18 PROBLEMS IN PHOTOSYNTHESIS 



H:6« and H:6:0« 



The excited state of a molecule after irradiation corresponds to the state 

 of a free radical. The irradiation of ethylene involves the formation of a free 

 biradical 



H H H H 



C::C J^ .C:C. 

 H H H H 



A free biradical is always produced when a double bond between two C atoms 

 is changed into a single bond 



C::C > •C:C« 



In § 9 we shall see that in the oxidation mechanism of nicotinic amide — 

 the active group of DPN+ and TPN+ — a monoradical is formed when the 

 double bond C=N+ is changed into the single bond C — N 



C::N+ > .C:N: 



The tendency to form electron pairs must be ascribed to the magnetic 

 properties of the electrons. The electron possesses a spin denoted by +V2 

 or — i/o- A spinning charged body must have a magnetic momentum and 

 the magnetic momentum of the electron is called the magneton which 

 equals eh/Awmc = 9.21 X 10-'" gauss-cm^ (e = elementary charge, m = 

 electron mass). According to Pauli's principle, two electrons in the same 

 orbit of an atom must have anti-parallel spin so that their magnetic momenta 

 neutralize each other. In the case of an electron pair there can thus be no 

 resulting spin as + V'2 — V2 = and there cannot be an excess of magnetons 

 as + 1 - 1 = 0. 



Conditions are different in the case of free radicals. The unpaired electron 

 of the free monoradical has a magnetic momentum which is not compensated 

 by the anti-parallel spin of a second electron. The resulting spin is then 

 V2 and the magneton excess is 1 . In the free biradical we find two unpaired 

 electrons having parallel spin so that the resulting spin is 1/2 + \ 2 = 1 

 and the magneton excess has the value 2. Owing to their magneton excess, 

 the free radicals behave as paramagnetic substances. When brought 

 into a strong magnetic field, the axis of the magnetic momenta in excess will 

 be orientated in the line joining the poles. As in the electron pairs the 

 magnetic momenta neutralize each other, chemical compounds are generally 

 diamagnetic. When brought into a magnetic field, diamagnetic substances 

 obtain magnetic momenta the axis of which will be orientated at right 

 angles to the line joining the poles. 



In contrast to other gases, O2 behaves like a paramagnetic substance. It 

 is therefore assumed that the molecule contains unpaired electrons 



