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Molecular orbital calculations at the ab initio, AM1, and PM3 levels have been carried out to investigate the lactam-lactim tautomerism of 1,2,4-triazolidine-3,5- dione(1) and 1,3,4-oxa(or thia)diazolidine-2,5-dione(2, 3). Most stable tautomer in 1 compound has been calculated to be a dilactam 1a and next one is lactam-lactim 1b. The relative energies between 1a and 1b are 4.1~12.6 kcal/mol depending on computational methods. The optimized 1a structure at ab initio level is in good agreement with X-ray structure. While the stabilities of 2 tautomers are in order of 2a>2b>2c, the stabilities of 3 tautomers are dependent on methods. According to 3-21G basis set, 3a tautomer is more stable by 4.9 kcal/mol over 3b tautomer. In contrast, the heat of formation of 3a at AM1 is higher by 2.71 kcal/mol than 3b.
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