nuclear magnetic resonance• magnetic resonance imaging, noninvasive • “nuclear” is omitted...
TRANSCRIPT
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Nuclear Magnetic Resonance Spectroscopy
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Introduction• NMR is the most powerful tool available for
organic structure determination.• It is used to study a wide variety of nuclei: 1H 13C 15N 19F 31P
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Nuclear Spin• A nucleus with an odd atomic number or
an odd mass number has a nuclear spin.• The spinning charged nucleus generates
a magnetic field.
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External Magnetic FieldWhen placed in an external field, spinning
protons act like bar magnets.
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Two Energy StatesThe magnetic fields of
the spinning nuclei will align either withthe external field, or against the field.
A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. =>
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∆E and Magnet Strength• Energy difference is proportional to the
magnetic field strength.• ∆E = hν = γ h B0
2π• Gyromagnetic ratio, γ, is a constant for
each nucleus (26,753 s-1gauss-1 for H).• In a 14,092 gauss field, a 60 MHz
photon is required to flip a proton.• Low energy, radio frequency. =>
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Magnetic Shielding• If all protons absorbed the same amount
of energy in a given magnetic field, not much information could be obtained.
• But protons are surrounded by electrons that shield them from the external field.
• Circulating electrons create an induced magnetic field that opposes the external magnetic field. =>
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Shielded ProtonsMagnetic field strength must be increased
for a shielded proton to flip at the same frequency.
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Protons in a MoleculeDepending on their chemical
environment, protons in a molecule are shielded by different amounts.
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NMR Signals• The number of signals shows how many
different kinds of protons are present.• The location of the signals shows how
shielded or deshielded the proton is.• The intensity of the signal shows the
number of protons of that type.• Signal splitting shows the number of
protons on adjacent atoms. =>
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The NMR Spectrometer
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The NMR Graph
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Tetramethylsilane
• TMS is added to the sample.• Since silicon is less electronegative
than carbon, TMS protons are highly shielded. Signal defined as zero.
• Organic protons absorb downfield (to the left) of the TMS signal.
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Si
CH3
CH3
CH3
H3C
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Chemical Shift
• Measured in parts per million.• Ratio of shift downfield from TMS (Hz)
to total spectrometer frequency (Hz).• Same value for 60, 100, or 300 MHz
machine.• Called the delta scale.
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Chapter 13 15
Delta Scale
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Location of Signals• More electronegative
atoms deshield more and give larger shift values.
• Effect decreases with distance.
• Additional electronegative atoms cause increase in chemical shift.
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Chapter 13 17
Typical Values
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Chapter 13 18
Aromatic Protons, δ7-δ8
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Vinyl Protons, δ5-δ6
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Acetylenic Protons, δ2.5
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Aldehyde Proton, δ9-δ10
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Electronegativeoxygen atom
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O-H and N-H Signals
• Chemical shift depends on concentration.• Hydrogen bonding in concentrated
solutions deshield the protons, so signal is around δ3.5 for N-H and δ4.5 for O-H.
• Proton exchanges between the molecules broaden the peak.
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Carboxylic Acid Proton, δ10+
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Chapter 13 24
Number of SignalsEquivalent hydrogens have the same
chemical shift.
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Intensity of Signals• The area under each peak is
proportional to the number of protons.• Shown by integral trace.
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How Many Hydrogens?When the molecular formula is known,
each integral rise can be assigned to a particular number of hydrogens.
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Spin-Spin Splitting• Nonequivalent protons on adjacent carbons
have magnetic fields that may align with or oppose the external field.
• This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.
• All possibilities exist, so signal is split. =>
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Chapter 13 28
1,1,2-TribromoethaneNonequivalent protons on adjacent carbons.
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Chapter 13 29
Doublet: 1 Adjacent Proton
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Chapter 13 30
Triplet: 2 Adjacent Protons
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The N + 1 Rule
If a signal is split by N equivalent protons,it is split into N + 1 peaks.
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Range of Magnetic Coupling
• Equivalent protons do not split each other.• Protons bonded to the same carbon will
split each other only if they are not equivalent.
• Protons on adjacent carbons normally will couple.
• Protons separated by four or more bonds will not couple.
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Splitting for Ethyl Groups
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Splitting for Isopropyl Groups
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Coupling Constants
• Distance between the peaks of multiplet• Measured in Hz• Not dependent on strength of the external
field• Multiplets with the same coupling
constants may come from adjacent groups of protons that split each other.
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Chapter 13 36
Values for Coupling Constants
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Complex Splitting
• Signals may be split by adjacent protons, different from each other, with different coupling constants.
• Example: Ha of styrene which is split by an adjacent H trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11 Hz).
=>
C CH
H
Ha
b
c
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Splitting TreeC C
H
H
Ha
b
c
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Spectrum for Styrene
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Stereochemical Nonequivalence
• Usually, two protons on the same C are equivalent and do not split each other.
• If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other.
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Some Nonequivalent Protons
C CH
H
Ha
b
cOH
H
H
H
a
b
c
d
CH3
H Cl
H H
Cl
a b =>
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Time Dependence• Molecules are tumbling relative to the
magnetic field, so NMR is an averaged spectrum of all the orientations.
• Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal.
• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule.
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Hydroxyl Proton
• Ultrapure samples of ethanol show splitting.
• Ethanol with a small amount of acidic or basic impurities will not show splitting.
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N-H Proton• Moderate rate of exchange.• Peak may be broad.
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3 45
Identifying the O-H or N-H Peak
• Chemical shift will depend on concentration and solvent.
• To verify that a particular peak is due to O-H or N-H, shake the sample with D2O
• Deuterium will exchange with the O-H or N-H protons.
• On a second NMR spectrum the peak will be absent, or much less intense.
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Carbon-13
• 12C has no magnetic spin.• 13C has a magnetic spin, but is only 1% of
the carbon in a sample.• The gyromagnetic ratio of 13C is one-
fourth of that of 1H.• Signals are weak, getting lost in noise.• Hundreds of spectra are taken, averaged.
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Fourier Transform NMR• Nuclei in a magnetic field are given a
radio-frequency pulse close to their resonance frequency.
• The nuclei absorb energy and precess (spin) like little tops.
• A complex signal is produced, then decays as the nuclei lose energy.
• Free induction decay is converted to spectrum. =>
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Chapter 13 48
Hydrogen and Carbon Chemical Shifts
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Combined 13C and 1H Spectra
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Differences in 13C Technique
• Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz.
• Peak areas are not proportional to number of carbons.
• Carbon atoms with more hydrogens absorb more strongly.
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Spin-Spin Splitting
• It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.
• 13C will magnetically couple with attached protons and adjacent protons.
• These complex splitting patterns are difficult to interpret.
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Proton Spin Decoupling
• To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping.
• The carbon nuclei see an average of all the possible proton spin states.
• Thus, each different kind of carbon gives a single, unsplit peak.
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Off-Resonance Decoupling
• 13C nuclei are split only by the protons attached directly to them.
• The N + 1 rule applies: a carbon with Nnumber of protons gives a signal with N + 1 peaks.
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Interpreting 13C NMR
• The number of different signals indicates the number of different kinds of carbon.
• The location (chemical shift) indicates the type of functional group.
• The peak area indicates the numbers of carbons (if integrated).
• The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =>
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Two 13C NMR Spectra
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MRI• Magnetic resonance imaging, noninvasive• “Nuclear” is omitted because of public’s
fear that it would be radioactive.• Only protons in one plane can be in
resonance at one time.• Computer puts together “slices” to get 3D.• Tumors readily detected.
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