Chemistry 341Chemistry 341

Spectroscopy of Organic Spectroscopy of Organic CompoundsCompounds

Modern Spectroscopic MethodsModern Spectroscopic Methods

Revolutionized the study of Organic ChemistryRevolutionized the study of Organic Chemistry

Determine the exact structure of small to Determine the exact structure of small to medium size molecules in a few minutes.medium 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.powerful techniques which we will focus on.

Interaction of Light and MatterInteraction of Light and MatterThe Physical Basis of SpectroscopyThe Physical Basis of Spectroscopy

Quantum properties of light (photons)Quantum properties of light (photons)

Quantum properties of matter (quantized Quantum properties of matter (quantized energy states).energy states).

Photons of light act as our “quantum Photons of light act as our “quantum probes” at the molecular level giving us probes” at the molecular level giving us back precise information about the energy back precise information about the energy levels within molecules. levels within molecules.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

ContinuousContinuous

Covers a wide range of wavelengths of Covers a wide range of wavelengths of “light” from radio waves to gamma rays.“light” from radio waves to gamma rays.

Wavelengths (Wavelengths () range from more than ten ) range from more than ten meters to less than 10meters to less than 10-12-12 meter. meter.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Relationship Between Wavelength, Relationship Between Wavelength, Frequency and EnergyFrequency and Energy

Speed of light (c) is the same for all wavelengths.Speed of light (c) is the same 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:

cc

Energy of a photon is directly proportional to frequency Energy of a photon is directly proportional to frequency and inversely proportional to wavelength:and inversely proportional to wavelength:

E = hE = h = hc/ = hc/(where h = Plank’s constant)(where h = Plank’s constant)

Wavelength/Spectroscopy Wavelength/Spectroscopy RelationshipsRelationships

Spectral RegionSpectral Region Photon EnergyPhoton Energy Molecular Molecular Energy Energy ChangesChanges

UV-VisibleUV-Visible ~ 100 kcal/mole~ 100 kcal/mole Electronic Electronic

Infrared (IR)Infrared (IR) ~ 10 kcal/mole~ 10 kcal/mole Bond vibrationsBond vibrations

RadioRadio < 0.1 kcal/mol< 0.1 kcal/mol Nuclear Spin Nuclear Spin states in a states in a magnetic fieldmagnetic field

Spin of Atomic NucleiSpin of Atomic Nuclei Spin 1/2 atoms: Spin 1/2 atoms: mass number is odd. mass number is odd.

examples: examples: 11H and H and 1313C.C.

Spin 1 atoms:Spin 1 atoms: mass number is even.mass number is even.examples: examples: 22H and H and 1414N.N.

Spin 0 atoms: Spin 0 atoms: mass number is even.mass number is even.examples: examples: 1212C and C and 1616O.O.

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

Energy States of Protons in a Magnetic Energy States of Protons in a Magnetic FieldField

E = absorbed light

Applied

Magnetic

Field

Hext

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 magnets.tiny 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 states.and – 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 states. photons as the protons change spin states.

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 or CCl or CCl44) is placed in the strong ) is placed in the strong magnetic field of the NMR.magnetic field of the NMR.

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 monitored.radio waves is monitored.

The data is collected and manipulated on a The data is collected and manipulated on a computer to obtain an NMR spectrum.computer to obtain an NMR spectrum.

The NMR SpectrometerThe NMR Spectrometer

The NMR SpectrometerThe NMR Spectrometer

The NMR SpectrumThe NMR Spectrum

The vertical axis shows the intensity of Rf The vertical axis shows the intensity of Rf absorption.absorption.

The horizontal axis shows relative energy at The horizontal axis shows relative energy at which the absorption occurs (in units of parts per which the absorption occurs (in units of parts per million = ppm)million = ppm)

Tetramethylsilane (TMS) in included as a Tetramethylsilane (TMS) in 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 peak.number of hydrogens represented by that peak.

