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Organic Chemistry

Organic Chemistry. William H. Brown & Christopher S. Foote. Nuclear Magnetic Resonance. Chapter 13. Molecular Spectroscopy.

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Organic Chemistry

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  1. Organic Chemistry William H. Brown & Christopher S. Foote

  2. Nuclear Magnetic Resonance Chapter 13

  3. Molecular Spectroscopy • Nuclear magnetic resonance (NMR) spectroscopy: a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of • hydrogen atoms using 1H-NMR spectroscopy • carbon atoms using 13C-NMR spectroscopy • phosphorus atoms using 31P-NMR spectroscopy

  4. Nuclear Spin States • An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 • this spinning charge creates an associated magnetic field • in effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment • Pauli exclusion principle:two electrons can occupy the same atomic or molecular orbital only if they have paired (opposite) spins

  5. Nuclear Spin States • Any atomic nucleus that has an odd mass, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment • The allowed nuclear spin states are determined by the spin quantum number, I, of the nucleus • A nucleus with spin quantum number I has 2I + 1 spin states. If I = 1/2, there are two allowed spin states

  6. Nuclear Spin States • Spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds

  7. Nuclear Spins in B0 • Within a collection of 1H and 13C atoms, nuclear spins are completely random in orientation • When placed in a strong external magnetic field of strength B0, however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed

  8. Nuclear Spins in B0 • For 1H and 13C, only two orientations are allowed.

  9. Nuclear Spins in B0 • In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for • 1H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz) • 13C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz)

  10. Nuclear Spin in B0 • The energy difference between allowed spin states increases linearly with applied field strength. • values shown here are for 1H nuclei.

  11. Nuclear Magnetic Resonance • When nuclei with a spin quantum number of 1/2 are placed in an applied field, a small majority of nuclear spins are aligned with the applied field in the lower energy state • The nucleus begins to precess and traces out a cone-shaped surface, in much the same way a spinning top or gyroscope traces out a cone-shaped surface as it precesses in the earth’s gravitational field • We express the rate of precession as a frequency in hertz

  12. Nuclear Magnetic Resonance • If the precessing nucleus is irradiated with electromagnetic radiation of the same frequency as the rate of precession, • the two frequencies couple, • energy is absorbed, and • the nuclear spin is flipped from spin state +1/2 (with the applied field) to -1/2 (against the applied field)

  13. Nuclear Magnetic Resonance • Coupling of precession frequency and the frequency of electromagnetic radiation

  14. Nuclear Magnetic Resonance • Resonance: the absorption of electromagnetic radiation by a precessing nucleus and the flip of its nuclear spin from a lower energy state to a higher energy state • The instrument used to detect this coupling of precession frequency and electromagnetic radiation records it as a signal

  15. Nuclear Magnetic Resonance • If we were dealing with 1H nuclei isolated from all other atoms and electrons, any combination of applied field and radiation that produces a signal for one 1H would produce a signal for all 1H. The same is true of 13C nuclei • But hydrogens in organic molecules are not isolated from all other atoms; they are surrounded by electrons, which are caused to circulate by the presence of the applied field

  16. Nuclear Magnetic Resonance • The circulation of electrons around a nucleus in an applied field is called diamagneticcurrentand the nuclear shielding resulting from it is called diamagnetic shielding • The difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small

  17. Nuclear Magnetic Resonance • The difference in resonance frequencies for hydrogens in CH3Cl compared to CH3F under an applied field of 7.05T is only 360 Hz, which is 1.2 parts per million (ppm) compared with the irradiating frequency

  18. Nuclear Magnetic Resonance • It is customary to measure the resonance frequency (signal) of individual nuclei relative to the resonance frequency (signal) of a reference compound • The reference compound now universally accepted is tetramethylsilane (TMS)

  19. Nuclear Magnetic Resonance • For a 1H-NMR spectrum, signals are reported by their shift from the 12 H signal in TMS • For a 13C-NMR spectrum, signals are reported by their shift from the 4 C signal in TMS • Chemical shift ():the shift in ppm of an NMR signal from the signal of TMS

  20. NMR spectrometer

  21. NMR Spectrometer • Essentials of an NMR spectrometer are a powerful magnet, a radio-frequency generator, and a radio-frequency detector • The sample is dissolved in a solvent, most commonly CDCl3 or D2O, and placed in a sample tube which is then suspended in the magnetic field and set spinning • Using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in about 2 seconds

