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Chapter 13

Chapter 13. Structure Determination: Nuclear Magnetic Resonance Spectroscopy. Introduction. Mass Spectrometry (MS) – determines the size and formula Infrared (IR) Spectroscopy – determines the kinds of functional groups present

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Chapter 13

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  1. Chapter 13 Structure Determination: Nuclear Magnetic Resonance Spectroscopy

  2. Introduction • Mass Spectrometry (MS) – determines the size and formula • Infrared (IR) Spectroscopy– determines the kinds of functional groups present • Nuclear Magnetic Resonance Spectroscopy (NMR) –– determines the carbon- hydrogen framework • Ultraviolet Spectroscopy (UV)– determines if a conjugated p electron system is present

  3. The Use of NMR Spectroscopy • It is used to determine relative location of atoms within a molecule • It is the most helpful spectroscopic technique in organic chemistry • It is related to Magnetic Resonance Imaging (MRI) in medicine • It maps carbon-hydrogen framework of molecules • It depends on very strong magnetic fields

  4. 1.Nuclear Magnetic ResonanceSpectroscopy (NMR) • Nuclear Magnetic Resonance Spectroscopy (NMR) – is a technique used to map the carbon-hydrogen framework • It detects the energy absorption accompanying the transition between nuclear spin states that occurs when a molecule is placed in a strong magnetic field and irradiated with radiofrequency waves.

  5. The 1H and 13C nuclei: • have spins (i.e behave as if they were spinning about an axis) • are magnetic (i.e interact with an external magnetic field, Bo)

  6. Other magnetic nuclei include: • all nuclei with an odd number of protons (1H, 2H, 14N, 19F, 31P,...) • all nuclei with an odd number of neutrons (13C) • Nonmagnetic nuclei include: • nuclei with even numbers of protons and neutrons (12C, 16O,…)

  7. In the absence of an external magnetic field, magnetic nuclear spins are oriented randomly • In the presence of an external field, Bo, magnetic nuclear spins orient with (parallel to) or against (antiparallel to) the external field. parallel antiparallel

  8. The parallel spin state (orientation) is slightly lower in energy and thereforefavored (more populated). parallel antiparallel

  9. When the oriented nuclei are irradiated with proper electromagnetic radiation frequency, energy is absorbed and the nuclei “spin-flip” from the lower-energy state(parallel)to the higher-energy state(antiparallel) • Magnetic nuclei are said to be in resonance with the applied radiation (hence the name of nuclear magnetic resonance). • The absorption of energy is detected, amplified and displayed as a nuclear magnetic resonance spectrum

  10. The exact frequency necessary for resonance depends on: • the strength of the external magnetic field • the identity of the nuclei

  11. The exact frequency necessary for resonance is proportional to field strength, Bo. • In the absence of an applied magnetic field, spin states have equal energies. In the presence of a magnetic field (Bo), spin states have unequal energies • If a very strong magnetic field is applied, the energy difference between the two spin states (DE) is larger, and higher-frequency (higher-energy) radiation is required for a spin-flip.

  12. Practice Problem: The amount of energy required to spin-flip a nucleus depends both on the strength of the external magnetic field and on the nucleus. At a field strength of 4.7 T, rf energy of 200 MHz is required to bring a 1H nucleus into resonance , but energy of only 187 MHz will bring a 19F nucleus into resonance. Use the equation below to calculate the amount of energy required to spin-flip a 19F nucleus. Is this amount greater or less than that required to spin-flip a 1H nucleus? c . 1.20 X 10-4 kJ/mol . E = and n = l l(m)

  13. Practice Problem: Calculate the amount of energy required to spin-flip a proton in a spectrometer operating at 300 MHz. Does increasing the spectrometer frequency from 200 to 300 MHz increase or decrease the amount of energy necessary for resonance?

  14. 2.The Nature of NMR Absorptions • The absorption frequency is not the same for all 1H or 13C nuclei in a molecule. • All nuclei in molecules are surrounded by electrons with their own magnetic fields Blocal • When an external field is applied to a molecule, the effective field (Beffective) felt by the nucleus is a bit smaller than the applied field (Bapplied): Beffective=Bapplied- Blocal

  15. Electrons setting up local magnetic fields shield nearby nuclei from the full effect of the applied field • Nuclei are shielded from the full effect of the applied field • Since each specific nucleus in a molecule is shielded to a slightly different extent, the effective magnetic field is not the same for each nucleus: • A distinct NMR signal for each chemically distinct 13C or 1H nucleus in a molecule can be detected. • Different signals appear for nuclei in different environments

