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Chapter 13 NMR Spectroscopy

Chapter 13 NMR Spectroscopy. NMR - Nuclear Magnetic Resonance NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds. Invented by physicists (1950’s), then used by chemists (1960’s).

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Chapter 13 NMR Spectroscopy

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  1. Chapter 13 NMR Spectroscopy NMR - Nuclear Magnetic Resonance NMR is a form of spectroscopy that uses an instrument with a powerful magnet to analyze organic compounds. Invented by physicists (1950’s), then used by chemists (1960’s). MRI – Magnetic Resonance Imaging (1980’s) A special form of NMR used in medicine.

  2. What is NMR? NMR – a tool to determine the structure of an organic compound. NMRSpectrometer magnet This instrument gives you information about an organic compound’s structure. 1H NMRSpectrum computer

  3. What is NMR? NMR Instruments computer Small, 60 mHz instrument for undergraduate student use. magnet student

  4. What is NMR? NMR Instruments Research grade instrument, 300 mHz magnet, that we use at Western. magnet

  5. What is NMR? NMR Instruments Research grade instrument, 300 mHz magnet, that we use at Western. computer

  6. What is NMR? NMR Instruments researcher State-of-the-art instrument, 950 mHz magnet. Rather large and expensive! magnet

  7. spinning proton bar magnet Why is it called NMR? Nuclear Magnetic Resonance Nuclear – because it looks at the nucleus of an atom, most commonly a hydrogen atom. A hydrogen atom nucleus consists of one proton with a +1 charge and “spin” of ½. It acts like a tiny bar magnet. proton generatesmagneticfield

  8. NMR – Effect of Magnetic Field Sample in Magnetic FieldSpins align with or against the external magnetic field No External Magnetic FieldNuclear spins are pointedin random directions

  9. hydrogennuclei NMR – Effect of Magnetic Field alignedagainst field(higher energy) aligned with field(lower energy) Sample placed in an external magnetic field H0 No external magnetic field applied to sample Random orientation of nuclear spins Spins align with or against field (most align with field)

  10. NMR: Absorption of Energy radio waves nucleusabsorbs energy Scan with RF field – nucleus absorbs energy, giving a signal in the NMR spectrum Initial State – nucleus at low energy level

  11. NMR: Information Obtained from a Spectrum • An NMR Spectrum will generally provide three types of information: • Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded) • Integration – gives the relative number of nuclei producing a given signal • Spin-Spin Coupling – describes the connectivity

  12. signal fromprotons in H20 1H NMR Spectrum – H2O scanning A sample of water is placed in an NMR instrument, and a proton spectrum is recorded (scanned from left to right). An NMR signal appears. This proves thatwater contains hydrogen atoms!

  13. When does nucleus absorb energy? Not all protons are the same! Magnetic Fields: 1. from spinning proton 2. from magnet 3. from electrons 3. 2, External Field (Ho)from magnet Absorption depends on shielding by electron cloud around the nucleus. More electron density = more shielding = signal shifted to the right.

  14. NMR: Simple 1H NMR Spectrum Showing Chemical Shift Chemical Shift: location of the signalon the spectrum. Right Side:high electrondensity Left Side:low electrondensity Two types of protons (a CH2 and a CH3) give two separate signals at two different chemical shifts.

  15. NMR: Chemical Shift Practice -OCH3 -CCH3 Cl3C-H EN Group 3 electronegative atoms -O-CH3 -Si-CH3 -C-CH3 Cl3C-H 3.5 1.8 2.5 3.0 Left Side:low electrondensity(high EN) -SiCH3 Assign the four groups shown to the four NMR singals, based on each element’s electronegativity.

  16. TMS = (silicon – low electroneg.) NMR: Chemical Shift Reference Chemical shift zero is set to TMS (tetramethylsilane). Chemical shift measured in ppm. For 1H: roughly 0 to 10 ppm.

  17. NMR: Chemical Shift Regions -CH2-CH3 Alkane region (high electron density) is from about .8 – 2.5 ppm.

  18. NMR: Chemical Shift Regions -O-CH3 Heteroatom region (low electron density) is from about 2.5 to 5.

  19. H C=C H NMR: Chemical Shift Regions Double bond region is on the left, from about 5 – 10 ppm.

  20. NMR: Chemical Equivalence and Number of Signals How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry) 2 4 5 2 4 7

  21. 4 2 1 3 NMR: Chemical Equivalence and Number of Signals How many signals should appear in the proton NMR spectrum for these compounds? In theory: 9 4 Signals actually resolved: 3-4 2

  22. NMR: Overlapping Proton Signals octane The -CH2- groups allappear in the same spot (not resolved) Protons b, c, and d are in roughly the same environment, and their chemical shifts are also about the same.

  23. These H’s are different Review: How Many NMR Signals? How many signals will the following compounds show in their 1H NMR Spectrum? (Hint: check for symmetry) 1 2 5 Fast chair flips at RT Fast rotation about C-C single bond No rotation about double bonds

  24. NMR: Chloroethane Fast rotation around single bonds gives an “averaged” spectrum for the three methyl hydrogens. An NMR spectrometer is like a camera with a slow shutter speed.

  25. NMR: Chair Cyclohexane Rapid chair flipping makes all H’s equivalent. Cylcohexane gives one peak in the 1H NMR spectrum. An NMR spectrometer is like a camera with a slow shutter speed.

