Proton tautomerism in the solid state

Even though rapid tautomerization is observed in an isolated system, it is not necessarily the case in the solid state, in which the symmetrical proton potential is sometimes deformed by an environmental effect. Therefore, the 

Naphthazarin [13] is known to afford polymorphic crystal structures (Cradrick and Hall, 1971). Among them, in the polymorphic form C, both the C=0 and 

Tautomerization of hetero-analogues of tropolone, such as 2-amino-troponimines [14] (Goldstein and Trueblood, 1967 Hexem et aL, 1982) and [15] (Machiguchi et aL, 1992) has been also investigated in the crystalline state. 

As seen from the above examples, intermolecular dipolar interactions and intermolecular hydrogen bonds are the major factors in the freezing of tautomeric exchange in the crystalline state. 

The deuterium isotope effect on the rate constant of the double-proton transfer in the solid state can be used as a criterion to judge whether the double-proton transfer proceeds in a simultaneous or in a stepwise manner. The difference between ratios of rate constants /crh/ hd and hd/ dd depends on the mechanism of the double-proton transfer. If a double-proton transfer occurs in a stepwise manner, the slower step in the /chd process, that is, a deuteron transfer, becomes rate-determining. Consequently the ratio /thh/ hd should be large. On the other hand, since deuteron transfers are ratedetermining in both / hd and / dd processes, the ratio / hd/ dd is expected to be small. By contrast, if a proton transfer and a deuteron transfer occur simultaneously, the ratios of / hh/ dd and /thd/ dd should be of the same order. The observed ratios for benzoic acid dimers in crystals were found to be /thh/ hd = 23, /thd/ dd = 10 at 15 K (Stockli et aL, 1990). Accordingly, the double-proton transfer is considered to occur concertedly in benzoic acid dimers [16]. 

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Before describing the physical properties derived from proton tautomerism in the solid state, the intrinsic nature of proton tautomerism in isolated systems, such as in the gas phase or in a dilute solution, is discussed. 

Proton tautomerism in the solid state has drawn attention from the aspect of application as well. Further investigations in related areas, such as photochromism, thermochromism, photochemical hole-burning and hydrogen-bonded dielectrics, may open a horizon of protonic molecular devices. 

The aim of this review is to describe the essence of proton tautomerism, focusing on the potential profile of proton transfer along the reaction coordinate of the tautomerization in the solid state. In Section 2, proton tautomerism in an isolated system is discussed based on spectroscopic... 

As mentioned earlier, only prototropic tautomerism is expected to occur in the solid state. Furthermore, as the motion of molecules is restricted to vibration about their equilibrium positions, the transfer of atoms from one part of a molecule to another must occur via a direct contact, that is, with the donor and the acceptor of the transferred atoms in immediate proximity. This is most easily satisfied when the atom that is to be transferred is a hydrogen atom, and the atoms donor and the acceptor are the donor and acceptor of an intra- or intermolecular hydrogen bond. Therefore, in the solid state tautomerization is equivalent to proton transfer in the solid state. 

For )V-unsubstituted pyrazoles the tautomeric proton was generally located without ambiguity. 3-Substituted tautomers were favoured in the solid state (45), (46) and (48) (Table 5). For the pyrazolyltriazole (47) the authors (77JHC65) concluded that the X-ray analysis indicates that the proton on the pyrazole ring populates either nitrogen atom to... 

The first observation of the proton transfer in pyrazoles in the solid state was made for the intermolecular tautomerism in 3,5-dimethylpyrazole 10b (85JA5290). The degenerate rearrangement was recorded using the... 

Tliere are several reasons for this great interest in the tautomerism of porphyrins (which could justify its own review) (1) their biological significance, (2) their applications in material science ( hole burning is related to their tautomerism), (3) the simplicity of the system (annular tautomerism involving intramolecular proton transfer both in solution and in the solid state), and (4) the possibility of elucidating the kinetic processes in great detail. 

Fig. 1. Combined proton tautomerism and 90° rotation of porphyrin molecule in the solid state, (a) Reduced scheme proposed by Frydman et al. (89JA7001). (b) Complete scheme according to Limbach et al. (94JA6593).

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