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Organic Chemistry

Organic Chemistry. William H. Brown Christopher S. Foote Brent L. Iverson. 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 Brent L. Iverson

  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 • The same effect holds for certain atomic nuclei • any atomic nucleus that has an odd mass number, 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

  5. Nuclear Spin States • a nucleus with spin quantum number I has 2I + 1 spin states; if I = 1/2, there are two allowed spin states • Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds

  6. 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

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

  8. 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)

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

  10. 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

  11. 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)

  12. Nuclear Magnetic Resonance • Figure 13.3 the origin of nuclear magnetic “resonance

  13. Nuclear Magnetic Resonance • Resonance: in NMR spectroscopy, resonance is the absorption of electromagnetic radiation by a precessing nucleus and the resulting “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 • signal: a recording in an NMR spectrum of a nuclear magnetic resonance

  14. 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 • the circulation of electrons around a nucleus in an applied field is called diamagneticcurrentand the nuclear shielding resulting from it is called diamagnetic shielding

  15. Nuclear Magnetic Resonance • the difference in resonance frequencies among the various hydrogen nuclei within a molecule due to shielding/deshielding is generally very small • 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

  16. Nuclear Magnetic Resonance • signals are measured relative to the signal of the reference compound tetramethylsilane (TMS) • 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

  17. NMR Spectrometer

  18. 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

  19. 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

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

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

  22. 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

  23. Chemical Shifts 1H-NMR

  24. Chemical Shift - 1H-NMR

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

  26. Chemical Shift • Hybridization of adjacent atoms

  27. Chemical Shift • Diamagnetic effects of pi bonds • 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

  28. Chemical Shift • magnetic induction in the pi bonds of a carbon-carbon triple bond (Fig 13.9)

  29. Chemical Shift • magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.10)

  30. Chemical Shift • magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11)

  31. Signal Splitting; the (n + 1) Rule • 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:if a hydrogen has n hydrogens nonequivalent to it but equivalent among themselves on the same or adjacent atom(s), its 1H-NMR signal is split into (n + 1) peaks

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

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

  34. Origins of Signal Splitting • 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): 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

  35. Origins of Signal Splitting

  36. Origins of Signal Splitting • because splitting patterns from spectra taken at 300 MHz and higher are often difficult to see, it is common to retrace certain signals in expanded form • 1H-NMR spectrum of 3-pentanone; scale expansion shows the triplet quartet pattern more clearly

  37. 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

  38. Origins of Signal Splitting

  39. 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

  40. 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

  41. Coupling Constants • 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°

  42. More Complex Splitting Patterns • thus far, we have concentrated on 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, the resulting coupling gives rise to a doublet of doublets

  43. More Complex Splitting Patterns • if Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets

  44. More Complex Splitting Patterns • because the angle between C-H bond determines the extent of coupling, bond rotation is a key parameter • 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

  45. More Complex Splitting Patterns • in ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent

  46. More Complex Splitting Patterns • a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate

  47. More Complex Splitting Patterns • 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

  48. More Complex Splitting Patterns • a tree diagram for the complex coupling in 2-methyl-2-vinyloxirane

  49. More Complex Splitting Patterns • 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

  50. More Complex Splitting Patterns • simplification of signal splitting occurs when coupling constants are the same

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