Chapter 14 APPLICATION OF ULTRAVIOLET/VISIBLE MOLECULAR ABSORPTION SPECTROMETRY

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Chapter 14APPLICATION OF ULTRAVIOLET/VISIBLE
MOLECULAR ABSORPTION SPECTROMETRY

• Absorption measurements based upon ultraviolet
  and visible radiation find widespread application
  for the identification and determination of
  myriad inorganic and organic species. Molecular
  ultraviolet/visible absorption methods are
  perhaps the most widely used of all quantitative
  analysis techniques in chemical and clinical
  laboratories throughout the world.

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• THE MAGNITUDE OF MOLAR ABSORPTIVITIES
• Molar absorptivities range from zero up to a
  maximum on the order of 105 are observed. The
  magnitude of ? depends upon the probability for
  an energy-absorbing transition to occur. Peaks
  having molar absorptivities less than about 103
  are classified as being of low intensity. They
  result from so-called forbidden transitions,
  which have probabilities of occurrence that are
  less than 0.01.

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• ABSORBING SPECIES
• The absorption of ultraviolet or visible
  radiation by a molecular species M can be
  considered to be a two-step process, excitation
• M h? M
• The lifetime of the excited species is brief
  (10-8 to 10-9 s). Relaxation involves conversion
  of the excitation energy to heat.
• M M heat
• The absorption of ultraviolet or visible
  radiation generally results from excitation of
  bonding electrons.

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• Electronic Transitions
• There are three types of electronic transitions.
  The three include transitions involving
• (1) ?, ?, and n electrons
• (2) d and f electrons
• (3) charge transfer electrons.

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• Types of Absorbing Electrons
• The electrons that contribute to absorption by a
  molecule are (1) those that participate directly
  in bond formation between atoms (2) nonbonding
  or unshared outer electrons that are largely
  localized about such atoms as oxygen, the
  halogens, sulfur, and nitrogen. The molecular
  orbitals associated with single bonds are
  designated as sigma (?) orbitals, and the
  corresponding electrons are ? electrons.

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• Types of Absorbing Electrons
• The double bond in a molecule contains two types
  of molecular orbitals a sigma (?) orbital and a
  pi (?) molecular orbital. Pi orbitals are formed
  by the parallel overlap of atomic p orbitals. In
  addition to ? and ? electrons, many compounds
  contain nonbonding electrons. These unshared
  electrons are designated by the symbol n.

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• Energy
• The energies for the various types of molecular
  orbitals differ significantly. The energy level
  of a nonbonding electron lies between the energy
  levels of the bonding and the antibonding ? and ?
  orbitals. Electronic transitions among certain of
  the energy levels can be brought about by the
  absorption of radiation. Four types of
  transitions are possible
• ? ??, n ??, n ??, and ? ??.

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Antibonding
?
Antibonding
?
n ??
? ??
n ??
? ??
n
Nonbonding
Bonding
?
Energy
?
Bonding
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• ? ?? Transition
• An electron in a bonding ? orbital of a molecule
  is excited to the corresponding antibonding
  orbital by the absorption of radiation. The
  energy required to induce a ? ?? transition is
  large. Methane can undergo only ? ??
  transitions, exhibits an absorption maximum at
  125 nm. Absorption maxima due to ? ??
  transitions are never observed in the ordinarily
  accessible ultraviolet region.

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• n ?? Transitions
• Saturated compounds containing atoms with
  unshared electrons are capable of n ??
  transitions. These transitions require less
  energy than the ? ?? type and can be brought
  about by radiation in the region of between 150
  and 250 nm, with most absorption peaks appearing
  below 200 nm. The molar absorptivities are low to
  intermediate in magnitude and range between 100
  and 3000 L cm-1 mol -1.

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• n ?? and ? ?? Transitions
• Most applications of absorption spectroscopy are
  based upon transitions for n or ? electrons to
  the ? excited state because the energies
  required for these processes bring the absorption
  peaks into an experimentally convenient spectral
  region (200 to 700 nm). Both transitions require
  the presence of an unsaturated functional group
  to provide the ? orbitals. The molar
  absorptivities for peaks associated with
  excitation to the n, ? state are generally low
  and ordinarily range from 10 and 100 L cm-1 mol
  -1 values for ? ?? transitions, on the other
  hand, normally fall in the range between 1000 and
  10,000.

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• Effect of Conjugation of Chromophores
• ? electrons are considered to be further
  delocalized by conjugation the orbitals involve
  four (or more) atomic centers. The effect of this
  delocalization is to lower the energy level of
  the ? orbital and give it less antibonding
  character. Absorption maxima are shifted to
  longer wavelengths as a consequence. Conjugation
  of chromophores, has a profound effect on
  spectral properties. 1,3-butadiene, CH2CHCHCH2,
  has a strong absorption band that is displaced to
  a longer wavelength by 20 nm compared with the
  corresponding peak for an unconjugated diene.

