Grotthuss-Draper law

The Grotthuss-Draper law states that only light which is absorbed by a chemical entity can bring about photochemical change. 

COMRUTER ALGORITHMS SOFTWARE GROTTHUSS CHAINS GROTTHUSS-DRAPER LAW GROTTHUSS MECHANISM... 

We can digress at this point to consider two laws of photochemistry thal feature in most introductions to the subject. First, the Grotthuss-Draper law states, in essence, that only light absorbed by a molecule can be effective in bringing about chemical change. This may seem obvious when considered at the molecular level—if the photon energy is not made available to the molecule by absorption, an electronically excited state cannot be produced, and no photochemical change can result. The law s importance is in its practical... 

Only light that is absorbed can produce a chemical change, a principle embodied in the Grotthuss-Draper law of photochemistry. This is true whether radiant energy is converted to some other form and then stored or is used as a trigger. Another important principle of photochemistry is the Stark-Einstein law, which specifies that each absorbed photon activates only one molecule. Einstein further postulated that all of the... 

The Stark-Einstein law of photochemical equivalence is in a sense simply a quantum-mechanical statement of the Grotthuss-Draper law. The Stark-Einstein law (1905) is another example of the break with classical physics. It states that each molecule which takes part in the photochemical reaction absorbs one quantum of the light which induces the reaction that is, one molecule absorbs the entire quantum the energy of the light beam is not spread continuously over a number of molecules. 

When a molecule or ion absorbs a photon, that photon s energy can be dissipated in several different ways, but one way is for that energy to cause a chemical reaction to occur. The first law of photochemistry is that a compound must absorb light for a photochemical reaction to occur (Grotthuss-Draper law). The second law of photochemistry is that each photon that is absorbed activates only one molecule for a subsequent reaction (Stark-Einstein law). The quantum yield ( ) is defined as the number of molecules that react divided by the number of photons absorbed. It can also be defined in terms of moles. 

The photochemical reaction of a material starts with photon absorption. In other words, only the photons absorbed by the molecule can bring about photochemical reactions. This is the first law of photochemistry, also called the Grotthuss-Draper law. The second law of photochemistry is one molecule is activated when one photon is absorbed. This is called the Stark-Einstein photochemical equivalence law. Generally, a particular group in an irradiated molecule absorbs a photon with an appropriate wavelength. When photoabsorption occurs, the molecule in the ground state is... 

The above statement was referred to as the Draper law or, when the Grotthuss paper was rediscovered, the Grotthuss-Draper law, generally stated in a qualitative, rather than quantitative, way. Notice that the quantitative aspect, indeed quite important in both of the original proposals by the two scientists, was rather referred to as the Bunsen-Roscoe law. This was stated as in two photochemical reactions, if the product of the intensity of illumination and the time of exposure are equal, the quantities of chemical material undergoing change will be equal [17]. ... 

Plotnikov later took pain in demonstrating [19] that taking the Grotthuss-Draper law in a quantitative sense, that is, that not only light has to be adsorbed, but that the effect is proportional to the radiation absorbed, could replace the Stark-Einstein law, but he obviously missed the point. The equivalence law states much more precisely that absorption of one quantum of light causes one photochemical act. Whether this applies in every single case or, more correctly, which is the mean probability that this statement applies to one type of acts or to another one, and under which conditions it can be affirmed that this applies to a chemical reaction, is another question (see below). That the effect is proportional to the absorbed flux— provided that the terms are properly defined—is obvious. 

These empirical adjustments were not xmiversally accepted, and as a matter of fact, a generally accepted theory was surely not yet available. Plotnikov, who certainly did not fail to manifest his idea, thought that [19] Einstein s law was in a sense simply a natural consequence of the Grotthuss-Draper law requiring that light was absorbed if it had to cause a chemical effect and in another sense wrong and generating confusion. To him it was inacceptable that photochemical processes were likewise subject to this purely photo-physical law, which is obviously nonsense, because it means the forcible elimination of the chemical principle from photochemistry, a statement that reveals how far many chemists of this time were from the concepts of the atomic structure introduced by quantum physics. 

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