Synthesis and Theoretical Study of Intramolecular Hydrogen Bond at Two Possible Positions in Pyrazolo[1,2-b]phthalazine

Properties of dimethyl 3-（alkylamino）-5,10-dioxo-5,10-dihydro-1H-pyraolo[1,20b]phthalazine-1,2-dicarboxy-late and its derivatives were studied by means of ab initio method. NO2 derivative of title compound was synthe- sized and the nature of its intramolecular hydrogen bond （HB） was investigated. Furthermore, the topological prop- erties of the electron density distributions for N--H＇＂O intramolecular bridges were analyzed in terms of the Bader theory of atoms in molecules （AIM）. The electron density （p） and Laplacian （V2p） properties, estimated by AIM calculations, indicated that O＇--H bond possesses low P and positive VZp values which are in agreement with elec- trostatic character of the HBs, whereas N--H bonds have covalent character （V2p〈0）. Moreover, steric effect of the t-Bu group on structure and topological parameters of pyrazolo[1,2-b]phthalazine conformers was studied. Fi- nally, the powerful method of Espinosa was used to obtain the H-bond energy.