  16. Nuclear Magnetic Resonance (NMR) Spectrum • NMR spectrum – plots the effective field strength felt by the nuclei (x-axis) versus the magnitude or intensity of NMR resonance signals (corresponding to the intensity of rf energy) (y-axis) • The intensity of NMR resonance signals is proportional to the molar concentration of the sample. • Each peak in the NMR spectrum corresponds to a chemically distinct 1H or 13C nucleus in the molecule • 1H or 13C spectra cannot be observed on the same spectrometer because of the difference in energy required to spin-flip the nuclei

  17. 1H NMR Spectrum 13C NMR Spectrum

  18. Chemically equivalent nuclei • are shielded to the same extent (i.e they have the same electronic environment) • always show a single absorption • Example: Methyl Acetate The 3H’s on CH3C=O are equivalent The 3H’s on –OCH3 are equivalent

  19. NMR Spectrometer • The Operation of a typical NMR spectrometer is illustrated:

  20. The NMR Measurement • The sample is dissolved in a solvent that does not have a signal itself and placed in a long thin tube • The tube is placed within the gap of a magnet and spun • Radiofrequency energy is transmitted and absorption is detected • If two species interconverting faster than 103 times per second are present in a sample, NMR records only a single, averaged spectrum, rather than separate spectra of the two distinct species: “blurring effect”

  21. Species that interconvert give an averaged signal that can be analyzed to find the rate of conversion • Example: Cyclohexane

  22. Practice Problem: 2-Chloropropene shows signals for three kinds of protons in its 1H NMR spectrum. Explain

  23. 3.Chemical Shifts • Chemical shift- is the relative energy of resonance of a particular nucleus resulting from its local environment - is the position on the NMR chart where a nucleus absorbs

  24. NMR spectra show the applied field strength increasing from left to right: • Left part is downfield (or low-field) • Right part is upfield (or high-field)

  25. NMR spectra show the applied field strength increasing from left to right: • Nuclei that absorb on the downfieldside are weakly shielded • Nuclei that absorb on the upfield side are strongly shielded

  26. NMR chart is calibrated versus a reference point, set as 0, tetramethylsilane [TMS] • TMS is used as a reference for both 1H and 13C

  27. Measuring Chemical Shift • Numeric value of chemical shift – is the difference between strength of magnetic field at which the observed nucleus resonates and field strength for resonance of a reference • Difference is very small but can be accurately measured • Taken as a ratio to the total field and multiplied by 106 so the shift is in parts per million (ppm) • Absorptions normally occur downfield of TMS, to the left on the chart

  28. Measuring Chemical Shift • NMR charts are calibrated using an arbitrary scale called the delta scale(d), where1d = 1 ppm (part per million) of spectrometer frequency •  is the number of parts per million (ppm) of the magnetic field expressed as the spectrometer’s operating frequency (it is a ratio and not a unit) Observed chemical shift (number of Hz away from TMS)  = Spectrometer frequency in MHz

  29. Measuring Chemical Shift • The chemical shift of an NMR absorption in d units is constant, regardless of the operating frequency of the spectrometer • It is independent of instrument’s field strength • Example: A 1H nucleus at d= 2.0 ppm on a 60 MHz instrument also absorbs at d = 2.0 ppm on a 300 MHz.

  30. Measuring Chemical Shift • Interpretation of a spectrum is easier with higher operatingspectrometer frequency (less likelihood of signal overlap) • For 60 MHz instrument, 1d = 60 Hz • For 300 MHz instrument, 1d = 300 Hz • Absorptions normally occur downfield of TMS: • Almost all1H NMR absorptions occur 0-10 ppm from TMS • Almost all 13C NMR absorptions occur 0-220 ppm from TMS.

  31. Practice Problem: When the 1H NMR spectrum of acetone, CH3COCH3, is recorded on an instrument operating at 200 MHz, a single sharp resonance at 2.1 d is seen. • How many hertz downfield from TMS does the acetone • resonance correspond to? • If the 1H NMR spectrum of acetone were recorded at • 500 MHz, what would be the position of the absorption • in d units? • How many hertz downfield from TMS does this 500 MHz • resonance correspond to?

