STRUCTURAL FORMULA 443 



of the molecule can be described, according to the quantum theory, as a 

 "mixture" of the two mesomeric forms, and the heavy lines in 16.III 

 represent equivalent bonds of an average strength of about 1.5. 



One could ask whether structures A, B, and C too, are mesomeric. 

 The answer depends on whether the magnesium is situated exactly in 

 the center of the molecule, and whether the equilibrium configurations 

 of all pyrrole nuclei are the same. If, in A and C, the equilibrium 

 position of the magnesium atom were closer to those two nitrogen atoms 

 to which it is bound by true valency bonds, this should prevent meso- 

 merism. However, from the probable size of the central "hole" and 

 the radius of magnesium in atomic binding (c/. page 448), there appears 

 not to be enough space for magnesium to be markedly displaced from 

 the central position; consequently the magnesium-nitrogen bonds prob- 

 ably can be switched from nitrogen to nitrogen without a change in the 

 position of the nuclei, and thus do not interfere with mesomerism. 



However, Fischer suggested that structure A is more stable than B 

 and C, because, in the latter, ring V is under strain. This implies that 

 the equilibrium configuration of the pyrrole nucleus with the semi- 

 isolated double bond is different from that of the other two nuclei. If 

 this difference is real, a mesomerism of structures A, B, and C becomes 

 impossible. 



The question of differences between the structures of the four pyrrole 

 nuclei in chlorophyll was debated between Haurowitz (1935, 1938) and 

 Fischer and Stern. Haurowitz argued in favor of a "biradical" formula 

 with a symmetrical all-round conjugated double-bond system (as in 

 16. VII B), and of all four pyrrole nuclei being in the same state. Since 

 we assumed that nucleus IV contains two extra hydrogen atoms, the 

 biradical formula is impossible, and the controversy reduces itself to the 

 question of identical or different structure of the three nonhydrogenated 

 nuclei, I, II, and III. 



Stern presented evidence that the introduction of certain substituents 

 (e g., carbonyl) has a different effect on the spectrum, depending on 

 whether they enter nucleus I, II, or III, and saw in this a proof of their 

 different double-bond structure (cf. however, Aronoff and Calvin 1943). 

 Although one may argue (with Haurowitz) that this difference arises 

 only after the introduction of the substituents, we are inclined to agree 

 with Fischer and Stern that the semi-isolated double bond is localized 

 in one of the three pyrrole nuclei. However, in trying to identify this 

 nucleus, we find that Stern's spectroscopic evidence favors nucleus II 

 (structure B), while Fischer's stability considerations point to nucleus 

 III (structure A). The spectroscopic data (discussed in more detail in 

 Chapter 21, Volume II) show a similarity between nuclei I and III, 

 as opposed to nucleus II, which is best expressed by formula 16. Ill B. 



