Electronic Absorption Spectroscopy of Organic Compounds

1
Electronic Absorption Spectroscopy of Organic
Compounds

• W. R. Murphy, Jr.
• Department of Chemistry and Biochemistry
• Seton Hall University

2
Course Topics

• UV absorption spectroscopy
• Basic absorption theory
• Experimental concerns
• Chromophores
• Spectral interpretation
• Chiroptic Spectroscopy
• ORD, CD
• Effects of inorganic ions (as time permits)

3
Electric and magnetic field components of plane
polarized light

• Light travels in z-direction
• Electric and magnetic fields travel at 90 to
  each other at speed of light in particular medium
• c ( 3 1010 cm s-1) in a vacuum

4
Characterization of Radiation
5
Wavelength and Energy Units

• Wavelength
• 1 cm 108 Å 107 nm 104 ? 107 m?
  (millimicrons)
• N.B. 1 nm 1 m? (old unit)
• Energy
• 1 cm-1 2.858 cal mol-1 of particles
• 1.986 ? 1016 erg molecule-1 1.24 ? 10-4 eV
  molecule-1
• ?E (kcal mol-1) ? ?(Å) 2.858 ? 105
• E(kJ mol-1) 1.19 ? 105/?(nm)297 nm 400 kJ

6
Absorption Spectroscopy

• Provide information about presence and absence of
  unsaturated functional groups
• Useful adjunct to IR
• Needed for chiroptic techniques
• Determination of concentration, especially in
  chromatography
• For structure proof, usually not critical data,
  but essential for further studies
• NMR, MS not good for purity

7
Importance of UV data

• Particularly useful for
• Polyenes with or without heteroatoms
• Benzenoid and nonbenzenoid aromatics
• Molecules with heteroatoms containing n electrons
• Chiroptic tool to investigate optically pure
  molecules with chromophores
• Practically, UV absorption is measured after NMR
  and MS analysis

8
UV Spectral Nomenclature
9
UV and Visible Spectroscopy

• Vacuum UV or soft X-rays
• 100 - 200 nm
• Quartz, O2 and CO2 absorb strongly in this region
• N2 purge good down to 180 nm
• Quartz region
• 200 350 nm
• Source is D2 lamp
• Visible region
• 350 800 nm
• Source is tungsten lamp

10
All organic compounds absorb UV-light

• C-C and C-H bonds isolated functional groups
  like CC absorb in vacuum UV therefore not
  readily accessible
• Important chromophores are R2CO, -O(R)CO,
  -NH(R)CO and polyunsaturated compounds

11
Spectral measurement

• usually dissolve 1 mg in up to 100 mL of solvent
  for samples of 100-200 D molecular weight
• data usually presented as A vs ?(nm)
• for publication, y axis is usually transformed to
  ? or log10? to make spectrum independent of
  sample concentration

12
Preparation of samples

• Concentration must be such that the absorbance
  lies between 0.2 and 0.7 for maximum accuracy
• Conjugated dienes have ? ? 8,000-20,000, so c ? 4
  ? 10-5 M
• n?? of a carbonyl have ? ? 10-100, so c ? 10-2 M
• Successive dilutions of more concentrated samples
  necessary to locate all possible transitions

13
UV cut-offs for common solvents
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Solvent choices

• Important features to consider are solubility of
  sample and UV cutoff of solvent
• Filtration to remove particulates is useful to
  reduce scattered light
• Solvent purity is very important

15
Chromophores

• Structures within the molecule that contain the
  electrons being moved by the photon of light
• Only those absorbing above 200 nm are useful
• n?? in ketones at ca 300 nm is only isolated
  chromophore of interest
• all other chromophores are conjugated systems of
  some sort

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Types of organic transitions (Chromophores)
??? Satd hydrocarbons Vacuum UV
n?? Satd hydrocarbons with heteroatoms Possibly quartz UV
??? Olefins UV
n?? Olefins with heteroatoms UV
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Modes of electronic excitation
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Simple lone pair system
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Simple olefin
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Simple chromophores
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Examples of n?? and ? ?? transitions
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Molecular orbitals for common transitions

