MAGNETOCHEMISTRY AND BOND TYPE 21 



to hold — one light quantum splitting one molecule of carbon monox- 

 ide from carbon monoxide pyridine hemochrome, though not from 

 carbon monoxide hemoglobin (S7J^). 



It may be noted that the light sensitivity of a carbon monoxide 

 compound is inversely proportional to its dissociation velocity con- 

 stant in the dark {cf. equation 5 above). Since this constant increases 

 with temperature, while 2, the velocity constant of a photochemical 

 reaction, remains unaltered, low temperatures are ideal for the meas- 

 urement. There is, however, a limit since the rate of respiration 

 decreases with temperature and becomes too small for convenient 

 measurement. 



An absorption spectrum determined by this means is known as a 

 "photochemical absorption spectrum." The method can obviously 

 be used only when the substance under consideration is a respiratory 

 catalyst which is inhibited by combination with some suitable, 

 readily measurable substance such as carbon monoxide, the resultant 

 compound being dissociated by light. 



6. MAGNETOCHEMISTRY AND BOND TYPE 



The determination of the type of bonds by which the iron or other 

 metal atom is linked to the pyrrolic nitrogen atoms of the porphyrin 

 and to other attached molecules is of great importance, and is fre- 

 quently solved by magnetochemical measurements. A brief resume 

 follows of those features of atomic structure and electronic valency 

 theory which are of special concern for the hematin compounds. For 

 more detailed treatment the reader is referred to standard texts and 

 reviews (622, 2125, 2529). 



6.1. Electronic Basis of Bond Formation 



The distribution of electrons in the outer orbitals of atoms of the 

 iron group which are concerned in metalloporphyrin complex forma- 

 tion, is shown in Table II. 



Pauli's Exclusion Principle requires that no more than two elec- 

 trons can occupy a single orbital, and that when two are present the 

 directions of spin must be opposed. It is not, however, necessary for 

 all available orbitals to be filled, or even occupied at all. In an atom 

 or monatomic ion, the electrons tend to occupy first the more stable 

 orbitals, two electrons of opposed spin entering each orbital. When 