The NMR SpectrumThe NMR Spectrum

Chemical Shift (Chemical Shift ()) The chemical shift (The chemical shift () in units of ppm is defined as:) in units of ppm is defined as:

== distance from TMS (in hz)distance 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)

Hydrogens in identical chemical environments Hydrogens in identical chemical environments (equivalent hydrogens) have identical chemical shifts. (equivalent hydrogens) have identical chemical shifts.

Shielding – The Reason for Shielding – The Reason for Chemical Shift DifferencesChemical Shift Differences

Circulation of electrons within molecular Circulation of electrons within molecular orbitals results in local magnetic fields that orbitals results in local magnetic fields that oppose the applied magnetic field.oppose the applied magnetic field.

The greater this “shielding” effect, the The greater this “shielding” effect, the greater the applied field needed to achieve greater the applied field needed to achieve resonance, and the further to the right resonance, and the further to the right (“upfield”) the NMR signal.(“upfield”) the NMR signal.

Structure Effects on ShieldingStructure Effects on Shielding

Electron donating groups increase the Electron donating groups increase the electron density around nearby hydrogen electron density around nearby hydrogen atoms resulting in increased shielding, atoms resulting in increased shielding, shifting peaks to the right.shifting peaks to the right.

Electron withdrawing groups decrease the Electron withdrawing groups decrease the electron density around nearby hydrogen electron density around nearby hydrogen atoms resulting in decreased shielding, atoms resulting in decreased shielding, (deshielding) shifting peaks to the left.(deshielding) shifting peaks to the left.

Structure Effects on ShieldingStructure Effects on Shielding

The Deshielding effect of an electronegative The Deshielding effect of an electronegative substituent can be seen in the NMR substituent can be seen in the NMR spectrum of 1-Bromobutanespectrum of 1-Bromobutane

Br – CHBr – CH22-CH-CH22-CH-CH22-CH-CH33

(ppm):(ppm): 3.43.4 1.81.8 1.51.5 0.90.9

No. of H’s:No. of H’s: 22 22 22 33

Some Specific Structural Effects on Some Specific Structural Effects on NMR Chemical ShiftNMR Chemical Shift

Type of HydrogenType of Hydrogen (ppm(ppm

Alkyl (C – H)Alkyl (C – H) 0.8 – 1.70.8 – 1.7

Alkyl Halide (RCHAlkyl Halide (RCH22X)X) 3 - 43 - 4

Alkene (RAlkene (R22C=CHC=CH22)) 4 - 64 - 6

Aromatic (e.g. benzene)Aromatic (e.g. benzene) 6 - 86 - 8

Carboxylic Acid Carboxylic Acid (RCOOH)(RCOOH)

10 - 1210 - 12

Spin-Spin SplittingSpin-Spin Splitting Non-equivalent hydrogens will almost always Non-equivalent hydrogens will almost always

have different chemical shifts.have different chemical shifts.

When non-equivalent hydrogens are on adjacent When non-equivalent hydrogens are on adjacent carbon atoms spin-spin splitting will occur due to carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the the hydrogens on one carbon feeling the magnetic field from hydrogens on the adjacent magnetic field from hydrogens on the adjacent carbon.carbon.

The magnitude of the splitting between two The magnitude of the splitting between two hydrogens (measured in Hz) is the coupling hydrogens (measured in Hz) is the coupling constant, constant, J.J.

Spin-Spin SplittingSpin-Spin SplittingOrigin of the DoubletOrigin of the Doublet

Spin-Spin SplittingSpin-Spin SplittingOrigin of the TripletOrigin of the Triplet

Spin-Spin SplittingSpin-Spin SplittingOrigin of the QuartetOrigin of the Quartet

Pascal’s TrianglePascal’s Triangle

PatternPattern Relative Peak HeightRelative Peak Height

SingletSinglet 11

DoubletDoublet 1 : 1 1 : 1

TripletTriplet 1 : 2 : 1 1 : 2 : 1

QuartetQuartet 1 : 3 : 3 : 1 1 : 3 : 3 : 1

QuintetQuintet 1 :4 : 6 : 4 : 11 :4 : 6 : 4 : 1

The n + 1 RuleThe n + 1 Rule

If Ha is a set of equivalent hydrogens and Hx is an adjacent If Ha is a set of equivalent hydrogens and Hx is an adjacent set of equivalent hydrogens which are not equivalent to set of equivalent hydrogens which are not equivalent to Ha:Ha:

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 hydrogens in the Hx set.) (where n = # of hydrogens 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 hydrogens in the Ha set.)(where n = # of hydrogens in the Ha set.)

