288 LIBERATION OF OXYGEN CHAP. 11 



none of the photochemical steps in reaction sequence (11.12) will require more energy 

 than is available in a quantum of red light. 



hp 



(11.12a) 0=C=0 + ROH > 0=C— O (R— performic acid) 



I i 

 R OH 



(11.12b) 0=C— O > ^02 + 0=C— OH (R— formic acid) 



R OH R 



hp 

 (11.12c) 0=C— OH + H2O > ROH H- 0=C— OH (formic acid) 



R H 



R OH OH R 



i i '^^ I I 

 (11.12d) 0=C— OH + ROH > 0— C— OH > O C— OH 



H H H 



(R — per formaldehyde hydrate) 



OH R R 



(11.12e) O C— OH ^^02 + OH— C— OH (R— formaldehyde hydrate) 



H H 



R H 



I hy I 



(I1.12f) OH— C— OH + H2O > ROH + OH— C— OH 



1 I 



H H 



(formaldehyde hydrate) 



Of the four photochemical reactions in (11.12), two are intramolecular H — OH 

 exchanges, and two intermolecular H — R exchanges. Franck and Herzfeld thought 

 the assumption of a weak R — OH bond is sufficient to equalize the energies of photo- 

 chemical steps (11.12a), (11.12b), (11.12d), and (11.12f); however, if R is an organic 

 radical, the C — R bond in (11.12a) is a C — C bond, which is 20 kcal weaker than a 

 C — H bond. Thus, while according to table Q.H the substitution of ROH for HOH 

 could bring a gain of 32 kcal in the energy required for dissociation into R and OH, 

 it should at the same time bring a loss of 20 kcal in the energy gained by the addition 

 of R and OH to C=0. Consequently, step (11.12a) should require about 73 kcal and 

 step (11.12c) only 13 kcal, a far cry from the desired equaUzation. 



These estimates are brought here in order to illustrate the diflEiculties 

 which "four quanta theories" of photosynthesis must invariably en- 

 counter — particularly if an attempt is made to interpret all intermediates 

 as valence-saturated molecules (instead of resonance-stabilized radicals, 

 as suggested in chapter 9, page 230). 



Two other schemes of photosynthesis also were based on the formation of peroxides 

 by the reduction substrate. Baur (1937) suggested that the first step in photosynthesis 

 may be the formation of percarbonic acid: 



OOH 



light I 



(11.13a) H2CO3 + ^02 > 0=C— OH (percarbonic acid) 



