CARBON DIOXIDE REDUCTION IN ULTRAVIOLET LIGHT 



81 



hydrogen atoms are transferred one by one, have the full energy of 

 their unsaturated bonds; while the corresponding radicals derived from 

 large organic molecules can often be stabilized by resonance, and there- 

 fore present much less of a barrier to a reversible reduction. We will 

 revert to this function of free radicals in chapter 9 (page 233). At 

 this point, we must state that we do not know of any reaction of carbon 

 dioxide or of the carboxyl group in vitro, which could be called a reversible 

 (or almost reversible) reduction of the C=0 double bond to a CH — OH 

 single bond, and that a closer inquiry into the possibilities of such a 

 reduction would be important for the study of artificial photosynthesis. 



2. Decomposition and Reduction of Carbon Dioxide 

 in Ultraviolet Light 



In describing the photochemical oxidation of water, we started w^ith 

 the direct effects of ultraviolet light; similarl}^, we begin now with the 

 nonsensitized photochemical decom- 

 position of carbon dioxide by ultra- 

 violet light. 



The spectrum of the molecule CO2 con- 

 sists of discrete bands, from 200 m/x to 103 m/u; 

 thus, the primary process is electronic exci- 

 tation rather than photochemical decompo- 

 sition. Figure 7 shows the extinction curves 

 of the ions, CO3 and HCOs" in water. In 

 this case, the primary process probably is an 

 electron transfer from the ion to water: 



hi- 

 (4.23) 



3.0 



2.5 



2.0 



1.5 



ctI.O 

 o 



HCOr-HzO 



->nco3H20- 



0.5 



-0 5 



that is, an oxidation of the carbonate and 

 reduction of water. This is hardly an appro- 

 priate initial step towards the reduction of the 

 carbonate and oxidation of water. 



The effect of ultraviolet light on 

 carbon dioxide was first observed 

 by Chapman, Chadwick and Rams- 

 bottom (1907) and Herschfinkel (1909). 

 They found that carbon dioxide gas 

 decomposes in light with an increase 

 in pressure (i. e., probably into car- 

 bon monoxide and oxygen), until a 

 stationary state is reached. Berthelot and Gaudechon (1910) found that 

 ultraviolet light accelerates both the dissociation of carbon dioxide and 



■1.0 



160 



180 



240 



260 



200 220 



Fig. 7. — Molar extinction curves 

 of CO3 — and HCOa" in water (after 

 Ley and Arends) . 



