Lactam-lactim isomerism

AHOMO and ALUMO are not very different from each other, the same procedure can be applied to such compounds in order to gain information about the protonation site, for example, in the various aza analogues of indolizine (10), or about the tautomerism in heterocycles as exemplified by the lactam-lactim isomerism in 2-pyridone (11) and about positional isomerism such as in the tautomeric 3,5- and 3,6-diazaindoles (12, 13). This is possible since the protonation of a nitrogen or the replacement of a carbon by a heteroatom can be viewed as a special case of substitution. 

Unlike phenols (Section 26-l), structural analysis of many of the hydroxy-substituted aza-aromatic compounds is complicated by isomerism of the keto-enol type, sometimes called lactim-lactam isomerism. For 2-hydro xypyrimidine, 19, these isomers are 19a and 19b, and the lactam form is more stable, as also is true for cytosine, 15, thymine, 16, and the pyrimidine ring of guanine, 18. 

In the same paper Baeyer concluded that isatin and indoxyl must first be converted into isomeric compounds before they can form members of the true indigo group. The isomers are known only in compounds, in the free state they pass back spontaneously into the original form. Their instability depends on the mobility of the hydrogen atom . He called the labile forms pseudocompounds and introduced the names lactam and lactim for the two forms ... 

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