Principles of Molecular Spectroscopy Electromagnetic Radiation

Electromagnetic radiation, of which visible light is but one example, has the properties of both particles and waves. The particles are called photons, and each possesses an amount of energy referred to as a quantum. In 1900, the German physicist Max Planck proposed that the energy of a photon (E) is directly proportional to its frequency (v). 

The SI units of frequency are reciprocal seconds (s ), given the name hertz and the symbol Hz in honor of the nineteenth-century physicist Heinrich R. Hertz. The constant of proportionality h is called Planck s constant and has the value 

Electromagnetic radiation travels at the speed of light (c = 3.0 X 10 m/s), which is equal to the product of its frequency v and its wavelength X  

FIGURE 13.1 The electromagnetic spectrum. (From M. Silberberg, Chemistry, 2d edition, WCB/McGraw-Hill, 2000, p. 260.) 

As diverse as these techniques are, all of them are based on the absorption of energy by a molecule, and all measure how a molecule responds to that absorption. In describing these techniques our emphasis will be on their application to structure determination. We ll start with a brief discussion of electromagnetic radiation, which is the source of the energy that a molecule absorbs in NMR, IR, and UV-VIS spectroscopy. Mass spectrometry is unique in that, instead of electromagnetic radiation, its energy source is a stream of charged particles such as electrons. 

The electromagnetic spectrum. (Reprinted, with permission, from M. Silberberg, Chemistry, 6th ed., McGraw-Hill Higher Education, 2009, p. 271.) 

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In this chapter, we discuss the basic principles that are necessary to understand measurements made with electromagnetic radiation, particularly those deeding with the absorption of UV, visible, and IR radiation. The nature of electromagnetic radiation and its interactions with matter are stressed. The next four chapters are devoted to. spectroscopic instruments (Chapter 25), molecular absorption spectroscopy (Chapter 26), molecular fluorescence spectroscopy (Chapter 27), and atomic spectroscopy (Chapter 28). 

Raman scatter, and excitation emission fluorescence spectroscopy (EEFS). They use interaction with radiation from different regions of the electromagnetic spectrum to identify the chemical nature of molecules. For example, absorption of UV and VIS radiation causes valence electron transitions in molecules which can be used to measure species down to parts per million concentrations for fluorophores (i.e., EEFS) determination can even go down to parts per billion levels. Whereas UV, VIS, and EEFS are limited to a smaller, select group of molecules, the NIR, IR, and Raman scatter spectroscopy techniques are probing molecular vibrations present in almost any species their quantification limits are somewhat higher but can still be impressive. The reader is referred to textbooks for further details on basic principles of these spectroscopic techniques [3]. 

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