Upload
others
View
13
Download
0
Embed Size (px)
Citation preview
Nuclear Magnetic Resonance Spectroscopy
Nuclear Spin Quantum Numbers and Allowed Nuclear Spin States
• Any atomic nucleus that has an odd mass number, an odd atomic number, or both has a spin and a resulting nuclear magnetic moment
• A nucleus with spin quantum number I has 2I + 1 spin states
Nuclear Magnetic Resonance (NMR) Spectroscopy • Spectroscopic technique that provides information about the number and
types of atoms in a molecule, such as the number and types of: • Hydrogen atoms using 1H-NMR spectroscopy• Carbon atoms using 13C-NMR spectroscopy• Phosphorus atoms using 31P-NMR spectroscopy
Origin of Nuclear Magnetic Resonance
(a) Precession of a spinning nucleus in an applied magnetic field (b) Absorption of electromagnetic radiation occurs when the frequency of radiation is equal to
the frequency of precessionResonancenuclear spin flips from a lower energy state to a higher energy state
NMR Spectrometer• An NMR spectrometer is composed of: a powerful magnet, a radio-frequency
generator, a radio-frequency detector, and a sample tube• The sample is dissolved in a solvent, most commonly CCl4, CDCl3, or D2O, and placed in a
tube, which is then suspended in the magnetic field and set spinning on its long axis
• By using a Fourier transform NMR (FT-NMR) spectrometer, a spectrum can be recorded in less than 2 seconds
1H NMR: Chemical Shift, what types of hydrogens
• Hydrogens in organic molecules are surrounded by electrons• Circulation of electron density in an applied magnetic field is called diamagnetic
current• Diamagnetic current creates a magnetic field that opposes the applied field,
referred to as diamagnetic shielding, leading to a weaker field, lower resonance frequency that can be measured, upfield shift on NMR spectra. A similar term deshielding means less shielding, a stronger field, higher resonance frequency, and downfield shift on NMR spectra.
• Chemical shift (δ) is the difference in resonance frequencies caused by differing amounts of shielding.
Diamagnetic Shielding and Chemical Shift
• Difference in resonance frequencies caused by shielding/deshielding is generally very small compared to the applied field
• Difference in resonance frequencies of hydrogens in CH3Cl compared to those in 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
• NMR signals are measured relative to the signal of the reference compound tetramethylsilane (TMS)
• Chemical shift (d): Shift in ppm of an NMR signal refer to the signal of TMS• 1H-NMR spectrum, the signal of the 12 equivalent H atoms in TMS is set to 0• 13C-NMR spectrum, the signal of the 4 equivalent C atoms in TMS is set to 0
6 6
360 Hz 1.2 = = 1.2 ppm300 10 Hz 10´
The NMR Spectrum: Chemical Shifts
1H-NMR Spectrum of Methyl Acetate
UpfieldDownfield
Where do different types of protons absorb?
Diamagnetic Shielding
Characteristic Functional Group Chemical Shifts in 1H NMR (ppm)
Chemical Shift• Depends on the extent of shielding a particular type of hydrogen
experiences• Shielding depends on the following factors:
• Electronegativity of nearby atoms• Hybridization of adjacent atoms• Magnetic induction within an adjacent pi bond
Causes nearby nuclei to resonate farther downfield
Shielding of Acetylenic Hydrogen and Shifting of Signal Upfield by the π Bond of C≡C, weaker local magnetic field, lower frequency
Deshielding of Vinylic Hydrogens and Shifting of Signal Downfield by the π Bond of C=C, stronger local magnetic field, higher frequency
• Magnetic field induced by circulation of the p electrons in an aromatic ring deshields the hydrogens of the aromatic ring and shifts their signal downfield
• Aryl hydrogens absorb even farther downfield than vinylic hydrogens owing to the existence of a ring current
Fast Exchange
• Hydrogen atoms bonded to oxygen or nitrogen atoms can exchange faster with each other than the time it takes to acquire a 1H-NMR spectrum
• Consequences: Signals for exchanging H atoms are generally broad singlets that do not take part in splitting with other signals
• Signal will disappear if D2O or a deuterated alcohol is added to the sample• H atoms will be replaced with D atoms, which are 1H-NMR silent
• Important affected functional groups - Carboxylic acids, alcohols, amines, and amides
Chemical Equivalent Hydrogens
• Hydrogens that have the same chemical environment• Molecule with one set of equivalent hydrogens gives one NMR signal as
each H atom is in the same environment
• Molecule with two or more sets of equivalent hydrogens gives a different NMR signal for each set
• Which of the following nuclei does not show magnetic behavior?