• Molecular orbital diagram for 2-butenal
• Shows n ? ? on right
• Shows ? ? ? on left
• Both peaks are broad due to multiple vibrational
  sublevels in ground and excited states

23
Energy level diagram for a carbonyl
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Beers Law

• Io Intensity of incident light
• I Intensity of transmitted light
• ? molar extinction coefficient
• l path length of cell
• c concentration of sample

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Transition Energies

• Electronic transitions are quantized, so sharp
  bands are expected
• In reality, absorption lines are broadened into
  bands due to other types of transitions occurring
  in the same molecules
• For electronic transitions, this means
  vibrational transitions and coupling to solvent

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Actual transition with vibrational levels
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Spectrum for energy level diagram shown on
previous slide
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Vibrational fine structure

• Rigid molecules such as benzene and fused benzene
  ring structures often display vibrational fine
  structure
• Example is benzene in heptane
• Usually only observed in gas phase, but rigid
  molecules do display this

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Benzene (note use of m? in this older data)
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Pyridine
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Mesityl oxide
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Intensities of transitions

• Strictly speaking, one should work with
  integrated band intensities
• However, overlap of bands prevents clean
  isolation of transitions (hence the popularity of
  fluorescence in photophysical studies)
• Therefore, intensities are used

33
Selection Rules

• After resonance condition is met, the
  electromagnetic radiation must be able to
  electrical work on the molecule
• For this to happen, transition in the molecule
  must be accom-panied by a change in the
  electrical center of the molecule
• Selection rules address the requirements for
  transitions between states in molecules
• Selection rules are derived from the evaluation
  of the properties of the transition moment
  integral (beyond scope of this course

34
Selection Rule Terminology

• Transitions that are possible according to the
  rules are termed allowed
• Such transitions are correspond-ingly intense
• Transitions that are not possible are termed
  forbidden and are weak
• Transitions may be allowed by some rules and
  forbidden by others

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Common Selection Rules

• Spin-forbidden transitions
• Transitions involving a change in the spin state
  of the molecule are forbidden
• Strongly obeyed
• Relaxed by effects that make spin a poor quantum
  number (heavy atoms)
• Symmetry-forbidden transitions
• Transitions between states of the same parity are
  forbidden
• Particularly important for centro-symmetric
  molecules (ethene)
• Relaxed by coupling of electronic transitions to
  vibrational transitions (vibronic coupling)

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Intensities

• P is the transition probability ranges from 0 to
  1
• a is the target area of the absorbing system (the
  chromophore)
• chromophores are typically 10 Å long, so a
  transition of P 1 will have an ? of 105

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Intensities, cont.

• this intensity is actually observed, and has been
  exceeded by very long chromophoric systems
• Generally, fully allowed systems have ? gt 10,000
  and those with low transition probabilities will
  have ? lt 1000
• Generally, the longer the chromophore, the longer
  wavelength is the absorption maximum and the more
  intense the absorption

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Intensities - Important forbidden transitions

• n??
• near 300 nm in ketones
• ? ca 10 - 100
• In benzene and aromatics
• band around 260 nm and equivalent in more complex
  systems
• ? gt 100
• Prediction of intensities is a very deep subject,
  covered in Physical Methods next year

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Fundamentals of spectral interpretation

• Examining orbital diagrams for simple conjugated
  systems is helpful (lots of good programs
  available to do these calculations)
• Wavelength and intensity of bands are both useful
  for assignments

40
Solvent effects

• Franck-Condon Principle
• nuclei are stationary during electronic
  transitions
• Electrons of solvent can move in concert with
  electrons involved in transition
• Since most transitions result in an excited state
  that is more polar than the ground state, there
  is a red shift (10 - 20 nm) upon increasing
  solvent polarity (hexane to ethanol)

41
Solvent effects

• Hydrocarbons ? water
• ???
• Weak bathochromic or red shift
• n??
• Hypsochromic or blue shift (strongly affected by
  hydrogen bonding solvents)
• Solvent effects due to stabilization or
  destabilization of ground or excited states,
  changing the energy gap

42
Solvent effects, cont

• n?? in ketones is the exception
• there is a blue shift
• this is due to diminished ability of solvent to
  hydrogen bond to lone pairs on oxygen
• example - acetone
• in hexane, ?max 279 nm (? 15)
• in water, ?max 264.5 nm

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Band assignments n??