  22. NMR Spectrum • 1H-NMR spectrum of methyl acetate • Downfield:the shift of an NMR signal to the left on the chart paper • Upfield:the shift of an NMR signal to the right on the chart paper

  23. Equivalent Hydrogens • Equivalent hydrogens:have the same chemical environment • a molecule with 1 set of equivalent hydrogens gives 1 NMR signal

  24. Equivalent Hydrogens • a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set

  25. Signal Areas • Relative areas of signals are proportional to the number of H giving rise to each signal • Modern NMR spectrometers electronically integrate and record the relative area of each signal

  26. Chemical Shift - 1H-NMR

  27. Chemical Shift - 1H-NMR

  28. Chemical Shift • Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds • electronegativity

  29. Chemical Shift • hybridization of adjacent atoms

  30. Chemical Shift • Magnetic induction in pi bonds of a • a carbon-carbon triple bond shields an acetylenic hydrogen and shifts its signal upfield (to the right) to a smaller  value • a carbon-carbon double bond deshields vinylic hydrogens and shifts their signal downfield (to the left) to a larger  value

  31. Chemical Shift • magnetic induction in the pi bonds of carbon-carbon triple bond (Fig 13.7)

  32. Chemical Shift • magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.8)

  33. Chemical Shift • magnetic induction of the pi electrons in an aromatic ring (Fig. 13.9)

  34. Signal Splitting (n + 1) • Peak:the units into which an NMR signal is split; doublet, triplet, quartet, etc. • Signal splitting:splitting of an NMR signal into a set of peaks by the influence of neighboring nonequivalent hydrogens • (n + 1) rule:the 1H-NMR signal of a hydrogen or set of equivalent hydrogens is split into (n + 1) peaks by a nonequivalent set of n equivalent neighboring hydrogens

  35. Signal Splitting (n + 1) • 1H-NMR spectrum of 1,1-dichloroethane

  36. Signal Splitting (n + 1) Problem: predict the number of 1H-NMR signals and the splitting pattern of each

  37. Origin 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

  38. Origin of Signal Splitting

  39. Origin of Signal Splitting • The signal of Ha is split into two peaks of equal area (a doublet) by its nonequivalent neighbor Hb.

  40. Signal Splitting • 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.

  41. Coupling Constants • Coupling constant (J):the distance between peaks in a split signal, expressed in hertz • the value is a quantitative measure of the magnetic interaction of nuclei whose spins are coupled

  42. Coupling Constants • Jab for a doublet and a triplet from splitting of Ha by nonequivalent hydrogen(s) Hb.

  43. Coupling Constants • 1H-NMR spectrum of 3-pentanone; scale expansion shows the triplet quartet pattern more clearly

  44. Stereochem & Topicity • Depending on the symmetry of a molecule, otherwise equivalent hydrogens may be • homotopic • enantiotopic • diastereotopic • The simplest way to visualize topicity is to substitute an atom or group by an isotope. Is the resulting compound • the same as its mirror image • different from its mirror image • are diastereomers possible

  45. Stereochem & Topicity • Homotopic atoms or groups • homotopic atoms or groups have identical chemical shifts under all conditions

  46. Stereochem & Topicity • Enantiotopic groups • enantiotopic atoms or groups have identical chemical shifts in achiral environments • they have different chemical shifts in achiral environments

  47. Stereochem & Topicity • Diastereotopic groups • H atoms on C-3 of 2-butanol are diastereotopic • substitution by deuterium creates a stereocenter • because there is already a stereocenter in the molecule, diastereomers are now possible • diastereotopic hydrogens have different chemical shifts under all conditions

  48. Stereochem & Topicity • The methyl groups on carbon 3 of 3-methyl-2-butanol are diastereotopic • if a methyl hydrogen of carbon 4 is substituted by deuterium, a new stereocenter is created • because there is already one stereocenter, diastereomers are now possible • protons of the methyl groups on carbon 3 have different chemical shifts

  49. Stereochem and Topicity • 1H-NMR spectrum of 3-methyl-2-butanol • the methyl groups on carbon 3 are diastereotopic and appear as two doublets

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