Download - CHEM 344
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CHEM 344
Spectroscopy of Organic Compounds
Lecture 118 June 2007
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Modern Spectroscopic MethodsModern Spectroscopic Methods
Revolutionized the study of organic chemistryRevolutionized the study of organic chemistry
Can determine the exact structure of small to Can determine the exact structure of small to medium size molecules in a few minutesmedium size molecules in a few minutes
Nuclear Magnetic Resonance (NMR) and Nuclear Magnetic Resonance (NMR) and Infrared Spectroscopy (IR) are particularly Infrared Spectroscopy (IR) are particularly powerful techniques which we will focus on and powerful techniques which we will focus on and use in this courseuse in this course
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Interaction of Light and MatterInteraction of Light and MatterThe Physical Basis of SpectroscopyThe Physical Basis of Spectroscopy
Spectroscopy: the study of molecular structure by the interaction of electromagnetic radiation with matter
Electromagnetic spectrum is continuous and covers a very wide range of wavelengths
Wavelengths () range from 103 to 10-15 meters
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The Electromagnetic SpectrumThe Electromagnetic Spectrum
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The Electromagnetic SpectrumThe Electromagnetic Spectrum
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Relationship Between Wavelength, Relationship Between Wavelength, Frequency and EnergyFrequency and Energy
Speed of light (c) is constant for all wavelengthsSpeed of light (c) is constant for all wavelengths
Frequency (Frequency (), the number of wavelengths per second, is ), the number of wavelengths per second, is inversely proportional to wavelength:inversely proportional to wavelength:
c
Energy of a photon is directly proportional to frequencyEnergy of a photon is directly proportional to frequency
E = hc/h(where h = Plank’s constant)(where h = Plank’s constant)
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Energy Levels in MoleculesEnergy Levels in Molecules
Energy levels within a molecule are discrete Energy levels within a molecule are discrete (quantized)(quantized)
Transitions between various energy levels occur Transitions between various energy levels occur only at discrete energiesonly at discrete energies
Transition caused by subjecting the molecule to Transition caused by subjecting the molecule to radiation of an energy that radiation of an energy that exactlyexactly matches the matches the difference in energy between the two levelsdifference in energy between the two levels
EEupperupper – E – Elowerlower = = ΔΔE = hE = h
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Wavelength/Spectroscopy Wavelength/Spectroscopy RelationshipsRelationships
Spectral RegionSpectral Region Photon EnergyPhoton Energy Molecular Molecular Energy ChangesEnergy Changes
UV-visibleUV-visible1010-7-7-10-10-8-8
mm
10101616-10-101717 Hz Hz
~ 100 kcal/mole~ 100 kcal/mole
~ 420 kJ/mole~ 420 kJ/mole
Electronic transitions Electronic transitions
(e.g. HOMO-LUMO)(e.g. HOMO-LUMO)
Infrared Infrared 1010-3-3-10-10-5-5
mm
10101212-10-101515 Hz Hz
~ 10 kcal/mole~ 10 kcal/mole Bond vibrationsBond vibrations
(e.g. C(e.g. C≡≡O stretching)O stretching)
Radio Radio (used for NMR)(used for NMR)
101033-10-1011 mm
101066-10-1077 Hz Hz
< 0.1 kcal/mol< 0.1 kcal/mol Flipping a nuclear Flipping a nuclear spin state in a spin state in a magnetic fieldmagnetic field
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Nuclear SpinsNuclear Spins
Spin Spin ½ ½ atoms: mass number is odd atoms: mass number is odd 11H, H, 1313C, C, 1919F, F, 2929Si, Si, 3131PP
Spin 1 atoms:Spin 1 atoms: mass number is evenmass number is even 22H, H, 1414NN
Spin 0 atoms: Spin 0 atoms: mass number is evenmass number is even1212C, C, 1616O, O, 3232SS NO NMR SIGNALNO NMR SIGNAL
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Magnetic Properties of the Proton Magnetic Properties of the Proton Related to SpinRelated to Spin
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Energy States of Protons in a Magnetic FieldEnergy States of Protons in a Magnetic Field
Spin states degenerate
Random orientations
Two allowed orientations (2I+1) = 2
Aligned with or against direction of Bo
No External Mag. Field External Mag. Field Bo
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Nuclear Magnetic Resonance Nuclear Magnetic Resonance (NMR)(NMR)
NuclearNuclear – spin ½ nuclei (e.g. protons) behave as – spin ½ nuclei (e.g. protons) behave as tiny bar magnetstiny bar magnets
MagneticMagnetic – a strong magnetic field causes a – a strong magnetic field causes a small energy difference between + ½small energy difference between + ½and – ½ spin statesand – ½ spin states
ResonanceResonance – photons of radio waves can match – photons of radio waves can match the exact energy difference between the + ½ the exact energy difference between the + ½ and – ½ spin states resulting in absorption of and – ½ spin states resulting in absorption of photons as the protons change spin statesphotons as the protons change spin states