• ? lt 2000
• Strong blue shift observed in high dielectric or
  hydrogen-bonding solvents
• n?? often disappear in acidic media due to
  protonation of n electrons
• Blue shifts occur upon attachment of an
  electron-donating group
• Absorption band corresponding to the n?? is
  missing in the hydrocarbon analog (consider H2CO
  vs H2CCH2
• Usually, but not always, n?? is the lowest
  energy singlet transition
• ??? transitions are considerably more intense

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Searching for chromophores

• No easy way to identify a chromophore
• too many factors affect spectrum
• range of structures is too great
• Use other techniques to help
• IR - good for functional groups
• NMR - best for C-H

45
Identifying chromophores

• complexity of spectrum
• compounds with only one (or a few) bands below
  300 nm probably contains only two or three
  conjugated units
• extent to which it encroaches on visible region
• absorption stretching into the visible region
  shows presence of a long or polycyclic aromatic
  chromophore

46
Identifying chromophores

• Intensity of bands - particularly the principle
  maximum and longest wavelength maximum
• Simple conjugated chromophores such as dienes and
  ?? unsaturated ketones have ? values from 10,000
  to 20,000
• Longer conjugated systems have principle maxima
  with correspondingly longer ?max and larger ?

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Identifying chromophores

• Low intensity bands in the 270 - 350 nm (with ?
  ca 10 - 100) are result of ketones
• Absorption bands with ? 1000 - 10,000 almost
  always show the presence of aromatic systems
• Substituted aromatics also show strong bands with
  ? gt 10,000, but bands with ? lt 10,000 are also
  present

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Next steps in spectral interpretation

• Look for model systems
• Many have been investigated and tabulated, so hit
  the literature
• Major references
• Organic Electronic Spectral Data, Wiley, New
  York, Vol 1-21 (1960-85)
• Sadtler Handbook of Ultraviolet Spectra, Heyden,
  London

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Substructure identification
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Substituted acyclic dienes

• ?max shifts
• Presence of substituents
• Length of conjugation

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Conjugated dienes

• Strong UV absorber
• ?max affected by geometry and substitution
  pattern
• S-trans ? 217 nm
• S-cis ? 253 nm
• Replacement of hydrogen with alkyl or polar
  groups red shift these base values
• Extending conjugation also red shifts ?max

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Conjugated Polyenes
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Diene example
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Energy levels for butadiene
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Distinguishing between polyenes
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Diene Examples 1
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Diene Examples 2
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Effects of Ring Strain
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Molecular orbitals for common transitions

• Molecular orbital diagram for 2-butenal
• Shows n ? ? on right
• Shows ? ? ? on left
• Both peaks are broad due to multiple vibrational
  sublevels in ground and excited states

60
Orbital Diagram for Carbonyl Group

• n?? bands are weak due to unfavorable
  orientation of n electrons relative to the ?
  orbitals

61
Rules for calculation of ??? ?max for conjugated
carbonyls
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Distinguishing between enones
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Selected References

• Harris, D. C., Bertolucci, M. D., Symmetry and
  Spectroscopy, Dover, 1978.
• Pasto, D. J., Johnson, C. R., Organic Structure
  Determination, Prentice-Hall, 1969.
• Drago, R. S., Physical Methods for Chemists,
  Surfside Publishing, 1992.
• Nakanishi, K., Berova, N., Woody, R. W., Circular
  Dichroism, VCH Publishers, 1994
• Williams, D. H., Fleming, I., Spectroscopic
  methods in organic chemistry, McGraw-Hill, 1987.