Nuclear Magnetic Resonance

1
Nuclear Magnetic Resonance

• Chapter 13

2
Molecular Spectroscopy

• Nuclear magnetic resonance (NMR) spectroscopy a
  spectroscopic technique that gives us information
  about the number of certain types of atoms and
  their environment in a molecule.
• Most commonly, about the number and types of
• hydrogen atoms using 1H-NMR spectroscopy
• carbon atoms using 13C-NMR spectroscopy
• The NMR study in Chem 3020 will be restricted to
  these two types of atoms.

3
13.1 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 has a net nuclear
  spin and a resulting nuclear magnetic moment.
• The allowed nuclear spin states are determined by
  the spin quantum number, I, of the nucleus.

4
Nuclear Spin States, Table 13.1

• 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.

5
13.2 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 Bo (or Ho), 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.

6
Nuclear Spins in B0

• for 1H and 13C, only two orientations are allowed
  (Fig 13.1)

B0
7
Nuclear Spins in B0

• In an applied field strength of 7.05T (Tesla),
  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).

8
Nuclear Spin in B0

• The energy difference between allowed spin states
  increases linearly with applied field strength.
• Values shown here are for 1H nuclei (Fig 13.2)

B0
300 MHz
60 MHz
9
13.3 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
  earths gravitational field.
• We express the rate of precession as a frequency
  in hertz.

10
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).

11
Nuclear Magnetic Resonance

• The origin of nuclear magnetic resonance
  (Fig 13.3)

12
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.
• Relaxation a loss of energy when the higher
  energy state returns to the lower 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.

13
13.4 NMR Spectrometer
(Fig 13.4)
14
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

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

16
NMR Spectrum

• 1H-NMR spectrum of methyl acetate (Fig 13.5)
• Downfield the shift of an NMR signal to the left
  on the chart paper downfield requires lower
  energy.
• Upfield the shift of an NMR signal to the right
  on the chart paper upfield requires higher
  energy.

17
NMR cycles/sec (Hertz) vs ppm (?)

• 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 (ppm is also called ?).

18
NMR Reference Signal

• 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.

19
13.5 Equivalent Hydrogens

• Equivalent hydrogens These have the same
  chemical environment.
• A molecule with 1 set of equivalent hydrogens
  gives 1 NMR signal.

20
Equivalent Hydrogens

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

21
13.6 Signal Areas (integration)

• 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 (Fig
  13.7).

22
13.7 Chemical Shift - 1H-NMR
Chemical Shifts 1H-NMR
O
O
O
O
O
O
23
Chemical Shift - 1H-NMR, Fig. 13.8
(Fig 13.8)
24
A. Chemical Shift, Table 13.2

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

25
B. Chemical Shift, Table 13.3

• Hybridization of adjacent atoms Greater s
  character in the hybrid holds shared electrons
  closer to carbon

26
C. 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.

27
Chemical Shift

• Magnetic induction in the pi bond of a
  carbon-carbon double bond (Fig 13.10)

28
Chemical Shift

• Magnetic induction of the pi electrons in an
  aromatic ring (Fig. 13.11).

29
Chemical Shift

• Magnetic induction in the pi bonds of a
  carbon-carbon triple bond (Fig 13.9)

30
Chemical Shift and Integration
31
13.8 Signal Splitting the (n 1) Rule

• NMR Signals not all appear as a single peak.
• Peak The units into which an NMR signal
  appears singlet, 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 (Fig 13.12)

33
13.9 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.

34
Origins of Signal Splitting
(Fig 13.13)
Ha and Hb are non-equivalent
35
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 and expand certain signals.
• 1H-NMR spectrum of 3-pentanone expansion more
  clearly shows the triplet/quartet (Fig 13.14).

36
Signal Splitting (n 1)

• Problem Predict the number of 1H-NMR signals
  and the splitting pattern of each.

37
Coupling Constants, Table 13.4

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

38
A. Origins of Signal Splitting
(Fig 13.15)
39
Signal Splitting

• Pascals Triangle
• As illustrated by the highlighted entries, each
  entry is the sum of the values immediately above
  it to the left and the right (Fig 13.16).

40
B. 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 (Fig 13.17).

41
Signal Splitting (n 1) example
42
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 (Fig 13.18).

43
C. 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.

(Fig 13.19)
44
More Complex Splitting Patterns

• If Hc is a set of two equivalent H, then the
  observed splitting is a doublet of triplets.

(Fig 13.20)
45
More Complex Splitting Patterns
46
D. More Complex Splitting Patterns

• 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.

(Fig 13.21)
47
More Complex Splitting Patterns

• In ethyl propenoate, an unsymmetrical terminal
  alkene, the three vinylic hydrogens are
  nonequivalent (Fig 13.22).

48
More Complex Splitting Patterns

• A tree diagram for the complex coupling of the
  three vinylic hydrogens in ethyl propenoate.

(Fig 13.23)
49
More Complex Splitting Patterns

• Cyclic structures often have restricted rotation
  about their C-C bonds and have constrained
  conformations (Fig 13.24).
• As a result, two H atoms on a CH2 group can be
  nonequivalent, leading to complex splitting.

50
More Complex Splitting Patterns

• A tree diagram for the complex coupling in
  2-methyl-2-vinyloxirane (Fig 13.25).

51
F. 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.

52
More Complex Splitting Patterns

• simplification of signal splitting occurs when
  coupling constants are the same (Fig 13.26).

53
More Complex Splitting Patterns

• 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.

(Fig 13.28)
54
13.10 Stereochemistry 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

55
Stereochemistry Topicity

• Homotopic atoms or groups
• Homotopic atoms or groups have identical chemical
  shifts under all conditions.

