NMR Spectroscopy

1. NMR Spectroscopy Part II. Signals of NMR

2. Free Induction Decay (FID) • FID represents the time-domain response of the spin system following application of an radio-frequency pulse. • With one magnetization at w0, receiver coil would see exponentially decaying signal. This decay is due to relaxation.

3. Fourier Transform The Fourier transform relates the time-domainf(t) data with the frequency-domainf(w) data.

4. Fourier Transform

5. Fourier Transform

6. NMR line shape Lorentzian line A amplitude W half-line width

7. Resolution • Definition For signals in frequency domain it is the deviation of the peak line-shape from standard Lorentzian peak. For time domain signal, it is the deviation of FID from exponential decay. Resolution of NMR peaks is represented by the half-height width in Hz.

8. Resolution

9. Resolution-digital resolution

10. Resolution • Measurement half-height width: 10~15% solution of 0-dichlorobenzene (ODCB) in acetone Line-shape: Chloroform in acetone

11. Resolution • Factors affect resolution Relaxation process of the observed nucleus Stability of B0(shimming and deuterium locking) Probe (sample coil should be very close to the sample) Sample properties and its conditions

12. Sensitivity • Definition signal to noise-ratio A : height of the chosen peak Npp : peak to peak noise

13. Sensitivity • Measurement 1H 0.1% ethyl benzene in deuterochloroform 13C ASTM, mixture of 60% by volume deuterobenzene and dioxan or 10% ethyl benzene in chloroform 31P 1% trimehylphosphite in deuterobenzene 15N 90% dimethylformamide in deutero-dimethyl- sulphoxide 19F 0.1% trifluoroethanol in deuteroacetone 2H, 17O tap water

14. Sensitivity • Factors affect sensitivity Probe: tuning, matching, size Dynamic range and ADC resolution Solubility of the sample in the chosen solvent

15. Spectral Parameters • Chemical Shift Caused by the magnetic shielding of the nuclei by their surroundings. d-values give the position of the signal relative to a reference compound signal. • Spin-spin Coupling The interaction between neighboring nuclear dipoles leads to a fine structure. The strength of this interaction is defined as spin-spin coupling constant J. • Intensity of the signal

17. Chemical Shift • Origin of chemical shift • sshielding constant • Chemically non-equivalent nuclei are shielded to different extents and give separate resonance signals in the spectrum

18. Chemical Shift

19. Chemical Shift • d – scale or abscissa scale

20. Chemical Shift Shielding s CH3Br < CH2Br2 < CH3Br < TMS 90 MHz spectrum

21. Abscissa Scale

22. Chemical Shift • d is dimensionless expressed as the relative shift in parts per million ( ppm ). • d is independent of the magnetic field • d of proton 0 ~ 13 ppm d of carbon-13 0 ~ 220 ppm d of F-19 0 ~ 800 ppm d of P-31 0 ~ 300 ppm

23. Chemical Shift • Charge density • Neighboring group Anisotropy Ring current Electric field effect Intermolecular interaction (H-bonding & solvent)

24. Chemical Shift –anisotropy of neighboring group c susceptibility r distance to the dipole’s center Differential shielding of HA and HB in the dipolar field of a magnetically anisotropic neighboring group

25. Chemical Shift –anisotropy of neighboring group d~2.88 d~9-10

26. Electronegative groups are "deshielding" and tend to move NMR signals from neighboring protons further "downfield" (to higher ppm values). • Protons on oxygen or nitrogen have highly variable chemical shifts which are sensitive to concentration, solvent, temperature, etc. • The -system of alkenes, aromatic compounds and carbonyls strongly deshield attached protons and move them "downfield" to higher ppm values.

27. Electronegative groups are "deshielding" and tend to move NMR signals from attached carbons further "downfield" (to higher ppm values). • The -system of alkenes, aromatic compounds and carbonyls strongly deshield C nuclei and move them "downfield" to higher ppm values. • Carbonyl carbons are strongly deshielded and occur at very high ppm values. Within this group, carboxylic acids and esters tend to have the smaller values, while ketones and aldehydes have values 200.

28. Ring Current • The ring current is induced form the delocalized p electron in a magnetic field and generates an additional magnetic field. In the center of the arene ring this induced field in in the opposite direction t the external magnetic field.

29. Ring Current -- example

30. Spin-spin coupling

31. Spin-spin coupling

32. AX system

33. AX2 system

34. Spin-spin coupling

35. AX3 system

36. Multiplicity Rule Multiplicity M (number of lines in a multiplet) M = 2n I +1 n equivalent neighbor nuclei I spin number For I= ½ M = n + 1

37. Example AX4 system I=1; n=3 AX4

38. Order of Spectrum Zero order spectrum only singlet First order spectrum Dn >> J Higher order spectrum Dn ~ J

39. AMX system

40. Spin-spin coupling • Hybridization of the atoms • Bond angles and torsional angles • Bond lengths • Neighboring p-bond • Effects of neighboring electron lone-pairs • Substituent effect

41. JH-H and Chemical Structure • Geminal couplings 2J (usually <0) H-C-H bond angle hybridization of the carbon atom substituents

42. Geminal couplings 2J bond angle

43. Geminal couplings 2J Effect of Neighboring p-electrons Substituent Effects

44. Vicinal couplings 3JH-H • Torsional or dihedral angles • Substituents • HC-CH distance • H-C-C bond angle

45. Vicinal couplings 3JH-Hdihedral angles • Karplus curves

46. Chemical Shift of amino acid http://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htm

48. Chemical Shift Prediction Automated Protein Chemical Shift Prediction http://www.bmrb.wisc.edu:8999/shifty.html BMRB NMR-STAR Atom Table Generator for Amino Acid Chemical Shift Assignments http://www.bmrb.wisc.edu/elec_dep/gen_aa.html

50. http://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htmhttp://bouman.chem.georgetown.edu/nmr/interaction/chemshf.htm