Properties of dimethyl 3-(alkylamino)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-1,2-dicarboxylate and its derivatives were studied by means of ab initio method. NO2 derivative of title compound was synthesized and the nature of its intramolecular hydrogen bond (HB) was investigated. Furthermore, the topological properties of the electron density distributions for N[BOND]H···O intramolecular bridges were analyzed in terms of the Bader theory of atoms in molecules (AIM). The electron density (ρ) and Laplacian (∇2ρ) properties, estimated by AIM calculations, indicated that O···H bond possesses low ρ and positive ∇2ρ values which are in agreement with electrostatic character of the HBs, whereas N[BOND]H bonds have covalent character (∇2ρ<0). Moreover, steric effect of the t-Bu group on structure and topological parameters of pyrazolo[1,2-b]phthalazine conformers was studied. Finally, the powerful method of Espinosa was used to obtain the H-bond energy.

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