The Electronic Structure of Haemoglobin 



we assume that it is possible for the actual state of the system to be a 

 hybrid of two or more Russell-Saunders states of the atom, with the 

 same J value, and presumably the same value of the spin quantum 

 number, S. 



For a complex such as that in the ferrihaemoglobin cyanide molecule 

 the odd electron might be expected to occupy one of the 3d orbitals, 

 the other eight outer orbitals of the iron atom being used in the forma- 

 tion of covalent bonds or occupied by electron pairs. The normal 

 spectroscopic state would then be 2 D 5!2y with magnetic moment 3-55. 

 The maximum value of J would be expected, according to Hund's 

 rule, because the electronic configuration corresponds to the second 

 half of a partially filled 3d subshell. If, however, the odd electron were 

 promoted to a 4/> orbital, the 3d orbital being made available for use 

 in bond formation, the spectroscopic state would be 2 P 3!2 , with 

 magnetic moment 2-58 Bohr magnetons, and hybridization would be 

 expected between this state and the state 3d 2 D 3j2 . The magnetic 

 moment for 2 D 3j2 is 1-55. (It seems likely that the s orbital would be 

 utilized entirely for bond orbitals ; if, however, the odd electron were 

 to occupy the 4s orbital, the spectroscopic state would be 2 S lj2 , with 

 moment 1-732, which might hybridize with 2 P 1!2 and 2 D V2 to produce 

 states with smaller moments.) The two reasonable alternatives thus 

 are 2 D 5t2 , with moment 3-55, and a hybrid of 2 P 3j2 and 2 D 3[2 , with 

 moment between 2-58 and 1-55, depending upon the relative amounts 

 of p and d character of the orbital occupied by the odd electron. We 

 see that the state with J = 5/2 is ruled out by the lack of agreement of 

 the predicted moment and the observed moment for the ferrihaemo- 

 globin compounds, and it seems reasonable to assume accordingly 

 that ferrihaemoglobin cyanide and similar compounds are in states 

 with total angular momentum quantum number / = 3/2. The reported 

 magnetic moments of ferrihaemoglobin cyanide and ferrihaemoglobin 

 hydrosulphide lie within the predicted range, and indicate that the odd 

 electron is mainly in the Ap orbital. The value for ferrihaemoglobin 

 azide is high, although possibly the discrepancy is within experimental 

 error. That for the ferrihaemoglobin hydroxide-ammonia complex 

 involves an extrapolation, and hence may be unreliable. 



It is interesting to mention that the moment of the ferricyanide ion, 

 which has been carefully determined, is 2-33, which corresponds to a 

 resonating structure v/ith 24 per cent d character and 76 per cent p 

 character for the odd electron. 



The moment for ferrihaemoglobin itself is 5-44 magnetons. 13 In this 

 structure there are six outer electrons. If they all occupied 3d orbitals, 

 the configuration being d 6 , the allowed spectroscopic state would be 

 5 Z>, presumably with J = 4, leading to moment 6-72. If one odd 



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