Azoles, mesomeric structures

Fusion of a pair of azoles gives aromatic systems isoconjugate with the heteropentalene dianion. Representative examples 21-24 are given in Figure 4 and these include further examples of conjugated mesomeric betaines, e.g., 23 and 24, which can only be represented by 1,3-dipolar structures (cf. Section 2.3.1.1.1.2). 

Measurement of the dipolar moments of the mesomeric betaines of azolium azolate 11 was extremely difficult. Of the various assays performed (extreme dilution, dioxane, 25°C), the best measurements were chosen. However, these quasi coplanar structures are highly associated when ti> > 0.0003, and the effect of self-association could not be eliminated completely. For example, for 2-(3-methyl-l-imidazolium)benzimidazolate 117 the was 11.35 D and the antiparallel orientation of 117 was confirmed by X-ray diffraction analysis (III,C). 

Ten mesomeric betaines, 430-432, of pyridinium azolates have been prepared from the corresponding pyridinium azole salts. Dipole moments, and NMR spectra. X-ray structures and theoretical calculations (MNDO) have been used to determine their molecular and electronic structures (87JOC5009). 

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