H
H
H
H
C
C
C
C
D
D
H
H
Achiral
Achiral
56
Stereochemistry Topicity

• Enantiotopic groups
• Enantiotopic atoms or groups have identical
  chemical shifts in achiral environments.
• They have different chemical shifts in chiral
  environments.

H
H
H
H
C
C
C
C
F
F
F
F
H
H
D
D
Chiral
Chiral
57
Stereochemistry Topicity

• Diastereotopic groups
• H atoms on C-3 of 2-butanol are diastereotopic.
• Substitution by deuterium creates a chiral
  center.
• Because there is already a chiral center in the
  molecule, diastereomers are now possible.
• Diastereotopic hydrogens have different chemical
  shifts under all conditions.

58
Stereochemistry 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 chiral center is created.
• Because there is already one chiral center,
  diastereomers are now possible.
• Protons of the methyl groups on carbon 3 have
  different chemical shifts.

3-Methyl-2-butanol
59
Stereochemistry and Topicity

• 1H-NMR spectrum of 3-methyl-2-butanol
• The methyl groups on carbon 3 are diastereotopic
  and appear as two doublets (Fig 13.29).

60
13.11 13C-NMR Spectroscopy

• Each nonequivalent 13C gives a different signal.
• A 13C signal is split by the 1H bonded to it
  according to the (n 1) rule .
• Coupling constants of 100-250 Hz are common,
  which means that there is often significant
  overlap between signals, and splitting patterns
  can be very difficult to determine.
• The most common mode of operation of a 13C-NMR
  spectrometer is a hydrogen-decoupled mode.

61
13C-NMR Spectroscopy

• In a hydrogen-decoupled mode, a sample is
  irradiated with two different radio frequencies.
• One to excite all 13C nuclei.
• A second broad spectrum of frequencies to cause
  all hydrogens in the molecule to undergo rapid
  transitions between their nuclear spin states.
• On the time scale of a 13C-NMR spectrum, each
  hydrogen is in an average or effectively constant
  nuclear spin state, with the result that 1H-13C
  spin-spin interactions are not observed they are
  decoupled.

62
13C-NMR 1H coupled and decoupled
63
13C-NMR Spectroscopy

• Hydrogen-decoupled 13C-NMR spectrum of
  1-bromobutane

64
Chemical Shift - 13C-NMR
65
Chemical Shift - 13C-NMR
(Fig 13.31)
66
13.12 The DEPT Method

• In the hydrogen-decoupled mode, information on
  spin-spin coupling between 13C and hydrogens
  bonded to it is lost.
• The DEPT method is an instrumental mode that
  provides a way to acquire this information.
• Distortionless Enhancement by Polarization
  Transfer (DEPT) An NMR technique for
  distinguishing among 13C signals for CH3, CH2,
  CH, and quaternary carbons.

67
The DEPT Method

• The DEPT methods uses a complex series of pulses
  in both the 1H and 13C ranges, with the result
  that CH3, CH2, and CH signals exhibit different
  phases
• Signals for CH3 and CH carbons are recorded as
  positive signals.
• Signals for CH2 carbons are recorded as negative
  signals.
• Quaternary carbons give no signal in the DEPT
  method.

68
Isopentyl acetate

• 13C-NMR (a) proton decoupled and (b) DEPT

(Fig 13.32)
69
13.13 Interpreting NMR Spectra

• A. Alkanes
• 1H-NMR signals appear in the range of ? 0.8-1.7.
• 13C-NMR signals appear in the considerably wider
  range of ? 10-60.
• B. Alkenes
• 1H-NMR signals appear in the range ? 4.6-5.7.
• 1H-NMR coupling constants are generally larger
  for trans vinylic hydrogens (J 11-18 Hz)
  compared with cis vinylic hydrogens (J 5-10 Hz)
• 13C-NMR signals for sp2 hybridized carbons.
  appear in the range ? 100-160, which is downfield
  from the signals of sp3 hybridized carbons.

70
Interpreting NMR Spectra

• 1H-NMR spectrum of vinyl acetate (Fig 13.33)

71
Interpreting NMR Spectra

• C. Alcohols
• 1H-NMR O-H chemical shifts often appears in the
  range ? 3.0-4.0, but may be as high as ? 0.5.
• 1H-NMR chemical shifts of hydrogens on the carbon
  bearing the -OH group are deshielded by the
  electron-withdrawing inductive effect of the
  oxygen and appear in the range ? 3.0-4.0.
• D. Ethers
• A distinctive feature in the 1H-MNR spectra of
  ethers is the chemical shift, ? 3.3-4.0, of
  hydrogens on carbon attached to the ether oxygen.

72
Interpreting NMR Spectra

• 1H-NMR spectrum of 1-propanol (Fig. 13.34)

73
Interpreting NMR Spectra

• E. Aldehydes and ketones
• 1H-NMR Aldehyde hydrogens appear at ? 9.5-10.1.
• 1H-NMR a-hydrogens of aldehydes and ketones
  appear at ? 2.2-2.6.
• 13C-NMR Carbonyl carbons appear at ? 180-215.
• G. Amines
• 1H-NMR Amine hydrogens appear at ? 0.5-5.0
  depending on conditions.

74
Interpreting NMR Spectra

• F. Carboxylic acids
• 1H-NMR Carboxyl hydrogens appear at ? 10-13,
  lower than most any other hydrogens .
• 13C-NMR Carboxyl carbons in acids and esters
  appear at ? 160-180 (Fig 13.35).

75
Interpreting NMR Spectra

• Spectral Problem 1 molecular formula C5H10O

76
Spectral Problem 1
molecular formula C5H10O
77
Interpreting NMR Spectra

• Spectral Problem 2 molecular formula C7H14O

78
Spectral Problem 2

• molecular formula C7H14O

79

• Nuclear
• Magnetic Resonance
• End Chapter 13