  26. bigger (9 H’s) smaller (2 H’s) NMR: A Second Proton Spectrum Note: the signal for the nine methyl H’s (red) is larger than the signal for the CH2 group (blue)

  27. NMR: Information Obtained from a Spectrum • An NMR Spectrum will generally provide three types of information: • Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded) • Integration – gives the relative number of nuclei that produces a given signal. The integral (area under the curve) is drawn on the spectrum by the instrument. • Spin-Spin Coupling – describes the connectivity

  28. Integral has relative height 9 Relative height 2 NMR: Integration Indicates Relative Number of Nuclei The height of the integration line (“integral”) gives you the relative number of nuclei producing each signal.

  29. NMR: Information Obtained from a Spectrum • An NMR Spectrum will generally provide three types of information: • Chemical Shift – indicates the electronic environment of the nucleus (shielded or deshielded) • Integration – gives the relative number of nuclei producing a given signal • Spin-Spin Coupling: - describes the carbon connectivity - follows the “n+1”rule”

  30. NMR: Splitting into a Doublet doublet Note that the red signal at 1.6 ppm for the methyl group is split into two peaks. Remember that this is one signal, composed of two separate peaks.

  31. NMR: Signal Splitting, n+1 Rule • A signal is often split into multiple peaks due to interactions with protons on carbons next door. Called spin-spin splitting • The splitting is into one more peak than the number of H’s on adjacent carbons (“n+1 rule”) • Splitting of a signal can give doublets (two peaks), triplets (three peaks), quartets (4 peaks), ect. • The relative intensities given by Pascal’s Triangle: doublet 1 : 1 triplet 1 : 2 : 1 quartet 1 : 3 : 3 : 1 pentet: 1 : 4 : 6 : 4 : 1

  32. NMR: Signal Splitting, n+1 Rule n+1 Rule: A signal in the proton NMR spectrum will be split into n+1 peaks, where n is the number of protons on adjacent carbons. Example: CH3-CH2-Br For the Methyl Group: There are two protons ‘next door’ (n=2), so the methyl signal will be split into three peaks (2+1), which is called a triplet. Chemical shift will be about 1.5 (alkane region), integration = 3. For the -CH2- Group: Three protons next door means the CH2 signal will be split into 4 (3+1) peaks, called a quartet. Chemical shift = 3.3 (heteroatom region), integration = 2.

  33. Three peaks, a triplet (1:2:1) Four peaks, a quartet (1:3:3:1) 1H NMR Spectrum for Bromoethane integration: 2 H 3 H Note the expansionsprinted above the spectrum

  34. NMR: Signal Splitting, n+1 Rule Peak Heights - Pascal’s Triangle singlet 1doublet 1 : 1triplet 1 : 2 : 1quartet 1 : 3 : 3 : 1pentet 1 : 4 : 6 : 4 : 1

  35. NMR: Signal Splitting, n+1 Rule many lines = “mulitplet”

  36. seven peaks How many neighbors? NMR: Signal Splitting, n+1 Rule H n + 1 = 7 n = 6

  37. NMR: Origin of Spin-Spin Splitting

  38. NMR: Origin of Spin-Spin Splitting

  39. NMR: Doublets and Triplets Triplet: for the two protons next door,there are four combinations possible:αααβ βββ α Doublet: the one proton next doorcan be either up or down (α or β)

  40. NMR: Signal Splitting, n+1 Rule

  41. NMR: Using the n+1 Rule Using the n+1 rule, predict the 1H NMR spectrum of 2-iodopropane.Give splitting pattern, integration, and approximate chemical shift. six neighbors one neighbor Note that the methyl groups are equivalent, so they will give one signal in the NMR spectrum.

  42. doublet Seven line pattern NMR: Spectrum of 2-iodopropane

  43. Equivalent protons do not split each other NMR: Rules for Spin-Spin Splitting • The signal of a proton with n equivalent neighboring H’s is split into n + 1 peaks • Protons that are farther than two carbon atoms apart do not split each other

  44. Common 1H NMR Patterns 1. triplet (3H) + quartet (2H) -CH2CH3 2. doublet (1H) + doublet (1H) -CH-CH- 3. large singlet (9H) t-butyl group 4. singlet 3.5 ppm (3H) -OCH3 group 5. large double (6H) + muliplet (1H) isopropyl 6. singlet 2.1 ppm (3H) methyl ketone

  45. Common 1H NMR Patterns 7. multiplet ~7.2 ppm (5H) aromatic ring, monosubstituted 8. multiplet ~7.2 ppm (4H) aromatic ring, disubstituted 9. broad singlet, variable -OH or –NH chemical shift (H on heteratom)

  46. Solving NMR Problems 1. Check the molecular formula and degree of unsaturation. How many rings/double bonds? 2. Make sure that the integration adds up to the total number of H’s in the formula. 3. Are there signals in the double bond region? 4. Check each signal and write down a possible sub-structure for each one. 5. Try to put the sub-structures together to find the structure of the compound.

  47. Proton NMR Spectrum: C9H12 aromatic, disubst. Degree of Unsat = 4

  48. 1H NMR Spectrum: C4H7O2Br Degree of Unsat = 1 s 3H t 2H t 2H 5.0 4.0 3.0 2.0 1.0 0

  49. Electronegative Substituents: Shift Left smalleffect ~noeffect Propane: heteroatomregion • CH3Cl 3.1 (one Cl) • CH2Cl2 5.3 (two Cl’s) • CHCl3 7.3 (three Cl’s) d 0.9 d 1.3 d 0.9 d 4.3 d 2.0 d 1.0 H3C—CH2—CH3 O2N—CH2—CH2—CH3 Effect is cumulative

  50. H NR H OR H OAr O C HO Hydrogens on Heteroatoms Chemical shifts for protons on heteroatoms are variable, and signals are often broad (not generally useful). Chemical shift (ppm) Type of proton 1-3 0.5-5 6-8 farleft may beuseful 10-13

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