Chapter 26 - Chapter 26 Molecular Absorption Spectrometry...

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Chapter 26 Molecular Absorption Spectrometry Molecular spectroscopic methods are among the most widely used of all instrumental analytical methods. Molecular spectroscopy is used for the identification and determination of a huge number of inorganic, organic and biochemical species. Molecular ultraviolet/visible absorption spectroscopy is employed primarily for quantitative analysis. Infrared absorption spectroscopy is one of the most powerful tools for determining the structure of both inorganic and organic compounds.
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ULTRAVIOLET/VISIBLE MOLECULAR ABSORTION SPECTROSCOPY In the UV/visible region, many types of inorganic compounds absorb radiation directly. Others can be converted to absorbing species by means of a chemical reaction. Absorption measurements in the UV/visible region of the spectrum provide qualitative and quantitative information about organic, inorganic, and biochemical molecules.
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Absorption by Organic Compounds: Two types of electrons are responsible for the absorption of ultraviolet and visible radiation by organic molecules: (1) shared electrons that participate directly in bond formation and (2) unshared outer electrons that are largely localized on atoms such as oxygen, the halogens, sulfur, and nitrogen. The shared electrons in single bonds are so firmly held that absorption occurs only with photons more energetic than normal UV photons. Electrons involved in double and triple bonds of organic molecules are more loosely held and are therefore more easily excited than electrons in single bonds. Thus, species with unsaturated bonds generally absorb in the UV. Unsaturated organic functional groups that absorb in the UV/visible region are known as chromophores .
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Absorption by Inorganic species: In general, the ions and complexes of elements in the first two transition series absorb broad bands of visible radiation in at least one of their oxidation states and are, as a consequence, colored. Absorption involves transitions between filled and unfilled d- orbitals of the metal ion with energies that depend on the bonded ligands. The energy differences between these d-orbitals and thus the position of the corresponding absorption maximum depend on the position of the element in the periodic table, its oxidation state, and the nature of the ligand bonded to it.
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Charge-Transfer Absorption: For quantitative purposes, charge-transfer absorption is particularly important because molar absorptivities are unusually large, a circumstance that leads to high sensitivity. Many inorganic and organic complexes exhibit this type of absorption and are therefore
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Chapter 26 - Chapter 26 Molecular Absorption Spectrometry...

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