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 ImagingLecture02Chemical Shift and J-Coupling(Spring Term, 2011)Department of ChemistryNational Sun Yat-sen University

2. Chemical Shift and J-Coupling

3. In the beginning…. All spins were of no difference…same, identical, equal, I/You/He/We/You/They were all the same…or believed to be so….then...the apple of discern came in… (Proctor says: "until it is clearly understood, the accuracy of magnetic moments determined under certain chemical conditions remains somewhat in doubt"). Valuable reading: http://www.ebyte.it/library/hist/ProctorWG_Reminiscences.html Proctor ? 1922-2006 Dickinson? Who can find his photo? Yu 1913-2003 W.G.Proctor, F.C.Yu(虞福春), Phys Rev 1950,77,717. W.C.Dickinson, Phys Rev 1950 77, 736. Norman Ramsey (Phys.Rev. 1950, 78,699):"Furthermore, with heavier nuclei the ratios of the resonance frequencies for the same nucleus in different molecules have been measured with high precision and discrepancies have been found by various observers that are sometimes called chemical shifts". Ramsey

4. Chemical Shift (Shielding) Induced shielding field chemical shift tensor

5. Chemical Shift:a molecule becomes a dipole The induced dipole moment shifts the resonance frequency of the nuclear spin. m chemical shift tensor

7. Shielding Depends on Chemical Environment Different environments cause different shieldings (Representing different local chemical environments, Proctor and Yu, 1951) Hence chemical shift (of resonance frequency relative to Zeeman frequency ω0.)

8. Anti-shielding Is Possible Anybody cares to find some interesting literature?

9. Downfield (high freq) Upfield (low freq) These words were from CW NMR. ‘Downfield’ means for a given resonance frequency, the magnetic field used is lower. ‘High frequency’ means at a fixed magnetic field, the spins in this region have higher resonance frequencies. Some proton chemical shifts The more localized AO/MO, the more shielding Stronger/more bonds mean smaller CS. Less shielded. (downfield) You can tell a lot from this diagram Increasing δ (these trends for σ,δ,B, |ν| are same for γ>0 or γ<0 nuclei. However, for γ<0, ν is negative.) Larger shift/small shielding Small shift/large shielding The OH bonding in vaporized water clearly differs from that in liquid water! (hydrogen bonding has significant effect on chemical shift) Reference shift

10. Why CHn have smaller CS than H atom? B0 H C H atom Some people said: An H in CHn seems to be less shielded because the C has larger electronegativity so it ‘draws’ electrons to its side. But why an H in CHn has smaller CS than H atom? Answer: The electron density at the C-H bonding area is larger than that of an H atom albeit the electron density at other places is smaller. Overall, the H in CHn is more shielded than in H atom. This can also explain why H > H2, H>OH>H2 More bonds, more shielded.

11. Why this CS order: CH>CH2>CH3? More bonds, more shielded. The bonding regions correspond to large shielding (small CS). The CS is smallest when the magnetic field is along the bonding direction.

12. Why this CS order: HF>HCl>HBr>HI? The fewer number of electrons of the bonding partner, the less the shielding the bonded H. (The more clothes you dress, the more you’re shielded.) The shielding of s orbitals is smaller than that of p orbitals which is even smaller than that of d orbitals etc. (The more localized the orbitals, the more shielding)

13. Isotopic Effect • Because CS is generated by electrons, nuclei of isotopic elements (e.g. H1/D2, N14/N15, Cl35/Cl37) have very similar chemical shift but isotope shift does exist: e.g., 1H CS of HOD is 0.035 ppm upfield (more shielded) of that of HOH (the electrons in HOD is a little ‘heavier’ than in HOH lower vib freq/amp  more shielding (you are more shielded by your clothes if you shake yourself less violently.) • There is a general rule which says that when one substitutes a nuclide in a chemical group with a heavier isotope then all other nuclides in the group become a bit more shielded (this has to do with an overall reduction in vibrational amplitudes). • Consequently, the chemical shift of protons in standard bulk water should be 4.795 + 0.035 = 4.830 ppm, give or take 0.02 ppm. Of course, heavy water and normal water do not even have the same bulk properties (such as density and magnetic susceptibility) and this introduces a further uncertainty when trying to deduce the chemical shift of normal water from the data on HDO traces in D2O.

14. You may be able to memorize this table or you may explain it based on your good scientific intuition

15. Most internal NMR referencing standards are pH and temperature sensitive. NMR Internal Referencing Standard Samples More info http://www.bmrb.wisc.edu/home/iupac.pdf. OTHER NMR RESOURCES

16. H1 and C13 Chemical Shifts of NMR Solvents

17. NOTES: • 1H chemical shifts are in PPM, relative to 0.05% TMS (v/v), at 295 K. • 13C chemical shifts are in PPM, relative to 1.0% TMS (v/v), at 295 K. • 'm' denotes broad peak with some fine structures (at 200 MHz). • HOD peak positions may vary depending upon concentration in solvent, pH and temperature. • M.P. and B.P. values are for the corresponding non-deuterated solvent (except for D2O). • (DSS) denotes chemical shifts relative to 2,2-Dimethyl-2-silapentane- 5-sulfonate sodium salt. • See NMR Referencing for more information

18. CHARACTERISTIC PROTON CHEMICAL SHIFTS

21. Carbon-13 Chemical Shifts

22. You may be able to memorize this table or you may explain it based on your good scientific intuition

25. Be aware of “abnormal” chemical shifts …... 1H • For most organic compounds, the 1H chemical shift is in the range of 12 ppm, but the chemical shift range for hydrides (organometallic compounds) is approximately +25 to -60 ppm, the largest range could possibly reach 200 ppm!. The downfield shifts are most common in d0, d10 and early transition metal cases whereas those with other dn counts and late transition metals tend to be upfield of zero. • Similar phenomenon occurs for other nuclei such as 13C, 31P etc. 1H 13C Harald Hilbig and Frank H. Koehler, New J Chem, 2001.

26. Phosphorous-31 Chemical Shifts

28. Phosphorous (III) Chemical Shift Table (from Bruker Almanac 1991)

29. Phosphorous (V) Chemical Shift Table (from Bruker Almanac 1991)

30. Fluorine-19 Chemical Shifts

32. Nitrogen-14 Chemical Shifts

34. B-11 Chemical Shift Almost all quadrupolar nuclei have rather small CS range.

35. Factors Affecting Chemical Shift • Temperature • Solvents (pH, concentration) • Pressure • Sample shape (susceptibility) • …… NMR can be used as a thermometer, a pH meter or a barometer. (Only very smart guys would like to buy an NMR spectrometer for those purposes though) * Relative to TMS.

36. Amide proton chemical shifts of NHA in CDCl2CDCl2 as a function of temperature and concentration. Derr et al. J. Chem. Soc., Perkin Trans. 1, 2000.

37. Chemical Shift The surrounding electrons cause a shielding magnetic field at the nucleus

38. Shielding Anisotropy (CSA) Electron clouds are seldom spherically symmetrical. They are anisotropic in almost all molecules. B0 B0 Chemical shift anisotropy (CSA) tensor In liquids, CSA is averaged out by rapid molecular tumbling; in solids, CSA is kept.

40. Oriented Molecules B0

41. Oriented Single Crystals B0

42. Powder (Polycrystalline Solid) B0

43. Chemical Shift Tensor

47. Applications of Chemical Shift

48. Applications of Chemical Shift http://www.bmrb.wisc.edu/data_access/outlier_selection_grid.html

49. Applications of Chemical Shift

50. Applications of Chemical Shift Relaxation, dynamics Solid state NMR CS Imaging ……