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The NMR ExperimentThe NMR Experiment
The sample, dissolved in a suitable NMR solvent The sample, dissolved in a suitable NMR solvent (e.g. CDCl(e.g. CDCl33, CCl, CCl44, C, C66DD66), is placed in the strong ), is placed in the strong magnetic field of the NMR spectrometermagnetic field of the NMR spectrometer
The sample is bombarded with a series of radio The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the frequency (Rf) pulses and absorption of the radio waves is monitoredradio waves is monitored
The data are collected and manipulated on a The data are collected and manipulated on a computer to obtain an NMR spectrumcomputer to obtain an NMR spectrum
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The NMR SpectrometerThe NMR Spectrometer
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The NMR SpectrometerThe NMR Spectrometer
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The NMR SpectrometerThe NMR Spectrometer
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The NMR SpectrumThe NMR Spectrum
The vertical axis shows the intensity of Rf absorptionThe vertical axis shows the intensity of Rf absorption The horizontal axis shows relative energy at which The horizontal axis shows relative energy at which
the absorption occurs (parts per million, ppm)the absorption occurs (parts per million, ppm) Tetramethylsilane (TMS, SiMeTetramethylsilane (TMS, SiMe44) is included as a ) is included as a
standard zero point reference (0.00 ppm)standard zero point reference (0.00 ppm) The area under any peak corresponds to the The area under any peak corresponds to the
number of hydrogens represented by that peaknumber of hydrogens represented by that peak
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The NMR SpectrumThe NMR Spectrum
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Chemical Shift (Chemical Shift ()) The chemical shift (The chemical shift () in units of ppm is defined as:) in units of ppm is defined as:
== shift from TMS (in Hz)shift from TMS (in Hz)
radio frequency (in MHz)radio frequency (in MHz)
A standard notation is used to summarize NMR spectral A standard notation is used to summarize NMR spectral data. For example data. For example pp-xylene:-xylene:
2.3 (6H, singlet)2.3 (6H, singlet) 7.0 (4H, singlet)7.0 (4H, singlet)
Hydrogen atoms in identical chemical environments have Hydrogen atoms in identical chemical environments have identical chemical shiftsidentical chemical shifts
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Shielding – The Reason for Shielding – The Reason for Chemical Shift DifferencesChemical Shift Differences
Circulation of electrons within molecular orbitals results in local magnetic fields that oppose the applied magnetic field
The greater this “shielding” effect, the greater the applied field needed to achieve resonance, and the further to the right (“upfield”) the NMR signal
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Structural Effects on ShieldingStructural Effects on Shielding
Electron donating groups increase the electron density around nearby hydrogen atoms resulting in increased shielding, shifting peaks to the right.
Electron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting in decreased shielding, (deshielding) shifting peaks to the left (downfield).
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Structural Effects on ShieldingStructural Effects on Shielding
The deshielding effect of an electronegative substituent can be seen in the 1H-NMR spectrum of 1-bromobutane:
Br – CH2-CH2-CH2-CH3
(ppm): 3.4 1.8 1.5 0.9
No. of H’s: 2 2 2 3
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Some Specific Structural Effects on Some Specific Structural Effects on NMR Chemical ShiftNMR Chemical Shift
Hydrogen Environment (ppm)
Alkyl (C – H) 0.8 – 1.7
Alkyl Halide (RCH2X) 3 - 4
Alkene (R2C=CH2) 4 - 6
Aromatic (e.g. benzene) 6 - 8
Carboxylic Acid (RCOOH)
10 - 12
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CHEM 344
Spectroscopy of Organic Compounds
Lecture 219 June 2007
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Review of Lecture 1 Spectroscopy: the study of molecular structure by the
interaction of electromagnetic radiation with matter
Energy levels in molecules quantized (ΔE = hv)
NMR uses magnetic fields and radio-waves to flip the spin-state of a nucleus (e.g. 1H, 13C)
Different local magnetic fields within the molecule give rise to different signals in the NMR spectrum
Local magnetic field influenced by local structure of molecule (e.g. electron withdrawing groups)
Equivalent hydrogen atoms = same chemical shift
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Spin-Spin Splitting Non-equivalent hydrogen atoms will (almost) always
have different chemical shifts.
When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent carbon.
This is the origin of signal multiplicity
The size of the splitting between two hydrogen atoms (measured in Hz) is the coupling constant, J.
