134 P. George, J. Beetlestone and J. S. Griffith 



Although this is far from direct evidence for the existence of the hydroxide 

 structure it is at least thoroughly consistent with it. 



The hypothesis of a thermal mixture is also fully borne out in the case of 

 ferrimyoglobin hydroxide by the temperature variation of its spectrum and 

 magnetic moment as described in Section V; and, in view of the self-consistent 

 results of the calculations using magnetic and spectroscopic data in Section 

 IV, it can be concluded that ferrihaemoglobin hydroxide is also a mixture of 

 high- and low-spin forms. This accounts equally well for its apparently 

 anomalous magnetic moment as the explanation in terms of the electronic 

 configuration with three unpaired electrons, which was shown to be unlikely 

 on theoretical grounds (see Section II). 



Thermodynamic data for the conversion of the high-spin to the low-spin 

 form can be obtained from the values of K^ for ferrimyoglobin in Table 5. 

 A plot of log Kg against \jT gives A// = — 1-5 ± 0-2 kcal/mole, and from 

 the equation AG° = A// - T^S°, with ^G° equal to 0-5 kcal/mole at 25°C, 

 AS" is found to be — 6-7 ±0-7 e.u. The conversion is thus favoured by the 

 enthalpy change, but is appreciably hindered by the entropy change to such 

 an extent that the resulting free energy change has a small positive value. In 

 other words, with respect to their heats of formation the low-spin form is the 

 more stable, whereas in terms of their entropies the high-spin form is the 

 more stable. 



The favourable value of A// may be regarded as purely fortuitous, because, 

 although the conversion to the low-spin form implies an increase in the 

 value of A, and hence extra stabilization, pairing energies have also to be 

 taken into consideration and in addition solvent interaction effects may be 

 important (see below). In order to discuss the entropy change accompanying 

 the conversion, it is convenient to distinguish the contribution arising from 

 the degeneracy of the electronic state of the iron in the two forms from the 

 remainder. The following estimate shows that this contribution is unlikely 

 to be more than about — 2 e.u. 



In the high-spin form the ferric ion has a ground term which is spatially 

 non-degenerate but has a sixfold degeneracy due to the spin 5* = 5/2. The 

 ligand field combined with the spin-orbit coupling lifts the degeneracy into 

 three Kramers doublets. If this splitting is large compared to kT, only one 

 Kramers doublet is occupied and the effective degeneracy of the ferric ion is 

 2. If it is small then the degeneracy is 6. This means that the entropy associ- 

 ated with the degeneracy of the electronic state of the iron in the high-spin 

 form lies between the two limits of i? log^ 2 = 1-38 e.u. and R log^ 6 = 3.56 e.u. 

 The actual magnitude of the splitting is unknown. If we assume that it may 

 be represented in a spin-Hamiltonian for the ground term with S = 5/2 by 

 the quadratic expression 



