Ionic isomers

These results are relevant to the tautomerism which is, in principle, possible between A -oxazolin-5-ones (azlactones) (76) and their meso-ionic isomers (77). Kille and Fleurj interpreted the spectroscopic... 

When Busch s endo-thiotriazolines were formulated as meso-ionic compounds, it was recognized that two constitutional possibilities (227 or 228) required consideration. Later studies - established that the endo-thiotriazolines were in fact the meso-ionic l,2,4-triazole-3-thiones (227). Recently, a specific synthesis of their meso-ionic isomers (228) has been reported, and under equilibration conditions in hot ethanol the rearrangement 228 - 227 occurs. This explains why the meso-ionic l,2,4-triazole-3-thiones (227) are the products of synthetic... 

In the early investigation of the nitrosation of 1,4-diphenyl-thiosemicarbazide (279, R = R = Ph), the product was obtained as deep red needles, m.p. 110°, and was allocated the remarkable 1,3-endoxyhydrazomethylene structure (280). Treatment of this deep red compound with warm aqueous alkali gave a colorless isomer, m.p. 157°, which was formulated as having one of the equally unacceptable structures 281 or 282. Recent investigations have established that nitrosation of 1,4-diarylthiosemicarbazides (279) yield the meso-ionic 1,2,3,4-oxatriazol-5-imines (277), which are transformed by base into the meso-ionic isomers (278). These alkaline transformation products formulated as meso-ionic l,2,3,4-tetrazol-5-ones (278) are in fact identical (see Section VII, J, 1) with substances prepared, but not formulated, by von Pechmann many years ago. ... 

VIII. The Existence and Interconversion of Type A Meso-ionic Isomers... 

When the term meso-ionic was put forward by Baker, Ollis, and Poole in 1949, the possible existence of meso-ionic isomers 302 and 303 was recognized in principle the symbols a, b, c, d, e, and f have the same meaning as that given earlier (Section II). A typical pair of meso-ionic isomers C ui be more specifically represented by 304 and 303. Single compounds having the type structures 304 or 305 have been known for a number of years, but it is only recently that (i) of the... 

Methods for the synthesis of meso-ionic isomers 304 and 305, their characterization and interconversion have been discussed in appropriate sections of the review. These results are summarized in Fig. 2, and in order to assist in the location of compounds they are associated with the formula numbers used in the text. 

In Fig. 2, the results of equilibration studies are indicated. In only one case (216 217), as might be expected, is an equilibrium established, and there is little difference in the relative thermodynamic stability of the two isomers (216 and 217). On current knowledge, it is not possible to provide a well-based understanding of the factors that determine the relative thermodynamic stability of meso-ionic isomers. 

Various methods and reagents have been used to effect the interconversion between meso-ionic isomers 304 305 including (i) heating in protic solvents such as ethanol or ethyl mercaptan, - (ii) treatment with hot ethanolic ammonia or ethanolic sodium hydroxide, - (iii) heating, (iv) heating with aryl isocyanates or isothiocyanates. ... 

Fig. 3. Equilibration of meso-ionic isomers in protic solvents (e.g., EtOH) involving a betaine intermediate.

Two rearrangements may well be regarded as further examples of interconversion between meso-ionic isomers. The 1,3-diazolium chloride (310) and warm dilute aqueous potassium hydroxide yield 4-anilino-l-methyl-2-phenylimidazole (311), presumably via the indicated meso-ionic intermediates. The transformation 312 - 313 by treatment of the thiazolium chloride with warm aniline may be similarly interpreted. ... 

The most clear-cut distinction between the pairs of meso-ionic isomers given in Fig. 2 is provided by comparison of their mass spectral fragmentation pattems. Comparison of the dipole moments of corresponding isomers is also informative. ... 

The spectroscopic properties of meso-ionic compounds have been discussed in detail elsewhere and the reviewers do not feel that it would be useful to include a comprehensive account here. Ultraviolet, infrared, and nuclear magnetic resonance spectra of meso-ionic heterocycles provide general support for the conjugative interaction that would be expected for aromatic heterocycles, " but detailed interpretation of their spectra is not justifiable. Mass spectrometry has been shown to be particularly useful for distinguishing between pairs of meso-ionic isomers... 

Finally, the mixed ligand complexes with both dithiocarbamate and 1,2-dithiolate ligands should be mentioned. Both neutral and ionic isomers (82) and (83) have been characterized.560-562... 

The Raman data indicate that Y-N2O4 has a molecular structure closer to a-N204 than to NO Os, with some but less pronounced ionic character and lower symmetry than D2h. Even when the pressure was increased up to 18 GPa, the prominent peak at 2208 cm" did not shift noticeably. This peak, the most distinctive for Y-N2O4, is appreciably lower in frequency but correlates with the stretching mode of the NO moiety seen in the ionic isomer NONO3. 

Figure 3 The HF/6-31G optimized geometries of the C(NH2)3C(CN)3 molecule (a) the covalent isomer, (b) the ionic isomer.

A promising candidate might be the solvated sodium dimer excited to its double-minimum state. Employing a pump and control scheme, as shown in Fig. 6.1, will enable the transfer of the initially covalent bond-length isomer, to its ionic isomer which should favor the... 

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