11H NMR Spectrum of BromoethaneH NMR Spectrum of Bromoethane

11H NMR Spectrum of H NMR Spectrum of 1-Nitropropane1-Nitropropane

Unknown AUnknown A(Figure 9.20 Solomons 7(Figure 9.20 Solomons 7thth ed.) ed.)

Formula: CFormula: C33HH77II

IHD = 0IHD = 0

No IR data providedNo IR data provided

11HNMR HNMR : 1.90 (d, 6H), 4.33 (sept., 1H): 1.90 (d, 6H), 4.33 (sept., 1H)

Unknown BUnknown B (Figure 9.20 Solomons 7 (Figure 9.20 Solomons 7thth ed.) ed.)

Formula CFormula C22HH44ClCl22

IHD = 0IHD = 0

No IR data givenNo IR data given

11HNMR HNMR : 2.03 (d, 3H), 4.32 (quartet, 1H): 2.03 (d, 3H), 4.32 (quartet, 1H)

Unknown CUnknown C (Figure 9.20 Solomons 7 (Figure 9.20 Solomons 7thth ed.) ed.)

Formula: CFormula: C33HH66ClCl22

IHD = 0IHD = 0

No IR data givenNo IR data given

11HNMR HNMR : 2.20 (pent., 2H), 3.62 (t, 4H) : 2.20 (pent., 2H), 3.62 (t, 4H)

Exceptions to the n+1 RuleExceptions to the n+1 Rule

The n+1 rule does not apply when a set of The n+1 rule does not apply when a set of equivalent H’s is split by two or more other equivalent H’s is split by two or more other non-equivalent sets with different coupling non-equivalent sets with different coupling constants.constants.

The n+1 rule does not apply to second The n+1 rule does not apply to second order spectra in which the chemical shift order spectra in which the chemical shift difference between two sets of H’s is not difference between two sets of H’s is not much larger than the coupling constant.much larger than the coupling constant.

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, and the chemical shift can no resolved splitting, and the chemical shift can vary greatly.vary 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)

NMR: Some Specific Functional NMR: Some Specific Functional Group CharacteristicsGroup Characteristics

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.

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.

Infrared SpectroscopyInfrared 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.molecules.

Vibrational energy levels are dependent on bond Vibrational energy levels are dependent on bond types and bond strengths.types and bond strengths.

IR is highly useful to determine if certain types of IR is highly useful to determine if certain types of bonds (functional groups) are present in the bonds (functional groups) are present in the molecule.molecule.

IR Spectrum of EthanolIR Spectrum of Ethanol

IR Correlation TableIR Correlation Table

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

IR Spectrum of BenzaldehydeIR Spectrum of Benzaldehyde

IR Spectrum of CyclohexanoneIR Spectrum of Cyclohexanone

IR Spectrum of Propanoic AcidIR Spectrum of Propanoic Acid

Unknown AUnknown A (Figure 14.27 Solomons 7 (Figure 14.27 Solomons 7thth 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

11HNMR HNMR : 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)

Unknown BUnknown B (Figure 14.27 Solomons 7 (Figure 14.27 Solomons 7thth 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

11HNMR HNMR : 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)

Unknown CUnknown C (Figure 14.27 Solomons 7 (Figure 14.27 Solomons 7thth 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)

Unknown HUnknown H (Figure 9.48 Solomons 7 (Figure 9.48 Solomons 7thth ed.) ed.)

Formula = CFormula = C33HH44BrBr22

IHD = 1IHD = 1

No IR data givenNo IR data given

1HNMR 1HNMR : 4.20 (2H), 5.63 (1H), 6.03 (1H) : 4.20 (2H), 5.63 (1H), 6.03 (1H)

Unknown YUnknown Y (Figure 14.34 Solomons 7 (Figure 14.34 Solomons 7thth 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)