Tautomerism, aromaticity, and electron density in the ring critical points were analyzed for the 1,3-dihydro-benzo[e][1,4]diazepin-2-one (BDA) molecule substituted at the position C7 of the benzo ring at the B3LYP/ 6-31G** level. We found that the N1H tautomer is much more stable (a matter of at leat 8 kcal/mol) than the N4H in the gas phase and in water. It is shown that the grater is the π-electron donation effect the more stable is the N1H tautomer. The benzo ring is aromatic and the diazepinone is antiaromatic in N1H tautomers while in the N4H tautomers the former is practically not aromatic and the latter is much more antiaromatic. This is the main reason why the N4H tautomers are so much less stable than N1H ones.  The AIM analysis of the electron density or its Laplacian in ring critical point of the seven-membered ring correlate well with the pEDA descriptor of the substituent effect on π-electron system, while for the benzo ring the analogous correlations are much weaker. This indicates that the susbstitution at the benzo ring modifies significantly the electron dencity of the diazepinone ring.

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