Mesohydric tautomerism

The concept of mesohydric tautomerism was advanced by Hunter and his associates in a series of papers which appeared between 1940 and 1950 (e.g., references 15 and 16). This concept was based on the fact that in all cases where the mobile hydrogen atom would be bonded to oxygen, sulfur, or nitrogen atoms in both possible tautomers, the individual forms had not been isolated. It was further established that many of these compounds were associated both in the liquid state and in solution, and it was concluded that the individual tautomers did not exist. The actual molecules were thought to be intermolec-ularly hydrogen-bonded, the mobile hydrogen atom being bonded equally to both of the hetero atoms. This concept has been useful and has led to clarification of the tautomerism which occurs in solids and... 

Fyrazoles and indazoles, imidazoles and benzimidazoles/ and benzotriazoles which possess a free NH group are associated and have been considered to exhibit mesohydric tautomerism see discussion in Volume 1, article I, Section I,C. 

This dramatic consequence of chelation has been interpreted theoretically by several workers. Hunter applied his views on mesohydric tautomerism (see our Chapter 6) (1001). Davison disagrees, arguing that the spectra do not show the H bonded ion (508). Nash (1492) proposed resonance forms different from Hunter s. 

Mesohydric Tautomerism Resonance. For completeness, brief reference should be made to proposals on the nature of the H bond which have not won general acceptance. 

The term mesohydric tautomerism was proposed by Hunter to describe the sharing of a tautomeric H atom by two atoms of oxygen, nitrogen, or sulfur (but not carbon) (1006). His review (1004) indicates that this terminology describes a model based on belief in a resonance description of the H bond. Hunter describes an associated substance B—A—H B—K—H in terms of the two structures (a) and (b) below. 

Any evidence that the individual tautomers do exist discredits one basic notion of mesohydric tautomerism. Surely the relatively small perturbations of the A—H vibrational bending modes indicate that the H atom remains attached to A though H bonded to B. (See Section 3.4.1.) Possibly more obvious evidence is that offered by spectroscopic meaisurements which locate the H atom. Both neutron diffraction and proton magnetic resonance studies show that the proton is usually nearer one atom than the other. (See Section 9.3.)... 

The resonance implicit in the concept mesohydric tautomerism has been specifically proposed by a number of authors both prior and subsequent to Hunter s proposal (Sidgwick, 1871 Sherman, 1844 Coates, 399 Wirtz, 2187 Vand, 2091). Coulson disposes of models based on resonance of a covalent character between two structures on the grounds that (i). . . it does not provide enough resonance energy unless the protons are central to each O - H- - O region and (ii). .. in order to get them there it would be necessary to expend too much energy on stretching the normal O—H bonds. (446). (See also 275.)... 

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