  32. Practice Problem: The following 1H NMR peaks were recorded on a spectrometer operating at 200 MHz. Convert each into d units. • CHCl3; 1454 Hz • CH3Cl; 610 Hz • CH3OH; 693 Hz • CH2Cl2; 1060 Hz]

  33. 4.13C NMR Spectroscopy: SignalAveraging and FT-NMR • Carbon-13, 13C – is the only naturally occurring carbon isotope with a nuclear spin • Natural abundance is only 1.1% of C’s in molecules • Sample is thus very dilute in this isotope • Two techniques have been developed to detect the 13C isotope in an organic sample by NMR: • Signal averaging(increases instrument sensitivity) • Fourier-transform NMR(increases instrument speed)

  34. Signal averaging: increases instrument sensitivity • Any individual 13C NMR spectrum is extremely “noisy”, but when hundreds of individual runs are added together by computer and then averaged, a greatly improved spectrum results. • Fourier-transform NMR: increases instrument speed • In the FT-NMR technique, all signals are recordedsimultaneously • The sample placed in a magnetic field of constant strength is irradiated with a short burst of rf energy. • All 1H and 13C in the sample resonate at once, and the complex composite signal is manipulated using so-called Fourier transforms before it can be displayed.

  35. Large amount of noise A single run A average of 200 runs

  36. 5.Characteristics of 13C NMRSpectroscopy • The carbon NMR spectrum of a compound provides the number of different types of electronic environments of carbon atoms in a molecule • It displays a single sharp signal for eachchemically distinct 13C nucleus in the molecule. • Most 13C resonances are between 0 to 220 ppm downfield from TMS

  37. 13C NMR spectrum displays a single sharp signal for eachchemically distinct 13C nucleus in the molecule. • Example: Methyl Acetate The C on –OCH3 The C on –CH3C=O The C of C=O

  38. Correlation of Environment with Chemical Shift • Most 13C resonances are between 0 to 220 ppm downfield from TMS • The exact chemical shift of each 13C depends on its electronic environment: • Electronegativity of nearby atoms • Hybridization of 13C atom

  39. Electronegativity of nearby atoms: C bonded to O, N, or halogen absorb downfield because O, N, or halogen pull electrons away from nearby 13C atoms, decreasing their electron density and “deshielding” them. • Hybridization of 13C atom: • sp3 C signal is in the range 0-90  • sp2C signal is in the range 110-220  • C=O signal is at the low-field end, in the range 160-220 

  40. C=O signal is at the low-field end, in the range 160-220  2-butanone para-bromo acetophenone

  41. Example: Only six C absorptions with different peak sizes

  42. Practice Problem: Identify the different 13C NMR absorptions Five absorptions: 14.1, 60.5, 128.5, 130.3, and 166.0d

  43. Practice Problem: Predict the number of carbon resonance lines you would expect in the 13C NMR spectra of the following compounds: • Methyl cyclopentane • 1,2-Dimethylbenzene • 1-Methylcyclohexene • 2-Methyl-2-butene

  44. Practice Problem: Propose structures for compounds that fit the following descriptions: • A hydrocarbon with seven lines in its 13C NMR • spectrum • A six-carbon compound with only five lines in its 13C • NMR spectrum • A four-carbon compound with three lines in its 13C • NMR spectrum

  45. Practice Problem: Assign the resonances in 13C NMR spectrum of methyl propanoate, CH3CH2CO2CH3 1 3 4 2

  46. 6.DEPT 13C NMR Spectroscopy • DEPT-NMR(distortionless enhancement by polarization transfer) • is one of the improved pulsing and computational methods that give additional information • can help distinguish among signals due to CH3, CH2, CH, and C • determines the number of H’s bonded to each C

  47. A DEPT experiment is done in three stages: • First stage: is to run an ordinary spectrum (called a broadband-decoupled spectrum): • It locates thechemical shifts of all C’s • Second stage: is called a DEPT- 90run: • onlysignals due to CH carbons appear • Third stage: is called DEPT- 135or INEPT(insensitive nuclear enhancement by polarization transfer) run: • positive signals for CH3 and CH carbons • negative signals for CH2 carbons. • zero signal for carbons having no hydrogen (C)

  48. Information from all three spectra can be used to determine the number of protons attached to each carbon.

  49. Example:DEPT-NMR spectrum for 6-methyl-5-hepten-2-ol CH3 CH3 CH3 Broadband-decoupled spectrum C 6 5 2 4 8 3 7 1 CH CH DEPT- 90 spectrum DEPT- 135 spectrum CH2 CH2

  50. Practice Problem: Assign a chemical shift to each carbon in 6- methyl-5-hepten-2-ol

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