Chapter 14 - Chapter 14 APPLICATION OF ULTRAVIOLET/VISIBLE...

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Chapter 14 APPLICATION 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 10 5 are observed. The magnitude of ε depends upon the probability for an energy-absorbing transition to occur. Peaks having molar absorptivities less than about 10 3 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 ν 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) π , σ , 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 ( σ ) orbitals, and the corresponding electrons are σ electrons.
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Types of Absorbing Electrons The double bond in a molecule contains two types of molecular orbitals: a sigma ( σ ) orbital and a pi ( π ) molecular orbital. Pi orbitals are formed by the parallel overlap of atomic p orbitals. In addition to σ and π 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 π and σ orbitals. Electronic transitions among certain of the energy levels can be brought about by the absorption of radiation. Four types of transitions are possible: σ σ *, n σ *, n π *, and π π *.
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This note was uploaded on 05/06/2010 for the course CHEM 4414 at Arkansas Tech.

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Chapter 14 - Chapter 14 APPLICATION OF ULTRAVIOLET/VISIBLE...

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