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CHEM 344. Spectroscopy of Organic Compounds Lecture 1 18 June 2007. Modern Spectroscopic Methods. Revolutionized the study of organic chemistry Can determine the exact structure of small to medium size molecules in a few minutes

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chem 344

CHEM 344

Spectroscopy of

Organic Compounds

Lecture 1

18 June 2007

modern spectroscopic methods
Modern Spectroscopic Methods
  • Revolutionized the study of organic chemistry
  • Can determine the exact structure of small to medium size molecules in a few minutes
  • Nuclear Magnetic Resonance (NMR) and Infrared Spectroscopy (IR) are particularly powerful techniques which we will focus on and use in this course
interaction of light and matter the physical basis of spectroscopy
Interaction of Light and MatterThe 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 (l) range from 103to 10-15 meters
relationship between wavelength frequency and energy
Relationship Between Wavelength, Frequency and Energy
  • Speed of light (c) is constant for all wavelengths
  • Frequency (n), the number of wavelengths per second, is inversely proportional to wavelength:

n = c/l

  • Energy of a photon is directly proportional to frequency

E = hc/l = hn

(where h = Plank’s constant)

energy levels in molecules
Energy Levels in Molecules
  • Energy levels within a molecule are discrete (quantized)
  • Transitions between various energy levels occur only at discrete energies
  • Transition caused by subjecting the molecule to radiation of an energy that exactly matches the difference in energy between the two levels

Eupper – Elower = ΔE = hn

nuclear spins
Nuclear Spins
  • Spin ½ atoms: mass number is odd

1H, 13C, 19F, 29Si, 31P

  • Spin 1 atoms: mass number is even

2H, 14N

  • Spin 0 atoms: mass number is even

12C, 16O, 32S

NO NMR SIGNAL

energy states of protons in a magnetic field
Energy States of Protons in a Magnetic Field

No External Mag. Field

External Mag. Field Bo

Spin states degenerate

Random orientations

Two allowed orientations (2I+1) = 2

Aligned with or against direction of Bo

nuclear magnetic resonance nmr
Nuclear Magnetic Resonance (NMR)
  • Nuclear – spin ½ nuclei (e.g. protons) behave as tiny bar magnets
  • Magnetic – a strong magnetic field causes a small energy difference between + ½

and – ½ spin states

  • Resonance – photons of radio waves can match the exact energy difference between the + ½ and – ½ spin states resulting in absorption of photons as the protons change spin states
the nmr experiment
The NMR Experiment
  • The sample, dissolved in a suitable NMR solvent (e.g. CDCl3, CCl4, C6D6), is placed in the strong magnetic field of the NMR spectrometer
  • The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored
  • The data are collected and manipulated on a computer to obtain an NMR spectrum
the nmr spectrum
The NMR Spectrum
  • The vertical axis shows the intensity of Rf absorption
  • The horizontal axis shows relative energy at which the absorption occurs (parts per million, ppm)
  • Tetramethylsilane (TMS, SiMe4) is included as a standard zero point reference (0.00 ppm)
  • The area under any peak corresponds to the number of hydrogens represented by that peak
chemical shift d
Chemical Shift (d)
  • The chemical shift (d) in units of ppm is defined as:

d =shift from TMS (in Hz)

radio frequency (in MHz)

  • A standard notation is used to summarize NMR spectral data. For example p-xylene:

d 2.3 (6H, singlet)

d 7.0 (4H, singlet)

  • Hydrogen atoms in identical chemical environments have identical chemical shifts
shielding the reason for chemical shift differences
Shielding – The Reason for Chemical 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
structural effects on shielding
Structural 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).
structural effects on shielding1
Structural 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

d (ppm): 3.4 1.8 1.5 0.9

No. of H’s: 2 2 2 3

slide24

CHEM 344

Spectroscopy of

Organic Compounds

Lecture 2

19 June 2007

slide25

Review of Lecture 1

  • Spectroscopy:thestudy 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
spin spin splitting
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.
pascal s triangle
Pascal’s Triangle

# eq. protons Multiplicity Relative Intensity

0 Singlet 1

1 Doublet 1:1

2 Triplet 1:2:1

3 Quartet 1:3:3:1

4 Quintet 1:4:6:4:1

5 Sextet 1:5:10:10:5:1

6 Septet 1:6:15:20:15:6:1

the n 1 rule
The n + 1 Rule

If Ha is a set of equivalent hydrogen atoms and Hx is an adjacent set of equivalent hydrogen atoms which are not equivalent to Ha: (i.e. Ha ≠ Hx)