1. 1H2. 2H3. 12C4. 13C5. 17O
• Each compound gives only one signal in its 1H-NMR spectrumPropose a structural formula for each compound
a. C2H6Ob. C3H6Cl2c. C6H12d. C4H6
• Chemically equivalent nuclei always show a single absorption
1. True2. False
• Order the following protons from lowest to highest chemical shift value
1. Ha < Hc < Hb < Hd2. Ha < Hc < Hd < Hb
3. Hc < Ha < Hd < Hb4. Hc < Ha < Hb < Hd
5. Hc < Hd < Ha < Hb
HbO
Ha
HdHc
Chemical and Stereochemical Equivalence • The following molecule has many hydrogen atoms. Let’s take a look at various pairs of
hydrogen atoms and ask: are they the same or not the same? The best way to do so is to perform a substitution test.
If the two compounds are entirely different, the groups are constitutionally inequivalent.
If the two compounds are diastereomers, the groups are diastereotopic.
If the two compounds are enantiomers, the groups are enantiotopic.
If the two compounds are identical, the groups are homotopic.
Let’s look at some examples:
• Homotopic hydrogens have identical chemical shifts under all conditions.• Enantiotopic hydrogens have identical chemical shifts in achiral
environments and different chemical shifts in chiral environments.• Diastereotopic hydrogens have different chemical shifts under all
conditions.
Stereochemistry and Topicity - Diastereotopic Groups
• Methyl groups on carbon-3 of 3-methyl-2-butanol are diastereotopic• If a H atom on one of the methyl groups on carbon-3 is substituted with a deuterium, a
new chiral center is created• Because there is already one chiral center, diastereomers are now possible• Diastereotopic hydrogens have different chemical shifts under all conditions
OH
3-Methyl-2-butanol
13
2 4
• Indicate whether the highlighted hydrogens in the following compounds are homotopic, enantiotopic, or diastereotopic
• What is the relationship between Ha and Hb in the following compound?
1. Chemically unrelated2. Homotopic3. Enantiotopic4. Diastereotopic5. None of these
ClHa
Hb
• What is the relationship between Ha and Hb in the following compound?
1. Chemically unrelated2. Homotopic3. Enantiotopic4. Diastereotopic5. None of these
Hc
HbHa
OH
1H NMR: Integration, how many hydrogens
Integrals — counting protons
Signal Areas• Modern NMR spectrometers electronically integrate and record the relative
area under each signal• Relative areas of signals are proportional to the number of equivalent hydrogens
giving rise to each signal
• Example - 1H-NMR spectrum of tert-butyl acetate
Structural Prediction• Following is a 1H-NMR spectrum for a compound of molecular formula
C9H10O2• From the integration, calculate the number of hydrogens giving rise to each signal
• 88 integration corresponds to 10 hydrogens• 44/88×10, or 5, hydrogens at δ 7.34• Similar calculations, two and three hydrogens at δ 5.08 and 2.06
• Molecular formula C7H14O
• Which of the following molecules best fits the following NMR spectrum?
1. 2. 3.
4. 5.
• Which of the following molecules best fits the following NMR spectrum?
1. 2. 3. 4. 5.O
O OH
OH
OH
OH
HO
HO
HO
HO
• Which of the following molecules best fits the following NMR spectrum?
1. 2. 3.
4. 5.
