13.9 Spin-Spin Splitting

1. 13.9 Spin-Spin Splitting

2. SPIN-SPIN SPLITTING Often a group of hydrogens will appear as a multiplet rather than as a single peak. Multiplets are named as follows: Singlet Quintet Doublet Septet Triplet Octet Quartet Nonet This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.

3. 1,1,2-Trichloroethane integral = 2 integral = 1 triplet doublet

4. n + 1 RULE

5. this hydrogen’s peak is split by its two neighbors these hydrogens are split by their single neighbor MULTIPLETS singlet doublet triplet quartet quintet sextet septet two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet

6. Some Common Patterns

7. SOME COMMON SPLITTING PATTERNS CH-CH3 ( x = y ) CH-CH2 -CH2-CH3 CH3 ( x = y ) CH CH3

8. tert-butyl group

9. EXCEPTIONS TO THE N+1 RULE IMPORTANT ! Protons that are equivalent by symmetry usually do not split one another 1) no splitting if x=y no splitting if x=y Protons in the same group usually do not split one another 2) or

10. SOME EXAMPLE SPECTRA WITH SPLITTING

11. NMR Spectrum of Bromoethane

12. in higher multiplets the outer peaks are often nearly lost in the baseline 1:6:15:20:16:6:1 NMR Spectrum of 2-Nitropropane

13. NMR Spectrum of Acetaldehyde offset = 2.0 ppm

14. The propyl group

15. INTENSITIES OF MULTIPLET PEAKS PASCAL’S TRIANGLE

16. The interior entries are the sums of the two numbers immediately above. PASCAL’S TRIANGLE Intensities of multiplet peaks 1 singlet 1 1 doublet 1 2 1 triplet 1 3 3 1 quartet 1 4 6 4 1 quintet 1 5 10 10 5 1 sextet 1 6 15 20 15 6 1 septet 1 7 21 35 35 21 7 1 octet

17. THE ORIGIN OF SPIN-SPIN SPLITTING HOW IT HAPPENS

18. THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED BY THE SPIN OF ITS NEIGHBORS aligned with Bo opposed to Bo +1/2 -1/2 50 % of molecules 50 % of molecules H H H H A A C C C C Bo downfield upfield neighbor aligned neighbor opposed At any given time about half of the molecules in solution will have spin +1/2 and the other half will have spin -1/2.

19. SPIN ARRANGEMENTS one neighbor n+1 = 2 doublet one neighbor n+1 = 2 doublet H H H H C C C C yellow spins blue spins The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.

20. SPIN ARRANGEMENTS two neighbors n+1 = 3 triplet one neighbor n+1 = 2 doublet methine spins methylene spins

21. H H H H H H C C C C H H H H SPIN ARRANGEMENTS three neighbors n+1 = 4 quartet two neighbors n+1 = 3 triplet methylene spins methyl spins

22. 13.10 The Coupling Constant

23. THE COUPLING CONSTANT J J J J J J The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet. J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

24. Separation is larger FIELD COMPARISON 100 MHz 200 Hz 100 Hz Coupling constants are constant - they do not change at different field strengths 7.5 Hz J = 7.5 Hz 6 5 4 3 2 1 200 MHz 400 Hz 200 Hz 7.5 Hz The shift is dependant on the field J = 7.5 Hz ppm 3 2 1

25. 50 MHz J = 7.5 Hz Why buy a higher field instrument? 3 2 1 Spectra are simplified! 100 MHz J = 7.5 Hz Overlapping multiplets are separated. 3 2 1 200 MHz J = 7.5 Hz Second-order effects are minimized. 3 2 1

26. NOTATION FOR COUPLING CONSTANTS The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. This is sometimes called vicinal coupling. It is designated 3J since three bonds intervene between the two hydrogens. 3J Another type of coupling that can also occur in special cases is 2J or geminal coupling ( most often 2J = 0 ) Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. 2J

27. LONG RANGE COUPLINGS Couplings larger than 2J or 3J also exist, but operate only in special situations, especially in unsaturated systems. Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”

28. SOME REPRESENTATIVE COUPLING CONSTANTS 6 to 8 Hz three bond 3J vicinal 11 to 18 Hz three bond 3J trans 6 to 15 Hz three bond 3J cis 0 to 5 Hz two bond 2J geminal

29. cis 6 to 12 Hz three bond 3J trans 4 to 8 Hz 4 to 10 Hz three bond 3J 0 to 3 Hz four bond 4J 0 to 3 Hz four bond 4J Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (<3 Hz)

30. 13.11 NMR Spectra of Carbonyl Compounds • Anisotropy in carbonyl compounds • Anisotropy deshields C-H on aldehydes: 9-10 ppm • Anisotropy also deshields methylene and methyl groups next to C=O: 2.0 - 2.5 ppm • Methylene groups directly attached to oxygen appear near 4.0 ppm

31. 1 2-Butanone (Methyl Ethyl Ketone) 60 MHz Spectrum

32. 2-butanone, 300 MHz spectrum WWU Chemistry

33. 2 Ethyl Acetate Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).

34. 3 t-Butyl Methyl Ketone (3,3-dimethyl-2-butanone)

35. 4 Phenylethyl Acetate

36. 5 Ethyl Succinate

37. a-Chloropropionic Acid 6

38. 13.12 and 13.13 Alkenes, Alkynes and Aromatic Compounds

39. CHEMICAL SHIFTS Alkenes and alkynes • vinyl protons appear between 5 to 6.5 ppm (anisotropy) • methylene and methyl groups next to a double bond appear at about 1.5 to 2.0 ppm • for terminal alkynes, proton appears near 2 ppm

40. BENZENE RING HYDROGENS Ring current causes protons attached to the ring to appear in the range of 7 to 8 ppm. Protons in a methyl or methylene group attached to the ring appear in the range of 2 to 2.5 ppm.

41. NMR Spectrum of Toluene 5 3

42. THE EFFECT OF CARBONYL SUBSTITUENTS When a carbonyl group is attached to the ring the o- and p- protons are deshielded by the anisotropic field of C=O Only the o- protons are in range for this effect.

43. Acetophenone (90 MHz) 3 2 3 deshielded

44. NMR Spectrum of 1-iodo-4-methoxybenzene 3 CHCl3 impurity 2 2

45. NMR Spectrum of 1-bromo-4-ethoxybenzene 3 4 2

46. THE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILAR All peaks move closer. Outer peaks get smaller …………………..… and finally disappear. Inner peaks get taller…………………………. and finally merge. all H equivalent X = X X = Y X ~ X’ same groups

47. NMR Spectrum of 1-amino-4-ethoxybenzene 3 4 2 2

48. NMR Spectrum of p-Xylene (1,4-dimethylbenzene) 6 4

49. 13.14 Hydroxyl and Amino Protons

50. Hydroxyl and Amino Protons Hydroxyl and amino protons can appear almost anywhere in the spectrum (H-bonding). Carboxylic acid protons generally appear far downfield near 11 to 12 ppm. These absorptions are usually broader than other proton peaks and can often be identified because of this fact.