In this work, two dispiro[3H -indole-3,2′-pyrrolidine-3′,3″-piperidines], C31H29Cl2N3O3 (4a) and C32H31Cl2N3O3 (4b), have been synthesized and characterized by IR, 1H NMR, 13C NMR, 1H 1H COSY, 1H 13C HSQC and single crystal X-ray diffraction. Both compounds were found to be crystallized in the triclinic space group View the MathML source with Z = 2. Molecules of the crystalline structure of 4a are linked with intermolecular Csingle bondH⋯O and Nsingle bondH⋯O hydrogen bonding. Meanwhile, an intermolecular Nsingle bondH⋯O hydrogen bonding was recognized in the crystalline structure of 4b. Molecular mechanics force field (MM+), followed by either semi-empirical AM1 or PM3, has been used to calculate the optimized geometrical parameters for the two compounds. The determined theoretical geometry parameters were found to be in a good agreement with the parameters obtained using X-ray studies. Moreover, the Molecular Electrostatic Potential (MEP) of both compounds have been calculated at B3LYP/6-31G(d,p) level of theory exhibiting that, the most electrophilic site of the synthesized compounds, is the piperidone O3 atom however, the most nucleophilic site is the indolyl N2. Additionally, the Frontier Molecular Orbitals (FMOs) 4a and 4b have been determined by the same technique exhibiting large energy gap between HOMO and LUMO orbitals (3.40, 3.43 eV for 4a and 4b, respectively), inferring good stability, high excitation energies and large chemical hardness of these molecules.
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