Solid-State Tautomerism, Proton Transfer, and Hydrogen Bonding

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. 

The most extensive studies of solid-state proton transfer have been undertaken on two types of systems - complexes (salts and cocrystals) of carboxylic acid with nitrogen bases, and carboxylic acid dimers. In the first case, the proton transfer occurs along the unsymmetrical H N, and in the other along a (usually) symmetrical O- -H- -O bond. As both these types of proton transfer can (in principle) also lead to tautomerization, we shall briefly expound the main finds of these studies. 

In the case of proton transfer in the carboxylic acid-nitrogen base complexes, the proton is transferred from the acid to a base in order to produce a more stable pair. Similarly as in solution, the occurrence of proton transfer is determined by the relative acidities and basicities of the donor and acceptor. However, the solution acid-base behavior is not repeated exactly in the solid state. On the one hand, the charge separation that results from the proton transfer is less favorable in the solid state than in the solution, as there is no additional stabilization of 

In crystals however, the symmetry is very often broken by the supramolecular surroundings. If the acid molecule is symmetric, and if there are no significant intermolecular interactions in which the carboxyl group participates, the motion of the proton in the crystal will lead to an equimolar mixture of both tautomers.  

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