Cl OH
NH2NH2
• 1H NMR will allow one to distinguish between the following two molecules:
1. True2. False
H BrBr H
1H NMR: Signal splitting, neighbors of hydrogens
Signal Splitting and the (n + 1) Rule• Signal splitting - Splitting of an NMR signal into a set of peaks by the
influence of neighboring nonequivalent hydrogens• Peaks are named by how a signal is split
• Singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), and so forth• Degree of signal splitting can be predicted on the basis of the (n + 1) rule
• (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
1,1-Dichloroethane
Example - Predicting 1H-NMR Spectra II• Predict the number of signals and the splitting pattern of each signal in the
1H-NMR spectrum of each molecule
Origins of Signal Splitting• Spin-spin coupling: Interaction in which nuclear spins of adjacent atoms
influence each other and lead to the splitting of NMR signals• Coupling constant (J) : Separation on an NMR spectrum (in hertz)
between adjacent peaks in a multiplet• Vicinal hydrogens: H atoms on two C atoms that are bonded to each
other • Coupling between vicinal hydrogens is referred to as vicinal coupling
Spin-Spin Splitting: Why Does It Happen? Total magnetic field at H (observed) = Applied field +
effect of H (neighbor)
• These two possibilities have different energies – so there are two lines• Each is equally probable – so the intensity of the lines is equal
Illustration of Spin-Spin Coupling That Gives Rise to Signal Splitting in 1H-NMR Spectra
Shielding vs. De-shielding
Spin-Spin Splitting: The Coupling Constant
Spin-Spin Splitting: Counting Neighbors
Spin-Spin Splitting: More Than One Neighbor
Pascal’s Triangle
Signature “Splitting” Patterns in 1H NMR Spectra
Complex Splitting Patterns
Jab ≠ Jbc
(n + 1)×(m + 1) peaks, a H atom that is coupled to a set of n H atoms with one coupling constant and m H atoms with another coupling constant
Bond Rotation
• Key parameter as the angle between C—H bonds determines the extent of coupling
• In molecules with relatively free rotation about C—C sigma bonds, H atoms bonded to the same C in CH3 and CH2 groups are generally equivalent
• If there is restricted bond rotation, as in alkenes and cyclic structures, H atoms bonded to the same C may not be equivalent
• Nonequivalent 1H nuclei on the same carbon will couple and cause signal splitting (referred to as geminal coupling)
Restricted Bond Rotation
Ethyl Propenoate
2-methyl-2-vinyloxirane
300 MHz 1H-NMR Spectrum of 1-Chloro-3-Iodopropane
• The central CH2 (c) has the possibility of splitting into 3×3 = 9 peaks (a triplet of triplets)
• Only 4 + 1 = 5 peaks are distinguishable as the values of Jab and Jbc are similar • m + n + 1 peaks, instead of (n + 1)×(m + 1) peaks.
Spin-Spin Splitting: More Complex Splitting Patterns
13C NMR
13C-NMR Spectroscopy• 13C atoms in a molecule rarely have 13C next to them
• 13C—13C signal splitting is not normally observed• According to the (n + 1) rule, a 13C signal is split by the hydrogens bonded to it • Coupling constants of 100 and 250 Hz are common, which means that there is often
significant overlap among signals and splitting patterns can be difficult to determine
• The most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode
• In the hydrogen-decoupled mode, a sample is irradiated with two different radio frequencies
• First radio frequency is used to excite all 13C nuclei• Second is a broad spectrum of frequencies that causes all hydrogens in the molecule to
undergo rapid transitions among their nuclear spin states
• On the time scale of a 13C-NMR spectrum, each hydrogen is in a time average of the two states, with the result that 1H-13C spin-spin interactions are not observed
• Process is known as spin-spin decoupling
Hydrogen-Decoupled 13C-NMR Spectrum of 1-Bromobutane
Characteristic Functional Group Chemical Shifts in 13C NMR (ppm)
• Which of the following is true of 13C-NMR spectra?
1. The number of carbon atoms in a molecule can be ascertained2. The number of hydrogen atoms in a molecule can be ascertained3. Certain functional groups can be deduced from the locations of the
peaks4. Both the number of carbon atoms and the number of hydrogen
atoms in a molecule can be ascertained5. All of these
• How many signals will appear in the 13C-NMR spectrum of the following molecule?