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Spin-Spin Splitting - Origin of the Doublet
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Spin-Spin Splitting - Origin of the Triplet
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Spin-Spin Splitting - Origin of the Quartet
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Pascal’s Triangle
# eq. protons Multiplicity# eq. protons Multiplicity Relative Intensity Relative Intensity
00 SingletSinglet 11
11 DoubletDoublet 1:1 1:1
22 Triplet 1:2:1Triplet 1:2:1
33 Quartet Quartet 1:3:3:1 1:3:3:1
44 QuintetQuintet 1:4:6:4:1 1:4:6:4:1
55 SextetSextet 1:5:10:10:5:1 1:5:10:10:5:1
66 SeptetSeptet 1:6:15:20:15:6:11:6:15:20:15:6:1
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The n + 1 RuleThe n + 1 Rule
If Ha is a set of equivalent hydrogen atoms and Hx is an If Ha is a set of equivalent hydrogen atoms and Hx is an adjacent set of equivalent hydrogen atoms which are not adjacent set of equivalent hydrogen atoms which are not equivalent to Ha: (i.e. Ha equivalent to Ha: (i.e. Ha ≠≠ Hx) Hx)
The NMR signal of Ha will be split into n+1 peaks by Hx. The NMR signal of Ha will be split into n+1 peaks by Hx. (where n = # of hydrogen atoms in the Hx set.) (where n = # of hydrogen atoms in the Hx set.)
The NMR signal of Hx will be split into n+1 peaks by Ha. The NMR signal of Hx will be split into n+1 peaks by Ha. (where n = # of hydrogen atoms in the Ha set.)(where n = # of hydrogen atoms in the Ha set.)
If there are n equivalent protons on an adjacent atom(s), If there are n equivalent protons on an adjacent atom(s), they will split a signal into n+1 peaks.they will split a signal into n+1 peaks.
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11H-NMR Spectrum of BromoethaneH-NMR Spectrum of Bromoethane
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Formula: CFormula: C33HH77II
11H-NMR H-NMR δδ: 1.90 (d, 6H), 4.33 (sept., 1H): 1.90 (d, 6H), 4.33 (sept., 1H)
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Formula: CFormula: C22HH44ClCl22
1H-NMR 1H-NMR δδ: 2.03 (d, 3H), 4.32 (quartet, 1H): 2.03 (d, 3H), 4.32 (quartet, 1H)
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Formula: CFormula: C33HH66ClCl22
1H-NMR 1H-NMR δδ: 2.20 (pent., 2H), 3.62 (triplet, 4H): 2.20 (pent., 2H), 3.62 (triplet, 4H)
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Infrared Spectroscopy
Energy of photons in the IR region corresponds Energy of photons in the IR region corresponds to differences in vibrational energy levels within to differences in vibrational energy levels within molecules (~10 kcal/mol = ~40 kJ/mol).molecules (~10 kcal/mol = ~40 kJ/mol).
Vibrational energy levels are dependent on bond Vibrational energy levels are dependent on bond types and bond strengths, and are quantized.types and bond strengths, and are quantized.
IR is useful to determine if certain types of bonds IR is useful to determine if certain types of bonds (functional groups) are present in the molecule.(functional groups) are present in the molecule.
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IR Spectrum of EthanolIR Spectrum of Ethanol
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IR Correlation TableIR Correlation Table
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Key Functional Groups by Region Key Functional Groups by Region of the IR Spectrumof the IR Spectrum
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IR Spectrum of BenzaldehydeIR Spectrum of Benzaldehyde
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IR Spectrum of CyclohexanoneIR Spectrum of Cyclohexanone
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IR Spectrum of Propanoic AcidIR Spectrum of Propanoic Acid
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CHEM 344
Spectroscopy of Organic Compounds
Lecture 320 June 2007
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Review of Lecture 2 Spin-spin splitting leads to multiplicity in NMR spectra
The size of the splitting between two hydrogen atoms is the coupling constant, J.
n+1 rule - doublet, triplet, quartet….Pascal’s triangle
Infrared radiation excites molecular vibrations
IR bands depend on bond type, strength etc.
IR spectroscopy good for functional group assignment
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NMR: Exceptions to the n+1 Rule
The n+1 rule does not apply when a set of equivalent H’s is split by two or more other non-equivalent sets with different coupling constants.
The n+1 rule does not apply to second order spectra in which the chemical shift difference between two sets of H’s is not much larger than the coupling constant.
Usually have to simulate 2nd order spectra
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NMR: Some Specific Functional NMR: Some Specific Functional Group CharacteristicsGroup Characteristics
O-H and N-H will often show broad peaks with O-H and N-H will often show broad peaks with no resolved splitting (acidic exchange no resolved splitting (acidic exchange broadening), and the chemical shift can vary broadening), and the chemical shift can vary greatly. greatly.
Aldehyde C-H is strongly deshielded.Aldehyde C-H is strongly deshielded.(( = 9-10 ppm) and coupling to alkyl H’s on = 9-10 ppm) and coupling to alkyl H’s on adjacent carbon is small.adjacent carbon is small.
