NMR Spin-Spin Coupling. Further Discussion

Transcription

1 NMR Spin-Spin oupling Further Discussion

2 Signs and Mechanisms Most important mechanism is Fermi contact mechanism (see below). J can be either positive or negative. Signs of J usually DO NOT affect the NMR spectra. Positive J Negative J Many exceptions

3 Signs of J (Another View) a b β β E α b E α a α Positive J β α Negative J β

4 Proton-arbon Direct oupling ( 1 J ) 1 J = 120 ~ 200 z

5 13 -Satellite Peaks in 1 -NMR O O O 13 O should be present in the sample (1.1%). 13 O O (? %)

6 1,3-Dioxolane 13 -Satellite J value?

7 Dimethyl Maleate Satellite Peaks

8 More 13-Satellite Peaks

9 - oupling and s-haracter 1 J (z) S-character 29% 32% 36% sp2.4 sp2 sp1.8

10 Two Bond ouplings α 2 J 2 J = 12 ~ 18 z 2 J = 0 ~ 3 z 2 J = 5 z α Geminal oupling J = γ γ D J D D = 6.51 x J D

11 Three Bond oupling φ 0 3 J Vicinal coupling 90 Karplus relationship 180

12 3,5-Diphenylbromocyclohexanes Br Br

13 Long Range oupling 4 J, 5 J, 6 J etc. oupling between protons that are separated by more than 3 bonds. In saturated systems, 4 J and 5 J are commonly observed when - and - bonds exist in the zig-zag arrangement. M or W arrangement

14 Long Range oupling in Saturated Systems J = 3 ~ 4 z 3 J = 7 ~ 8 z J = 1.5 z J = 18 z

15 Long Range oupling in Unsaturated Systems 4 J = 1.33z 5 J = 9.63z 4 J = 1.75z 5 J = 8.04z 3 6 J = 0.8z 6 J = 0.2z J = 2.7z O J = 1.3z 5 J = 0.4z 3 2 O 9 J = 0.4z (!)

16 Peak Broadening Due to Benzylic oupling

17 Determining J-values - Vinyl Acetate -

18 dd Patterns and Examples 1 J 12 J Two Js and a total of 4 lines

19 ddd Patterns and Examples Three Js and a total of 8 lines

20 dddd Pattererns and Examples Four Js and a total of 16 lines

21 Key Observations d dd ddd dddd # of Js # of lines Some lines may overlap, giving higher intensity Splitting is symmetrical Spacing between line 1 and 2 (or line N-1 and N) gives the smallest coupling constant.

22 Inverted Tree Generation Determine (1) Total# of lines (2) relative line intensities For dddd, (relative intensities) = 16 Thus, four Js, J L, J ML, J MS and J S (i) Determine J S first (ii) Identify the full set of pairs of lines that are seprated by J S. (iii) Identify the centers of each of the pairs. (iv) Now, you have a ddd system to analyze! Determine J MS. (v) You now have a dd system, and determine J ML. (vi) Finally, determine J L.

23 Example - ddd J S J M J L

24 Example - dddd J S J MS J ML J L

25 Which ydrogens are oupling? J values a l l l a b Spin decoupling 2D NMRs such as OSY

26 Spin Decoupling A X ν A ν A ν X ν A ν X ν X ν A ν X

27 Ethyl rotonate

28 Mannosan Triacetate

29 Let s onsider Problem 3.6(b)

30 3.6(b) 3 l 3 l No symmetry 1 2 2

31 3.6(b) What are they? 3 l a l a They do not seem to follow the N+1 rule????

32 They are 2 nd Order Spectra

33 1 st Order and 2 nd Order Spectra Second-order spectra are chracterized by peak spacings that do not correspond to coupling constants, by nonbinominal intensities, by chemical shifts that are not at resonance midpoints, or by resonance multiplicities that do not follow the n+1 rule.

