY spectrum could be generated in about 2-4 hours of data accumulation, a good DQF-COSY requires about 16-24 hours to allow complete relaxation during d1.