Nmr ch-19-part-4
- 1. Chapter 19 Part IV Nuclear Magnetic Resonance Dr. Nizam M. El-Ashgar Chemistry Department Islamic University of Gaza 03/12/18 1Chapter 19
- 2. 2 Spin-Spin Splitting in 1 H NMR Spectra Peaks are often split into multiple peaks due to magnetic interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting. The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule” The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5=pentet, 6=sextet, 7=heptet…..)
- 3. 03/12/18 Chapter 19 3 SPIN–SPIN COUPLING (SPLITTING): (n + 1) Rule NMR Signals:NMR Signals: not all appear as a single peak. Peak:Peak: The units into which an NMR signal appears: singlet, doublet, triplet, quartet, etc. Signal splitting:Signal splitting: Splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens. ((nn + 1) rule:+ 1) rule: If a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1 H- NMR signal is split into (n + 1) peaks.
- 4. 03/12/18 Chapter 19 4 Origins of Signal Splitting Signal coupling:Signal coupling: An interaction in which the nuclear spins of adjacent atoms influence each other and lead to the splitting of NMR signals. Coupling constant (J):Coupling constant (J): The separation on an NMR spectrum (in hertz) between adjacent peaks in a multiplet. A quantitative measure of the influence of the spin- spin coupling with adjacent nuclei.
- 5. 03/12/18 Chapter 19 5 Physical Basis for (n + 1) Rule Coupling of nuclear spins is mediated through intervening bonds. H atoms with more than three bonds between them generally do not exhibit noticeable coupling. For H atoms three bonds apart, the coupling is referred to as vicinal coupling.
- 6. 03/12/18 Chapter 19 6 Rules for Spin-Spin Splitting Equivalent protons do not split each other Protons that are farther than two carbon atoms apart do not split each other
- 7. 03/12/18 Chapter 19 77 1 H NMR—Spin-Spin Splitting • Splitting is not generally observed between protons separated by more than three σ bonds. • If Ha and Hb are not equivalent, splitting is observed when:
- 8. 03/12/18 Chapter 19 88 • Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. The Origin of 1 H NMR—Spin-Spin Splitting Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs: • When placed in an applied field, (B0), the adjacent proton (CHBr2) can be aligned with (↑) or against (↓) B0. The likelihood of either case is about 50% (i.e., 1,000,006↑ vs 1,000,000↓). • Thus, the absorbing CH2 protons feel two slightly different magnetic fields —one slightly larger than B0, and one slightly smaller than B0. • Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet, where the two peaks of the doublet have equal intensity.
- 9. 03/12/18 Chapter 19 9 Origins of Signal Splitting Ha and Hb are non-equivalent
- 10. 03/12/18 Chapter 19 10 n = 3. Its signal is split into (3 + 1) or 4 peaks; a quartet n = 1. Their signal is split into (1 + 1) or 2 peaks ; a doublet CH3 -CH-Cl Cl
- 11. 03/12/18 Chapter 19 11 Origins of Signal Splitting When the chemical shift of one nucleus is influenced by the spin of another, the two are said to be coupled. Consider nonequivalent hydrogens Ha and Hb on adjacent carbons. The chemical shift of Ha is influenced by whether the spin of Hb is aligned with or against the applied field C C Ha Hb
- 12. 03/12/18 Chapter 19 12 MagneticfieldofHbadds totheappliedfield;Ha signalappearsatalower appliedfield MagneticfieldofHb subtractsfromtheapplied field;Hbsignalappearsat ahigherappliedfield Hb Hb Ha B0
- 13. 03/12/18 Chapter 19 13
- 14. 03/12/18 Chapter 19 1414
- 15. 03/12/18 Chapter 19 15 Spin-Spin Splitting Non equivalent protons on adjacent carbons always interact each other. Equivalent protons do not not split each other. CH3 – CO - CH3 Do not split CH3 – CH2 - Cl Split each other
- 16. 03/12/18 Chapter 19 16 Spin-Spin Splitting
- 17. 03/12/18 Chapter 19 17 Spin-Spin Splitting
- 18. 03/12/18 Chapter 19 18 Spin-Spin Splitting
- 19. 03/12/18 Chapter 19 19 Spin-Spin Splitting If a signal is split by N equivalent protons, it is split into N + 1 peaks.
- 20. 03/12/18 20 Pascal’s Triangle: As illustrated by the highlighted entries, each entry is the sum of the values immediately above it to the left and the right .
- 21. 03/12/18 Chapter 19 21 Coupling Constants (J) Distance between the peaks of multiplet measured in Hz (usually 0-18) called coupling constant. J is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled. Not dependent on strength of the external field. Gives info on type of H. Multiplets with the same coupling constants may come from adjacent groups of protons that split each other. Structural features.