1. 102. 113. 124. 145. 15
• How many signals will appear in the 13C-NMR spectrum of the following molecule?
1. 12. 23. 34. 45. 5
• How many signals will appear in the 13C-NMR spectrum of the following molecule?
1. 32. 43. 54. 65. 7
OCH3 N
O
CH3
O
painkiller Demerol
• Predict the number of signals in a proton-decoupled 13C-NMR spectrum of each compound
Interpreting NMR Spectra
Interpreting NMR Spectra• Alkanes
• 1H-NMR chemical shifts fall within the range of δ 0.8–1.7 • 13C-NMR chemical shifts fall within the considerably wider range of δ 10–60
• Alkenes• 1H-NMR signals appear in the range δ 4.6–5.7• Coupling constants are generally larger for trans vinylic hydrogens (11–18 Hz) when
compared with cis vinylic hydrogens (5–10 Hz)• Signal of each vinylic hydrogen in vinyl acetate is predicted to be a doublet of
doublets• 13C-NMR signals for sp2 hybridized carbons appear in the range δ 100–150 ppm,
which is considerably downfield from sp3 hybridized carbons
Vinyl Acetate
Interpreting NMR Spectra• Alcohols
• Chemical shift of a hydroxyl hydrogen in a 1H-NMR spectrum is variable and depends on the purity of the sample, the solvent, the concentration, and the temperature
• Often appears in the range δ 3.0–4.0, but may be as low as δ 0.5• Hydrogens on the carbon bearing the —OH group are deshielded by the electron-
withdrawing inductive effect of the oxygen atom and their signals appear in the range δ 3.4–4.0
1-Propanol
Interpreting NMR Spectra• Ethers
• A distinctive feature in the 1H-NMR spectra of ethers is the chemical shift of hydrogens, δ 3.3–4.0, on the carbons bonded to the ether oxygen
• Range corresponds to a downfield shift of approximately 2.4 units compared with their normal position in alkanes
• Aldehydes and ketones• Signal for aldehyde hydrogens appears between δ 9.5 and δ 10.1 in the 1H-NMR
spectrum• H atoms on a-carbons of aldehydes and ketones appear around δ 2.2 to 2.6• In 13C-NMR, carbonyl carbons have characteristic positions between δ 180 and δ 215
• Amines• In the 1H-NMR spectrum, amine hydrogens appear from δ 0.5 to 5.0 depending on
experimental conditions due to hydrogen bonding• Amine hydrogens generally appear as broad singlets• Carbons bonded to nitrogen appear in the 13C-NMR spectrum approximately 20 ppm
higher than in alkanes of comparable structure, but about 20 ppm below carbons attached to oxygen in ethers or alcohols
Interpreting NMR Spectra• Carboxylic acids and esters
• Signals for hydrogens on the α-carbon to a carboxyl group in acids and esters appear in a 1H-NMR spectrum in the range δ 2.0 to 2.6
• Hydrogen of a carboxyl group gives a very distinctive signal in the range δ 10 to 13• 13C resonance of the carboxyl carbon in acids and esters appears in the range δ 165
to 185 • Hydrogens α to an ester oxygen are strongly deshielded and resonate between δ 3.7 and
4.7
2-Methylpropanoic Acid (Isobutyric Acid)
• Which of the following molecules best fits the following 13C-NMR data?
• 13C-NMR data: 20, 22, 32, 44, and 67 ppm
1. 2.
3. 4.
HO
H3C
CH3 HO
H3C
CH3
HO
H3C CH3
HO
H3CCH3
• Molecular formula C5H10O
Spectral Problem
• Molecular formula C7H14O
Spectral Problem
• Following is the 1H-NMR spectrum of compound O, molecular formula C7H12
• Compound O reacts with bromine in carbon tetrachloride to give a compound with the molecular formula C7H12Br2
• The 13C-NMR spectrum of compound O shows signals at d 150.12, 106.43, 35.44, 28.36, and 26.36
• Deduce the structural formula of O
Spectral Problem