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

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organic chemistry

Organic Chemistry

William H. Brown

Christopher S. Foote

Brent L. Iverson

molecular spectroscopy
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
nuclear spin states
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
nuclear spin states5
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
nuclear spins in b 0
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
nuclear spins in b 07
Nuclear Spins in B0
  • for 1H and 13C, only two orientations are allowed
nuclear spins in b 08
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)
nuclear spin in b 0
Nuclear Spin in B0
  • the energy difference between allowed spin states increases linearly with applied field strength
  • values shown here are for 1H nuclei
nuclear magnetic resonance10
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
nuclear magnetic resonance11
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)
nuclear magnetic resonance12
Nuclear Magnetic Resonance
  • Figure 13.3 the origin of nuclear magnetic “resonance
nuclear magnetic resonance13
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
nuclear magnetic resonance14
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
nuclear magnetic resonance15
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
nuclear magnetic resonance16
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
nmr spectrometer18
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
nmr spectrum
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
equivalent hydrogens
Equivalent Hydrogens
  • Equivalent hydrogens:have the same chemical environment
    • a molecule with 1 set of equivalent hydrogens gives 1 NMR signal
equivalent hydrogens21
Equivalent Hydrogens
  • a molecule with 2 or more sets of equivalent hydrogens gives a different NMR signal for each set
signal areas
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
slide23

Chemical

Shifts

1H-NMR

chemical shift
Chemical Shift
  • Depends on (1) electronegativity of nearby atoms, (2) the hybridization of adjacent atoms, and (3) diamagnetic effects from adjacent pi bonds
  • Electronegativity
chemical shift26
Chemical Shift
  • Hybridization of adjacent atoms
chemical shift27
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
chemical shift28
Chemical Shift
  • magnetic induction in the pi bonds of a carbon-carbon triple bond (Fig 13.9)
chemical shift29
Chemical Shift
  • magnetic induction in the pi bond of a carbon-carbon double bond (Fig 13.10)
chemical shift30
Chemical Shift
  • magnetic induction of the pi electrons in an aromatic ring (Fig. 13.11)
signal splitting the n 1 rule
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
signal splitting n 1
Signal Splitting (n + 1)
  • 1H-NMR spectrum of 1,1-dichloroethane
signal splitting n 133
Signal Splitting (n + 1)

Problem: predict the number of 1H-NMR signals and the splitting pattern of each

origins of signal splitting
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
origins of signal splitting36
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
coupling constants
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
signal splitting
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
physical basis for n 1 rule
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
coupling constants41
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°
more complex splitting patterns
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
more complex splitting patterns43
More Complex Splitting Patterns
  • if Hc is a set of two equivalent H, then the observed splitting is a doublet of triplets
more complex splitting patterns44
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
more complex splitting patterns45
More Complex Splitting Patterns
  • in ethyl propenoate, an unsymmetrical terminal alkene, the three vinylic hydrogens are nonequivalent
more complex splitting patterns46
More Complex Splitting Patterns
  • a tree diagram for the complex coupling of the three vinylic hydrogens in ethyl propenoate
more complex splitting patterns47
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
more complex splitting patterns48
More Complex Splitting Patterns
  • a tree diagram for the complex coupling in 2-methyl-2-vinyloxirane
more complex splitting patterns49
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
more complex splitting patterns50
More Complex Splitting Patterns
  • simplification of signal splitting occurs when coupling constants are the same
more complex splitting patterns51
More Complex Splitting Patterns
  • an example of peak overlap occurs in the spectrum of 1-chloro-3-iodopropane
  • the central CH2 has the possibility for 9 peaks (a triplet of triplets) but because Jab and Jbc are so similar, only 4 + 1 = 5 peaks are distinguishable
stereochemistry topicity
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
stereochemistry topicity53

Substitute

Substitute

Substitution does not

Substitution does not

C

C

l

l

C

C

l

l

one H by D

one H by D

produce a stereocenter;

produce a stereocenter;

C

C

l

l

therefore hydrogens

therefore hydrogens

C

C

l

l

are homotopic.

are homotopic.

Dichloro-

Dichloro-

methane

methane

(achiral)

(achiral)

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

stereochemistry topicity54

Substitution produces a

Substitution produces a

Substitute

Substitute

C

C

l

l

stereocenter;

stereocenter;

one H by D

one H by D

C

C

l

l

therefore, hydrogens are

therefore, hydrogens are

enantiotopic. Both

enantiotopic. Both

hydrogens are prochiral;

hydrogens are prochiral;

one is pro-R-chiral, the

one is pro-R-chiral, the

Chlorofluoro-

Chlorofluoro-

other is pro-S-chiral.

other is pro-S-chiral.

methane

methane

(achiral)

(achiral)

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

stereochemistry topicity55
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
stereochemistry topicity56

O

H

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

stereochemistry and topicity
Stereochemistry and Topicity
  • 1H-NMR spectrum of 3-methyl-2-butanol
    • the methyl groups on carbon 3 are diastereotopic and appear as two doublets
13 c nmr spectroscopy
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
13 c nmr spectroscopy59
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
13 c nmr spectroscopy60
13C-NMR Spectroscopy
  • hydrogen-decoupled 13C-NMR spectrum of 1-bromobutane
the dept method
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
the dept method64
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
isopentyl acetate
Isopentyl acetate
  • 13C-NMR: (a) proton decoupled and (b) DEPT
interpreting nmr spectra
Interpreting NMR Spectra
  • 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
  • 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
interpreting nmr spectra67
Interpreting NMR Spectra
  • 1H-NMR spectrum of vinyl acetate (Fig 13.33)
interpreting nmr spectra68
Interpreting NMR Spectra
  • Alcohols
  • 1H-NMR O-H chemical shifts often appears in the range  3.0-4.0, but may be as low 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
  • 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
interpreting nmr spectra69
Interpreting NMR Spectra
  • 1H-NMR spectrum of 1-propanol (Fig. 13.34)
interpreting nmr spectra70
Interpreting NMR Spectra
  • 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
  • Amines
    • 1H-NMR: amine hydrogens appear at  0.5-5.0 depending on conditions
interpreting nmr spectra71
Interpreting NMR Spectra
  • 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
interpreting nmr spectra72
Interpreting NMR Spectra
  • Spectral Problem 1; molecular formula C5H10O
interpreting nmr spectra73
Interpreting NMR Spectra
  • Spectral Problem 2; molecular formula C7H14O
slide74
Nuclear

Magnetic Resonance

End Chapter 13