- 22. 03/12/18 Chapter 19 22 An important factor in vicinal coupling is the angle a between the C-H sigma bonds and whether or not it is fixed. Coupling is a maximum when a is 0° and 180°; it is a minimum when a is 90°
- 23. 03/12/18 Chapter 19 23 Typical coupling constant
- 24. 03/12/18 Chapter 19 24 8-11 Hz 8-14 Hz 0-5 Hz 0-5 Hz6-8 Hz 11-18 Hz 5-10 Hz 0-5 Hz CC Ha C C HbHa C Hb C Ha Hb Ha Hb Ha Hb Hb Ha Hb Ha C C Ha Hb
- 25. 03/12/18 Chapter 19 25 Spin Decoupling It’s a powerful tool for determining 1. The connectivity of the protons. 2. Assigning proton peaks Irradiation of one proton in a spin coupled system removes its coupling effect on the neighboring protons to which it had coupled. CH3 - CH2 - CH2 OH Triplet Triplet Sextet
- 26. 03/12/18 Chapter 19 26 CH3 - CH2 - CH2 OH Irradiate CH3 - CH2 - CH2 OH Triplet Quartet Irradiate CH3 - CH2 - CH2 OH Singlet CH3 - CH2 - CH2 OH Triplet Triplet
- 27. 03/12/18 Chapter 19 27 Ethyl acetate (HW)
- 28. 03/12/18 Chapter 19 28
- 29. 03/12/18 Chapter 19 29 Ethyl Bromide
- 30. 03/12/18 Chapter 19 30 1 H-NMR spectrum of 1,1-dichloroethane CH3-CH-Cl Cl For these hydrogens, n = 1; their signal is split into (1 + 1) = 2 peaks; a doublet For this hydrogen, n = 3; its signal is split into (3 + 1) = 4 peaks; a quartet
- 31. 03/12/18 Chapter 19 31 Expansion of Spectrum (HW) Because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see, it is common to retrace and expand certain signals. 1 H-NMR spectrum of 3-pentanone; expansion more clearly shows the triplet/quartet
- 32. 03/12/18 Chapter 19 32 Spectra Involving Chemical Exchange Processes Pure dry liquid ethanol. Ethanol containing a very small amount of HCl. Note: in this case there is no change in chemical shift(s), only in splitting pattern(s).
- 33. 03/12/18 Chapter 19 33 Spectra Involving Chemical Exchange Processes The observed signal is the result of the weighted average of the nucleus in its different magnetic environments. Fast exchanges show up as sharp signals. Exchanges on the NMR timescale (“intermediate”) show up as broad signals. (presence of acid/base catalyst, temperature, nature of the solvent, etc.) Slow exchanges will show two separate lines.
- 34. 03/12/18 Chapter 19 34 O-H and N-H Signals Chemical shift depends on concentration. Hydrogen bonding in concentrated solutions deshield the protons, so signal is around δ 3.5 for N-H and δ 4.5 for O-H. Proton exchanges between the molecules broaden the peak.
- 35. 03/12/18 Chapter 19 35 Carboxylic Acid Proton, δ10+ =>
- 36. 03/12/18 Chapter 19 36 Splitting of Hydroxyl Proton Ultrapure samples of ethanol show splitting Ethanol with a small amount of acidic or basic impurities will not show splitting
- 37. 03/12/18 Chapter 19 37 N-H Proton Moderate rate of proton transfer Peak may be broad
- 38. 03/12/18 Chapter 19 3838 1 H NMR—Spin-Spin Splitting Whenever two (or three) different sets of adjacent protons are not equivalent to each other, use the n + 1 rule to determine the splitting pattern only if the coupling constants (J) are identical: a a b c Free rotation around C-C bonds averages coupling constant to J = 7Hz Jab = Jbc
- 40. 03/12/18 Chapter 19 40
- 42. 03/12/18 Chapter 19 42 Signal Splitting (n + 1) example
- 43. 03/12/18 Chapter 19 43 More Complex Splitting Patterns Thus far, we have observed spin-spin coupling with only one other nonequivalent set of H atoms. More complex splittings arise when a set of H atoms couples to more than one set H atoms. A tree diagram shows that when Hb is adjacent to nonequivalent Ha on one side and Hc on the other, coupling gives rise to a doublet of doublets.
- 44. 03/12/18 Chapter 19 44 If Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets.
- 45. 03/12/18 Chapter 19 45
- 46. 03/12/18 Chapter 19 46 Because the angle between C-H bond determines the extent of coupling, bond rotation is a factor. In molecules with relatively free rotation about C-C sigma bonds, H atoms bonded to the same carbon in CH3 and CH2 groups generally are equivalent. If there is restricted rotation, as in alkenes and cyclic structures, H atoms bonded to the same carbon may not be equivalent. Nonequivalent H on the same carbon will couple and cause signal splitting, this type of coupling is called geminal coupling.geminal coupling.
- 47. 03/12/18 Chapter 19 47 In ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent.
- 48. 03/12/18 Chapter 19 48 A tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate.
- 49. 03/12/18 Chapter 19 49 Cyclic structures often have restricted rotation about their C-C bonds and have constrained conformations As a result, two H atoms on a CH2 group can be nonequivalent, leading to complex splitting.
- 50. 03/12/18 Chapter 19 50 A tree diagram for the complex coupling in 2-methyl-2- vinyloxirane
- 51. 03/12/18 Chapter 19 51 Complex coupling in flexible molecules: Coupling in molecules with unrestricted bond rotation often gives only m + n + I peaks. That is, the number of peaks for a signal is the number of adjacent hydrogens + 1, no matter how many different sets of equivalent H atoms that represents. The explanation is that bond rotation averages the coupling constants throughout molecules with freely rotation bonds and tends to make them similar; for example in the 6- to 8-Hz range for H atoms on freely rotating sp3 hybridized C atoms.
- 52. 03/12/18 Chapter 19 52 simplification of signal splitting occurs when coupling constants are the same.
- 53. 03/12/18 Chapter 19 53 simplification of signal splitting occurs when coupling constants are the same.
- 54. 03/12/18 Chapter 19 54 An example of peak overlap occurs in the spectrum of 1- chloropropane. The central CH2 has the possibility for 12 peaks (a quartet of triplets) but because Jab and Jbc are so similar, only 5 + 1 = 6 peaks are distinguishable.