Early Attempts to Rationalize Photochemical Reactions

This point of view was strongly developed by Bancroft, who thought probable that two of the laws of photochemistry, both of them he attributed to Grotthuss, were  

At the beginning of the twentieth century, Bancroft reviewed essentially all of the photochemistry known at the time and found that in every case, Grotthuss theory was useful for the rationalization of the results [24—32]. 

As an example, Plotnikov thought that water was photochemically unsaturated—and actually photoreactive—because oxygen was bonded to two atoms of hydrogen only, leaving four electrons free for interactions, while the photostability of methane (in the absence of molecules that were themselves photounstable, such as chlorine or oxygen) was related to the fact that all of the electrons were involved in bonds. On the basis of this reasoning he expected that linear alkanes as well as non-tensioned cycloalkanes would not react but doubted that this would apply to tensioned compounds, such as cyclopropane. 

More down to earth, Plotnikov examined the effect that different atoms had on the spectra and noticed that metal iodides absorbed at longer wavelength than bromides and even more of chlorides, again an indication of the periodicity of photochemical properties, and likewise X increased with the atomic weight of the cation. 

Summing up, attempts to correlate stmcture and photoreactivity, as indeed reactivity in general, were repeatedly done, and the feeling that this had something to do with electron removal/shift was diffuse, but a real step forward woirld have been possible only when physical theories would have been not only formulated but also assimilated by chemists. 

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