Electromagnetic radiation, photochemistry

From comparison of the data presented in Table 2.2 [8], it is obvious that the energy of the microwave photon at a frequency of 2.45 GHz (0.0016 eV) is too low to cleave molecular bonds and is also lower than Brownian motion. It is therefore clear that microwaves cannot induce chemical reactions by direct absorption of electromagnetic energy, as opposed to ultraviolet and visible radiation (photochemistry). 

There have also been attempts to affect photochemical reactions by use of other sources of nonclassical activation, for example ultrasound [20, 21], Combined chemical activation by use of two different types of electromagnetic radiation, microwave (MW) and ultraviolet (UV)/visible (VIS), is covered by the field broadly called micro-wave photochemistry. The energy of MW radiation is considerably lower than that of... 

There are several reasons for starting this account with a discussion of electromagnetic radiation. Historically, it was in this area that the quantum theory first developed. It is easier here to understand the evidence for the theory, and to appreciate some of its paradoxical consequences, than it is in the quantum theory of matter. The applications of the light-quantum hypothesis, as it was first called, also provide key pieces of evidence for the quantization of energy in atoms and molecules. Studies of the absorption and emission of radiation—the field of spectroscopy—and of the effect of light on chemical reactions—photochemistry—are very important areas of modem chemistry, in which the quantum nature of radiation is crucial. 

A true unimolecular reaction is induced by electromagnetic radiation. That is, only one molecule takes part in the reaction and the energy is provided by the electromagnetic field. In fact, chemical reactions induced by electromagnetic radiation form such an important subfield of chemistry that it has its own designation photochemistry. 

A second important law of photochemistry, known as the Stark-Einstein Law, follows directly from the particle behaviour of electromagnetic radiations. 

Although the first law of photochemistry had already been recognized by Grot-thus in 1817 and Draper in 1843, the quantum nature of UV/VIS radiation and their consequences for photochemistry were established only decades later by Planck and Einstein (see Tab. 2-2). Nowadays, the observation by Grotthus and Draper that only electromagnetic radiation that is absorbed by a molecule leads to a... 

Dark photochemistry (photochemistry without light) Chemical reactions involving electronically excited molecular entities which are generated thermally rather than by absorption of electromagnetic radiation. The use of this term is discouraged. 

Spectroscopy The interaction of electromagnetic radiation with materials including scattering, absorption, and emission. It does not include chemical effects such as bond formation or free radical formation. It does include some aspects of photochemistry, which is a specialized form of energy transduction. See Campbell, I.D. and Dwek, R.A., Biological Spectroscopy, Benjamin Cummings, Menlo Park, CA, 1984 Stuart, B., Infrared Spectroscopy, John Wiley Sons, Chichester, UK, 2004. 

Activation for chemical transformations can be accomplished by different methods, traditionally by heating the reaction vessel with an oil bath. Other methods are used for shorter periods, e.g. heating by microwave irradiation or ultrasound treatment. Another possibility is irradiation by electromagnetic radiation in the visible and UV region of the spectrum, which is referred to as photochemistry. This kind of activation is used in nature for building up carbohydrates by photosynthesis. 

Photochemistry encompasses the study of chemical changes generated by the absorption of electromagnetic radiation by molecules. Energies of photons vary... 

Energies of electromagnetic radiation relevant to photochemistry in different units... 

Spectroscopic methods are used in the structural characterization of biomolecules (Bell, 1981 Campbell and Dwek, 1984 Greve et al, 1999 Hammes, 2(X)5). These methods are usually rapid and noninvasive, require small amount of samples, and can be adapted for analytical purposes. Spectroscopy is defined as the study of the interaction of electromagnetic radiation with matter, excluding chemical effects (photochemistry refers to the interaction with chemical effects). The electromagnetic spectrum covers a wide range of wavelengths (Figure 7.1). 

Photochemistry is inherently related to photophysics, the study of those radiative and nonradiative processes that convert one electronic state into another electronic state without chemical change. Central to both photochemistry and photophysics is the classification of UV-vis radiation in terms of its energy. Because electromagnetic radiation is quantized, it has properties like those of a particle, and a mole of photons is called an einstein. Electromagnetic radiation also has the properties of a wave, and equation 12.1 gives the relationship between the energy of UV-vis radiation in kcal/mol and its wavelength in run. ... 

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