Solid state tautomerism

The solid-solid transformation of 2-amino-3-hydroxy-6-phenylazopyridine, 57a, to 57b proceeds through two intermediate phases (119). X-Ray and IR studies of the former, low-temperature, and the latter, high-temperature phase show that they are the phenolazo and quinone hydrazone forms, respectively. This solid-state tautomerism can be accounted for by a cooperative intermolecular shift of protons across the various hydrogen bonds. However, because of the complexity of the hydrogen-bond network, the actual pathway of the proton shift has not been uniquely defined. 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

The solid state tautomerism of solid phthalocyanine was discovered [66a] and studied by Limbach et al. [66b[. As illustrated in Fig. 6.22(b), there are two forms which differ in the arrangement of the central nitrogen atoms. They are arranged in a square in the a- form but in a rectangular way in the yS-form [66b]. Thus, the latter contains two weak inner NHN-hydrogen bonds. 

Figure 6.32 Arrhenius curves of the solid state tautomerism of benzoic acid dimers. Data for the HH and HD reactions taken from Ref [74], and for the DD reaction from Ref [72cj. The solid lines were calculated using the Bell—Limbach tunneling model with the parameters listed in Table 6.4.

Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in,... 

In the solid state, tautomeric structures could be frozen. But tautomerism could be still fast on the NMR timescale and even polymorphism and different tautomers in the unit cell could be observed. In the latter case, as in solution, low-temperature measurements can complement tautomerism studies. Characteristic representative examples will be given in this chapter. 

Tautomerism in the solid state can be studied by or CP-MAS NMR spectroscopy. Crystals (but also those of substantial defects and amorphous samples) contain usually one predominant form but tautomeric exchange can occur also in the solid state especially if the NMR spectra are recorded at not low enough temperatures. In the former case, solid-state tautomerism can be compared with the corresponding solution-state tautomerism. In case of agreements, tautomer assignments (requiring often in addition computed NMR spectra) are most likely, taking the specific solid-state shift effects (see above) into account. 

Direct Evidence of Solid-State Tautomerism by Diffraction Methods... 

Solid-State Tautomerism, Proton Transfer, and Hydrogen Bonding... 

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. 

Mechanochemical grinding reactions are known for providing a pathway to products that are not accessible by any other procedures due to the essentially solid reaction environment. In the context of solid-state tautomerism, the mechanochemistry has recently almost become an unavoidable technique, yielding new tautomeric forms as in the case of the keto form of thiobarbituric acid [25] or the enol form of barbituric acid [78]. Milling was also used for facilitating the solid-state transformation between the solid tautomeric forms, such as the mechanochemical conversion of keto-amino tautomer of 6-[(3-hydroxy-pyridin-2-ylamino)-methylene]-cyclohexa-2,4-dienone to more stable enol form. Scheme 13.10 [45]. These examples will be discussed in more detail in the latter sections. 

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