Carboxylic Acid O-H is very strongly deshielded. Carboxylic Acid O-H is very strongly deshielded. (( = 10-12 ppm) = 10-12 ppm)
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NMR: Some Specific Functional NMR: Some Specific Functional Group CharacteristicsGroup Characteristics
Cis and trans H’s on alkenes usually show Cis and trans H’s on alkenes usually show strong coupling, but geminal H’s on strong coupling, but geminal H’s on alkenes show little or no resolved alkenes show little or no resolved coupling.coupling.
Ortho splitting on aromatic rings is often Ortho splitting on aromatic rings is often resolved, but meta and para splitting is resolved, but meta and para splitting is not.not.
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NMR: Mixtures of CompoundsNMR: Mixtures of Compounds
Unlike most textbook examples, “real world” Unlike most textbook examples, “real world” NMR samples commonly contain mixtures of NMR samples commonly contain mixtures of compounds.compounds.
It is very common to see extra signals due to It is very common to see extra signals due to impurities. The impurities can often be impurities. The impurities can often be identified.identified.
In some cases it is useful (or necessary) to In some cases it is useful (or necessary) to analyze mixtures that contain comparable analyze mixtures that contain comparable amounts of two or more compounds. amounts of two or more compounds.
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NMR: Mixtures of CompoundsNMR: Mixtures of Compounds
Synthesis of Ethyl Acetate by Fischer Synthesis of Ethyl Acetate by Fischer Esterification Results in an Equilibrium Mixture:Esterification Results in an Equilibrium Mixture:
H3C O
O
H
H3C
H2C
OH
H3C O
H2C
CH3
O
++
H2O
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Unknown AUnknown A (Figure 14.27 Solomons 8 (Figure 14.27 Solomons 8thth ed.) ed.)
Formula = CFormula = C99HH1212
IHD = 4IHD = 4
IR shows no medium or strong bands above IR shows no medium or strong bands above 1650 cm1650 cm-1-1 except C-H stretching bands around except C-H stretching bands around 3,000 cm3,000 cm-1-1
11H NMR H NMR : 1.26 (d, 6H), 2.90 (sept., 1H), : 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H)7.1-7.5 (m, 5H)
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Unknown BUnknown B (Figure 14.27 Solomons 8 (Figure 14.27 Solomons 8thth ed.) ed.)
Formula = CFormula = C88HH1111NN
IHD = 4IHD = 4
IR shows two medium peaks between 3300 and IR shows two medium peaks between 3300 and 3500 cm3500 cm-1-1 . No other medium or strong bands . No other medium or strong bands above 1650 cmabove 1650 cm-1 -1 except C-H stretching bands except C-H stretching bands around 3,000 cmaround 3,000 cm-1-1
11H NMR H NMR : 1.4 (d, 3H), 1.7 (s, br, 2H), : 1.4 (d, 3H), 1.7 (s, br, 2H), 4.1(quart., 1H), 7.2-7.4 (m, 5H)4.1(quart., 1H), 7.2-7.4 (m, 5H)
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Unknown CUnknown C (Figure 14.27 Solomons 8 (Figure 14.27 Solomons 8thth ed.) ed.)
Formula = CFormula = C99 H H1010
IHD = 5IHD = 5
IR shows no medium or strong bands above IR shows no medium or strong bands above 1650 cm1650 cm-1-1 except C-H stretching bands around except C-H stretching bands around 3,000 cm3,000 cm-1-1
11H NMR H NMR : 2.05 (pent., 2H), : 2.05 (pent., 2H), 2.90 (trip., 4H), 7.1-7.3 (m, 4H)2.90 (trip., 4H), 7.1-7.3 (m, 4H)
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Unknown HUnknown H (Figure 9.50 Solomons 8 (Figure 9.50 Solomons 8thth ed.) ed.)
Formula = CFormula = C33HH44BrBr22
IHD = 1IHD = 1
No IR data givenNo IR data given
11H NMR H NMR : 4.20 (2H), 5.63 (1H), 6.03 (1H) : 4.20 (2H), 5.63 (1H), 6.03 (1H)
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Unknown YUnknown Y (Figure 14.34 Solomons 8 (Figure 14.34 Solomons 8thth ed.) ed.)
Formula = CFormula = C99HH1212OO
IHD = 4IHD = 4
IR shows a strong, broad, absorbance centered IR shows a strong, broad, absorbance centered at 3400 cmat 3400 cm-1-1
11HNMR HNMR : 0.85 (t, 3H), 1.75 (m, 2H), : 0.85 (t, 3H), 1.75 (m, 2H), 4.38 (s, br, 1H), 4.52 (t, 1H), 4.38 (s, br, 1H), 4.52 (t, 1H), 7.2-7.4 (m, 5H)7.2-7.4 (m, 5H)