A Ultraviolet and Visible Molecular Absorption Spectroscopy

Molecular spectroscopy based on ultraviolet, visible, and infrared radiation is widely used for the identification and detennination of many inorganic, organic, and biochemical species. Molecular ultraviolet/visible absorption spectroscopy is used primcirily for quantitative analysis and is probably more extensively applied in chemical and clinical laboratories throughout the world than any other single method. Infrared absorption spectroscopy is a poweiful too for determining the structure of both inorganic and organic compounds. In addition, it now plays an important role in quantitative analysis, particularly in the area of environmental pollution. 

In this section, we first consider types of molecular species that absorb ultraviolet and visible radiation. We then describe qualitative and quantitative applications of 

As noted in Section 24C-2, absorption of ultraviolet and visible radiation by molecules generally occurs in one or more electronic absorption bands, each of which is made up of many closely packed but discrete lines. Each line arises from the transition of an electron from the ground state to one of the many vibrational and rotational energy states associated with each excited electronic energy state. Because there are so many of these vibrational and rotational states and because their energies differ only slightly, the number of lines contained in the typical band is quite large and their displacement from one very small. 

Absorption of radiation by organic molecules in the wavelength region between 180 and 780 nm results from interactions between photons and electrons that either participate directly in bond formation (and are thus associated with more than one atom) or are localized about such atoms as oxygen, sulfur, nitrogen, and the halogens. 

A band consists of a large number of closely spaced vibrational and rotational lines. The energies associated with the lines differ little from one another. 

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