CHEM 344

CHEM 344

Spectroscopy of Organic Compounds

Lecture 118 June 2007

22

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

33

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

44

The Electromagnetic SpectrumThe Electromagnetic Spectrum

55

The Electromagnetic SpectrumThe Electromagnetic Spectrum

66

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)

77

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

88

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

99

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

1010

Magnetic Properties of the Proton Magnetic Properties of the Proton Related to SpinRelated to Spin

1111

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

1212

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

1313

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

1414

The NMR SpectrometerThe NMR Spectrometer

1515


1616


1717

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

1818

The NMR SpectrumThe NMR Spectrum

1919

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

2020

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

2121

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).

2222

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

2323

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

2424

CHEM 344



2525

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

2626

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.

2727

Spin-Spin Splitting - Origin of the Doublet

2828

Spin-Spin Splitting - Origin of the Triplet

2929

Spin-Spin Splitting - Origin of the Quartet

3030

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

3131

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.

3232

11H-NMR Spectrum of BromoethaneH-NMR Spectrum of Bromoethane

3333

Formula: CFormula: C33HH77II

11H-NMR H-NMR δδ: 1.90 (d, 6H), 4.33 (sept., 1H): 1.90 (d, 6H), 4.33 (sept., 1H)

3434

Formula: CFormula: C22HH44ClCl22

1H-NMR 1H-NMR δδ: 2.03 (d, 3H), 4.32 (quartet, 1H): 2.03 (d, 3H), 4.32 (quartet, 1H)

3535

Formula: CFormula: C33HH66ClCl22

1H-NMR 1H-NMR δδ: 2.20 (pent., 2H), 3.62 (triplet, 4H): 2.20 (pent., 2H), 3.62 (triplet, 4H)

3636

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

3939

IR Correlation TableIR Correlation Table

4040

Key Functional Groups by Region Key Functional Groups by Region of the IR Spectrumof the IR Spectrum

4141

IR Spectrum of BenzaldehydeIR Spectrum of Benzaldehyde

4242

IR Spectrum of CyclohexanoneIR Spectrum of Cyclohexanone

4343

IR Spectrum of Propanoic AcidIR Spectrum of Propanoic Acid

4444

CHEM 344



4545

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

4646

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

5151

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)

5353

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.

6060

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.

6161

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)

6666

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)

6868

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)

7070

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)

7171

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)

CHEM 344

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