34 Trans-3-methoxyacrylonitrile A 1st order spectrum A B

35 Bromostilbene A 2nd order spectrum a Br a = ppm b = ppm b

36 onsider a Spin-Spin oupling δ a = ν a - ν ref ν instrument x 10 6 a b δ b = ν b - ν ref ν instrument x 10 6 ν a J ab ν b δ a - δ b = ν a - ν b ν instrument x 10 6 Δν = ν a - ν b = (δ a - δ b ) x ν instrument x 106 When Δν >> J ab, the 1st order splitting pattern is observed. J ab J ab ν a ν b

37 What are the haracteristics of 1st Order Spectra?

38 1st and 2nd Order Spectra 1st order spectra Δν/J > 10 Use A, M, X etc (Pople notation) 2nd order spectra Δν/J 10 Use A, B, etc (Pople notation) O 3 N AX A 3 Δδ = = 2.605ppm Δν = x = 1041 z J ab ~ 16 z Δν/J = 65 b a Br AB Δδ = = 0.048ppm Δν = x = 19.2 z J ab ~ 16 z Δν/J = 1.2

39 A 2 to AB to AX A 2 a a AB A Br B AX A O 3 N X

40 AB Spin System Δν AB = (4 2 - J 2 ) ν A = ν 2 + ν Δν AB, ν B = ν 2 + ν Δν AB

41 Simple Spin Systems l l l F F l A 2 A 2 X 2 A 2 F l 3 O 2 l l A 2 X A 2 X 3 A 4 3 O 3 O 3 A 2 X 2 A 3 AX 6

42 More Spin Systems ABX 3 O 2 B X O A A A 2 B 2 X A O 2 O 3 Br B B O 2 3 A 2 X ABX 3

43 3-Phenyl Glutaric Acid E, ppm F, ppm

44 Simulated AB 2 /AX 2 and A 2 B 2 /A 2 X 2 Systems Both chemical environment (J) and spectral frequency affect the spectra. Δν = ν a - ν b = (δ a - δ b ) x ν instrument x 106

45 Magnetic Equivalence hemically equivalent OR enantiotopic protons (same δ) Magnetically equivalent protons (same δ) Show no splitting between them Magnetically non-equivalent protons (same δ) Show spin-spin coupling interactions δν/j = 0, and thus 2nd order spectra

46 Magnetically Equivalent Nuclei (1) They are chemically equivalent. (2) They have equal coupling (same J-values) to ALL other nuclei in the spin system. 2 J F 3 J F (trans) Magnetically equivalent protons 19 F 19 F Magnetically non-equivalent protons 19 F 19 F 2 J F A 2 X 2 3 J F (cis) AA XX

47 Difluoromethane and Difluoroethene 2 J F 2 J F

48 Ortho-Disubstituted Benzene A, A X, X

49 Furan X X A A A & A ppm X & X ppm 90 Mz z from TMS Lunazzi, Mol. Phys. 1970,3, 413

50 Para-Disubstituted Benzene A O 3 A X X O

51 Para-disubstituted Benzens

52 Y Z (AA XX )System

53 yclic Acetal

54 More AA BB Systems

55 Virtual oupling X is NOT simple doublet A B Strongly coupled AND ν A ~ ν B J BX large J AX ~ 0 X A and B are strongly coupled AND ν A ~ ν B Virtual oupling (2nd order)

56 Linear Alkanes First order Virtual coupling O 3 O 2 N 3 2 -O Same shift 3 broad 2 β

57 3-Methyl Glutaric Acid Me broad doublet OO B B OO A Me A A 2 B 2 X 3

58 Dimethylbenzoquinones O J ~ O Two AX 3 spin systems X 3 3 O 3 X 3 ' A O A' J AA XX 3 system

59 Dimethyl Adipiate 3) O 2 Me O 2 Me A X X A A, A X, X

60 Which is the orrect One?

61 AA XX 2 nd Order Spectra