NMR spectra of some simple molecules Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)

1. NMR spectra of some simple molecules Effect of spinning: averaging field inhomogeneity (nmr1.pdf pg 2)

3. Ho Because the protons have a magnetic field associated with them, the field changes as across the nmr tube. Diffusion tends to offset this field gradient

5. Chemical Shifts Heff = The magnetic field felt at the proton Heff = Hext + Hlocal; Heff: magnetic field felt by the nuclei Hext: external magnetic field Hlocal: local field induced by the external field Hlocal: Electrons in a chemical bond are considered to be in motion and are charged. This induces a local magnetic field which can shield (oppose) or deshield (enhance) the magnetic field experienced by the nucleus. Since the precessional frequency of the nucleus is governed by Heff, changes in this field as a result of local fields caused by bonding electrons, the resonance frequency of magnetically and chemically non-equivalent nuclei differ resulting in slightly different values of . This is the origin of the chemical shift. The local magnetic field is induced by the external field and is directly proportional to the external field

6. Hlocal: the effect of the external magnetic field on the bonding electrons depends on electron density and molecular structure. Hlocal isdirectly proportional to Hext Remember H is a vector. This property has both magnitude and direction

9. Increasing frequency Typical chemical shifts for protons: 0 –10 ppm In a 300 MHz instrument, differences in  range about 3000 Hz (3000 Hz shifts relative to a total of 300*106 cycles /sec)

10. aromatic CH CH2 CH3 -CH= Typical chemical shifts for protons: 0 –10 ppm

11. >C=C< CR4 CHR3 R2CH2 CH3 >C=O aromatic Typical chemical shifts for 13C: 0 to 220 ppm

12. Common terms used in NMR (terms originating from use of CW instruments) Shielded: the induced local field opposes the external field Deshielded: the induced local field field augments the external field Upfield shift: shift toward lower frequency; higher magnetic field, lower energy Downfield shift: shift toward higher frequency; lower magnetic field higher energy

13. Frequency sweep instruments: Hext= constant;  swept 10 ppm Heff < than Hext  must decrease for resonance lower frequency, lower energy, nucleus is shielded, upfield shift Hext Hlocal Heff > than Hext  must increase for resonance higher frequency, higher energy, nucleus is deshielded, downfield shift Hext Hlocal

14. Field sweep instruments: At 600 MHz • ω = constant; Hext swept from • “140000 to 146000 gauss” • Heff < than Hext  must decrease for resonance • lower frequency, lower energy, nucleus is shielded, upfield shift Hext Hlocal • Then resonance would occur at a lower value of Hext • nucleus is deshielded, downfield shift Hext Hlocal

15. Sigma bonds Field due to circulating e- Hexternal field electron cloud nucleus All protons have the same precessional frequency in a vacuum Field felt by the nucleus Heff = Hext - Hlocal For resonance either Hext must be increased or  decreased relative to the situation where Hlocal= 0

16. π bonds in acetylenes Hext Hlocal

17. shielding cone deshielding region π bonds in alkenes and aldehydes Hlocal Hext

18. π bonds in aromatic compounds Hext Hlocal Field felt by the nucleus Heff = Hext + Hlocal For resonance either Hextmust be decreased or  increased relative to the situation where Hlocal= 0

19. Hext  9.3  -3.0  0.3

20. An Example of A Simple Spectrum Area: 9:1:2

21. Other Factors Influencing Hlocal Hlocalis influenced by all local fields; the field effect of the bonding electrons results in the chemical shift, a relatively small perturbation Hlocal is induced by the external field and depends on its magnitude What about the field effects of the local protons? Suppose we have two identical protons attached to the same carbon. What are the possible spin states of this system and how do they effect the local magnetic field?

22. Nomenclature used to describe spin-spin coupling First Order Spectra: Chemical shift difference ∆ > 10 J AX ; A2X; A3X; AMX; A3MX; A3M2X; … J is a measure of the effective magnetic field of neighboring protons. The effect is generally considered to be transmitted through chemical bonds and not through space Non-first Order Spectra: Chemical shift ∆ < 10 J AB ; A2B; A3B; ABC; A3CB; A3B2X; A3B2C …

23. A2 Case, J = 0 H-C-C-C-C-H Energy or H Remember: Ne/Ng = e-H/RT 1

24. A2 Case H-C-H For positive J +J/4 A +J/4 -3J/4 A +J/4 J = 0 H – H interaction No H – H interaction

25. A2 Case For negative J -J/4 A +3J/4 -J/4 A J =0 -J/4 H – H interaction No H – H interaction

26. AX; X >A J = 0 A Relative ordering of energy levels without AX interactions X X Energy A Both opposed to magnetic field A X

27. AX; X >A +J/4 A + J/2 X +J/2 -J/4 Relative ordering of energy levels with AX interactions X -J/2 -J/4 A – J/2 Both opposed to magnetic field +J/4 A X For positive J

28. In the absence of coupling, ie J = 0 In the presence of coupling, ie J ≠ 0 J J X A

29. AX; X >A -J/4 A – J/2 +J/4 X -J/2 X +J/2 Relative ordering of energy levels with AX interactions +J/4 A + J/2 Both opposed to magnetic field -J/4 A X For negative J

30. X A J J

31. A2X X >A No AX interaction, JAA ≠ 0 A2 X

32. A2X X >A A +J/2 X+J/2 0 A -J/2 No AX interaction X X X-J/2 X A -J/2 0 A+J/2 For positive JAX A2 X

33. A2X X >A A +J/2 A+J/2 0 A+J/2 A -J/2 AX interaction Note that the A transitions are twice as intense A -J/2 A-J/2 0 A-J/2 A+J/2 J = 0 For positive J A2 X

34. No A2X coupling A2X coupling A X

35. The 2nS +1 Rule The number of lines observed for a particular nucleus as a result of n “identical” neighbors is 2nS + 1 where S is the spin of the neighboring nucleus. For most nucleus, S = ½, the relationship simplifies to n+1 lines “identical” in this context refers to nuclei that have the same or very similar coupling constants to the nucleus being observed. number of “identical neighbors” multiplicity of nucleus observed 1 2 (1:1) 2 3 (1:2:1) 3 4 (1:3:3:1) 4 5 (1:4:6:4:1) 5 6 (1:5:10:10:5:1)

36. Examples of First Order Spectra

37. CH3CH2OH

39. What information do you get out of a 1H NMR spectrum? Chemical Shift? An indication of the type of proton and its environment Multiplicity? An indication of the number of nearest neighbors and their proximity Area? A measure of the relative number of hydrogen nuclei in the molecule

41. The compound has a IR frequency of 1720 cm-1 and a molecular formula of C4H8O. What is its structure? 3 2 3

43. geminal 2J vicinal 3J 4J 5J

48. Magnitude of the Vicinal Coupling Constant J Karplus Equation 3JCHCH = 10 cos2(φ) where φ is the dihedral angle

50. Summary of the Field Dependence of  and J • is the local field that is induced by the magnitude of the external field, Ho.  is therefore chemical shift dependent. J is dependent on the magnetic moment of the proton and is therefore independent of the external field, Ho.