  • The NMR signal of Ha will be split into n+1 peaks by Hx. (where n = # of hydrogen atoms in the Hx set.)
  • The NMR signal of Hx will be split into n+1 peaks by Ha. (where n = # of hydrogen atoms in the Ha set.)
  • If there are n equivalent protons on an adjacent atom(s), they will split a signal into n+1 peaks.
slide33

Formula: C3H7I

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

slide34

Formula: C2H4Cl2

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

slide35

Formula: C3H6Cl2

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

infrared spectroscopy
Infrared Spectroscopy
  • Energy of photons in the IR region corresponds to differences in vibrational energy levels within molecules (~10 kcal/mol = ~40 kJ/mol).
  • Vibrational energy levels are dependent on bond types and bond strengths, and are quantized.
  • IR is useful to determine if certain types of bonds (functional groups) are present in the molecule.
slide44

CHEM 344

Spectroscopy of

Organic Compounds

Lecture 3

20 June 2007

slide45

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
nmr exceptions to the n 1 rule
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
nmr some specific functional group characteristics
NMR: Some Specific Functional Group Characteristics
  • O-H and N-H will often show broad peaks with no resolved splitting (acidic exchange broadening), and the chemical shift can vary greatly.
  • Aldehyde C-H is strongly deshielded.

(d = 9-10 ppm) and coupling to alkyl H’s on adjacent carbon is small.

  • Carboxylic Acid O-H is very strongly deshielded. (d = 10-12 ppm)
nmr some specific functional group characteristics1
NMR: Some Specific Functional Group Characteristics
  • Cis and trans H’s on alkenes usually show strong coupling, but geminal H’s on alkenes show little or no resolved coupling.
  • Ortho splitting on aromatic rings is often resolved, but meta and para splitting is not.
nmr mixtures of compounds
NMR: Mixtures of Compounds
  • Unlike most textbook examples, “real world” NMR samples commonly contain mixtures of compounds.
  • It is very common to see extra signals due to impurities. The impurities can often be identified.
  • In some cases it is useful (or necessary) to analyze mixtures that contain comparable amounts of two or more compounds.
nmr mixtures of compounds1
NMR: Mixtures of Compounds

Synthesis of Ethyl Acetate by Fischer Esterification Results in an Equilibrium Mixture:

unknown a figure 14 27 solomons 8 th ed
Unknown A (Figure 14.27 Solomons 8th ed.)
  • Formula = C9H12
  • IHD = 4
  • IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1
  • 1H NMR d: 1.26 (d, 6H), 2.90 (sept., 1H), 7.1-7.5 (m, 5H)
unknown b figure 14 27 solomons 8 th ed
Unknown B (Figure 14.27 Solomons 8th ed.)
  • Formula = C8H11N
  • IHD = 4
  • IR shows two medium peaks between 3300 and 3500 cm-1 . No other medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1
  • 1H NMR d: 1.4 (d, 3H), 1.7 (s, br, 2H),

4.1(quart., 1H), 7.2-7.4 (m, 5H)

unknown c figure 14 27 solomons 8 th ed
Unknown C (Figure 14.27 Solomons 8th ed.)
  • Formula = C9 H10
  • IHD = 5
  • IR shows no medium or strong bands above 1650 cm-1 except C-H stretching bands around 3,000 cm-1
  • 1H NMR d: 2.05 (pent., 2H),

2.90 (trip., 4H), 7.1-7.3 (m, 4H)

unknown h figure 9 50 solomons 8 th ed
Unknown H (Figure 9.50 Solomons 8th ed.)
  • Formula = C3H4Br2
  • IHD = 1
  • No IR data given
  • 1H NMR d: 4.20 (2H), 5.63 (1H), 6.03 (1H)
unknown y figure 14 34 solomons 8 th ed
Unknown Y (Figure 14.34 Solomons 8th ed.)
  • Formula = C9H12O
  • IHD = 4
  • IR shows a strong, broad, absorbance centered at 3400 cm-1
  • 1HNMR d: 0.85 (t, 3H), 1.75 (m, 2H),

4.38 (s, br, 1H), 4.52 (t, 1H),

7.2-7.4 (m, 5H)