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• Absorption Involving d and f Electrons
• Most transition-metal ions absorb in the
  ultraviolet or visible region of the spectrum.
  For the lanthanide and actinide series, the
  absorption process results from electronic
  transition of 4f and 5f electrons for elements
  of the first and second transition-metal series,
  the 3d and 4d electrons are responsible.

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• Absorption by Lanthanide and Actinide Ions
• The ions of most lanthanide and actinide
  elements absorb in the ultraviolet and visible
  regions. Their spectra consist of narrow,
  well-defined, and characteristic absorption
  peaks. The transitions responsible for absorption
  by elements of the lanthanide series appear to
  involve the various energy levels of 4f
  electrons, while it is the 5f electrons of the
  actinide series that interact with radiation.

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• Absorption by Elements of the First and
• Second Transition-Metal Series
• The ions and complexes of the first two
  transition series tend to absorb visible
  radiation in one if not all of their oxidation
  states. The absorption bands are often broad and
  are strongly influenced by chemical environmental
  factors. The spectral characteristics of
  transition metals involve electronic transitions
  among the various energy levels of d orbitals.

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• Charge-Transfer Absorption
• Species that exhibit charge-transfer absorption
  are of particular importance because their molar
  absorptivities are very large (?max gt 10,000).
  Thus, these complexes provide a highly sensitive
  means for detecting and determining absorbing
  species. Complexes exhibit charge transfer
  absorption are called charge-transfer complexes.
  In order for a complex to exhibit a
  charge-transfer spectrum, it is necessary for one
  of its components to have electron-donor
  characteristics and for the other component to
  have electron-acceptor properties. Absorption of
  radiation then involves transfer of an electron
  from the donor to an orbital that is largely
  associated with the acceptor.

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• APPLICATION OF ABSORPTION MEASREMENT TO
  QUALITATIVE ANALYSIS
• Methods of Plotting Spectral Data Several
  different types of spectral plots are encountered
  in qualitative molecular spectroscopy. The
  ordinate is most commonly percent transmittance,
  absorbance, log absorbance, or molar
  absorptivity. The abscissa is usually wavelength
  or wavenumber, although frequency is occasionally
  employed.

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• Solvents In choosing a solvent, consideration
  must be given not only to its transparence, but
  also to its possible effects upon the absorbing
  system. Polar solvents such as water, alcohols,
  esters, and ketones tend to obliterate spectral
  fine structure arising from vibrational effects
  spectra that approach those of the gas phase are
  more likely to be observed in nonpolar solvents
  such as hydrocarbons. In addition, the positions
  of absorption maxima are influenced by the nature
  of the solvent. Clearly, the same solvent must be
  used when comparing absorption spectra for
  identification purposes.

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• QUANTITATIVE ANALYSIS BY
• ABSORPTION MEASUREMENTS
• Absorption spectroscopy is one of the most
  useful and widely used tools available to the
  chemist for quantitative analysis. Important
  characteristics of spectrophotometric method
  include (1) wide applicability to both organic
  and inorganic systems, (2) typical sensitivities
  of 10-4 to 10-5 M, (3) moderate to high
  selectivity, (4) good accuracy, (5) ease and
  convenience of data acquisition.

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• Scope The applications of quantitative,
  ultraviolet/visible absorption methods not only
  are numerous, but also touch upon every field in
  which quantitative chemical information is
  required. It has been estimated that in the field
  of health alone, 95 of all quantitative
  determinations are performed by
  ultraviolet/visible spectrophotometry and this
  number represents well over 3,000,000 daily tests
  carried out in the United States.

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• Procedural Details The first steps in a
  photometric or spectrophotometric analysis
  involve the establishment of working conditions
  and the preparation of a calibration curve
  relating concentration to absorbance.
• Selection of Wavelength Spectrophotometric
  absorbance measurements are ordinarily made at a
  wavelength corresponding to an absorption peak
  because the change in absorbance per unit of
  concentration is greatest at this point the
  maximum sensitivity is thus realized.

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• Variables That Influence absorbance Common
  variables that influence the absorption spectrum
  of a substance include the nature of the solvent,
  the pH of the solution, the temperature,
  electrolyte concentrations, and the presence of
  interfering substances.
• Cleaning and Handling of Cells Accurate
  spectrophotometric analysis requires the use of
  good-quality, matched cells. These should be
  regularly calibrated against one another to
  detect differences that can arise from scratches,
  etching, and wear.

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• Determination of the Relationship Between
  Absorbance and Concentration It is necessary to
  prepare a calibration curve from a series of
  standard solutions that bracket the concentration
  range expected for the samples. Ideally,
  calibration standards should approximate the
  composition of the samples to be analyzed not
  only with respect to the analyte concentration
  but also with regard to the concentrations of the
  other species in the sample matrix in order to
  minimize the effects of various components of the
  sample on the measured absorbance.

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• The Standard Addition Method The standard
  addition method involves adding one or more
  increments of a standard solution to sample
  aliquots of the same size. Each solution is then
  diluted to a fixed volume before measuring its
  absorbance. It is possible to perform a standard
  addition analysis using only two increments of
  sample. Here, a single addition of Vs mL of
  standard would be added to one of the two
  samples. This approach is based upon the equation

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