CHLOROPHYLL-SENSITIZED REACTIONS ITl VltrO 27 



electron. The hydrogen ion, i.e., a bare proton, can, however, hydrate to forma 

 hydronium ion. This hydration liberates about 270 kcal. Roughly, one can re- 

 move an electron from a hydrogen atom and make a (hydrated) hydrogen ion 

 with 30 to 40 kcal. only if one hydrates the proton. Similar considerations ex- 

 plain why the heat of ionization of water is about 13 kcal.: both the H+ and 

 the 0H~ are hydrated — an exothermic process. Dr. Franck intended to point out 

 that there are no reactions known of the type: 



(1) dye + h^ > dye* 



(2) dye* + H^O > dye + H+ + OH- 



where hv is 30 to 60 kcal. He also wished to mention the reactions: 



(3) (Fe ++)a<, + h. >(Fe + + +OH-)a<, + H 



(4) (Fe + + +)aq + h. >(Fe ++)a<, + H+ +0H 



The second of these does not proceed using visible radiation because there is not 

 time to hydrate the H-*- during the photochemical act and thus to gain the energy 

 of hydration. A quantum of visible light does not have sufficient energy to liber- 

 ate the bare, unhydrated proton. The Franck-Condon principle (conservation of 

 momentum) restricts movements of atomic nuclei during photochemical acts. 

 The energy requirement for the reaction liberating a free proton (H"^) is thus 

 very great. 



Bassham: The electron is not necessarily taken from a hydrogen atom. It may 

 be taken from water. 



Brugger : What difference does it make? You still have a bare proton sitting 

 around which you have to hydrate. 



Bassham : Yes, but we can take several electrons at the same time from water, 

 or we can get away from the radical in between. And, since we have an aggre- 

 gated chlorophyll system where we can have a high population of exited chloro- 

 phylls at the same time, it is conceivable that we don't have to form any radicals 

 in between. 



Brugger: I don't mean to dodge the issue but I think it would take too long to 

 go into it. Dr. Franck's theory was specifically designed to provide a mechanism 

 whereby hydrogen atoms could be transferred. 



Rabinowitch: Whenever you transfer electrons, you immediately create an 

 uncomfortable position which requires a subsequent adjustment of the nuclei to 

 fit the new distribution of charges and you lose some of the energy. In other 

 words, electron transfer requires a high activation energy. It is better to trans- 

 fer hydrogen atoms with only the energy needed for the transfer and without 

 extra energy of activation, whther it is one or two atoms at a time. 



Brugger: I can make one more point w^hile I think of it. Often the electron 

 transfer reactions involve two electrons at a time. The energy is calculated per 

 electron by just dividing the energy by two. One has absolutely no guarantee 

 that the energy required to transfer the first electron is half the sum required to 

 transfer both. So if you can just squeeze by theoretically in transferring one 

 electron at half the energy of the two-electron process, you have no guarantee 

 that this actuallv works. 



