162 



PRIMARY PHOTOCHEMICAL PROCESS 



CHAP. 7 



native (Y as a photocatalyst), since it removes both carbon dioxide 

 and water from the direct participation in the primary photochemical 

 process, and can thus be considered as the least specific of all. The 

 resulting system of reactions, (7.11), represented in scheme 7.V, is a 

 logical generalization of system (7.10) and scheme 7. IV. 



(7.11a) 



(7.11b) 

 (7.11c) 

 (7.11d) 



(7.11) 



4Y + 4HZ 

 4 HY + 4 X 



4 A;- 



-> 4 HY + 4 Z 



'ihp 



> 4 HX + 4 Y 



4 {CO2I + 4 HX > iCHjO) + 3 CO2 + H2O + 4 X 



4Z + 4 {H2OJ 



-^ 4 HZ + O2 + 2 H2O 



4{C02} +4 IH2O} 



8hy 



-^02+ {CH2OJ + 3 CO2 + 3 H2O 



Another scheme of the same type was suggested by Franck and 

 Herzfeld (1941) to replace the older "four quanta theories" of Franck 



4CO2 



4X 



^{cOa} 



4Y 



r 



4HX 

 _J 



I 



r 



4-HY 



I 



4HZ 



-I 



Ahv\(l.\\a) 



4Z 



L 



4{h20) 



4hv|(7.ll b) 



4Y 



(7.|ld) 



4HZ 1- {Oj} + SHjO 



(7.11c) 



4{hC02} 



i 

 4X 



(7. lid) 



{CHjOJ + HzO+aCOa 



Scheme 7.V. — Photosynthesis, with oxidation-reduction reactions between three 

 intermediary catalysts as the two primary photochemical processes. (The central 

 catalyst, which participates in both photochemical reactions, may be chlorophyll.) 

 (First eight quanta theory.) 



(1935) and Franck and Herzfeld (1937). In this scheme, the "photo- 

 catalyst" was identified with X in scheme 7.1, that is, it was assumed to 

 react directly with the carbon dioxide-acceptor complex in the "photo- 

 reduction," and to be restored by an intermediate hydrogen donor in the 

 "photoxidation." Two additional specific assumptions were made by 

 Franck and Herzfeld. In the first place, they assumed that the reduced 

 photocatalyst, HX, hydrogenates not only the complex, {CO2}, but also 

 its three reduction intermediates, {HCO2}, {H2CO2}, and {H3CO2} — 

 in other words, they assumed four different photoreduction processes 

 (c/. 7.7a, b, c, and d), and combined them with four identical primary 



