158 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



If water does not participate directly in the primary process, the photo- 

 chemical reduction of carbon dioxide must occur at the cost of an inter- 

 mediate hydrogen donor (designated by HX in Scheme 7.1). 



It has often been assumed that the reduction of carbon dioxide must 

 of necessity involve several successive photochemical steps, e. g. : 



(7.7a) {CO2I + HX > {HCO2} + X 



(7.7b) {HCO2) + HX "—^ {H2CO2} + X 



(7.7c) {H2CO2I + HX "-^ {H3CO2) + X 



(7.7d) {H3CO2} + HX "—^ {H4CO2! + X > {CH2OI + H2O + X 



(7.7) {C02J+4HX ^ {CH2OI +H2O + 4X 



The first and third stage lead to "odd" molecules (that is, free 

 radicals), while the second one produces an intermediate of the reduction 

 level of formic acid. 



In equations (7.7) we postulated four differ ejit 'primary 'photochemical 

 reactions, and this may be considered as a setback compared with van 

 Niel's theory. However, the assumption of a single primary process is 

 possible in van Niel's theory only through combination of this primary 

 process with the catalytic dismutation of the primary oxidation product, 

 {OH}, by reactions (7.4). A similar scheme, with a single primary 

 photochemical reaction, also can be substituted for (7.7), with the only 

 difference that, because of the participation of four hydrogen atoms in 

 the reduction of oifie molecule of carbon dioxide, two successive dismuta- 

 tions are required to complete the reaction. In the first one, two radicals, 

 {HCO2}, dismutate into two "even" molecules, {CO2) and {H2CO2I, 

 while in the second one, two molecules, (H2CO2}, dismute into {CO2} 

 and {H4CO2}: 



(7.8a) 4{C02l+4HX ^4|HC02l+4X 



catalysts 



(7.8b) 4 {HCO2I > 2 {CO2! + 2 {H2CO2} 



catalysts 



(7.8c) 2 {HaCOz) > {CO2} + {H4CO2} > {002} + {CH2O} + H2O 



(7.8) 4 {CO2} +4HX "-^ {CH2OI +H2O + 3 {CO2} +4X 



catalysts 



If (7.8a) is considered to be the only photochemical reaction in 

 photosynthesis, the process must be completed by a nonphotochemical 

 oxidation of water by the oxidized intermediate X, possibly involving 

 the intermediate catalysts Y and Z (c/. Scheme 7.1) : 



catalysts 



(7.8d) 4X + 2{H20} > 2 HX + O2 



