Gas-phase conformational and intramolecular π–π interaction studies on some pyrazolo[3,4-d]pyrimidine derivatives

The conformational stabilities and computations of optimized geometrical parameters of 1,3-bis(4,6-diethylthio-1H-pyrazolo[3,4-d]pyrimidin-1-yl)propane (1), 1,3-bis(4-ethoxy-6-methylsulfanyl-1H-pyrazolo[3,4-d]pyrimidin-1-yl)propane (2), 4,6-bis(methylsulfanyl)-1-phthalimidopropyl-1H-pyrazolo[3,4-d]pyrimidine (3) and 6-methylsulfanyl-1-phthalimidopropyl-4(pyrollidin-1-yl)-1H-pyrazolo[3,4-d]pyrimidine (4) molecules have been carried out using density functional B3LYP/6-31G** and dispersion corrected density functional DFT-D/B97D methods. To study different conformations of the molecules, potential energy surface scan (PES) investigations were performed using both theoretical methods. Torsion angles τ1(N1-C8-C9-C10) and τ2(C8-C9-C10-N11) corresponding to the central methylene group were varied in all compounds at steps of 30°. Five most probable conformations in each case for all the four molecules were obtained. Geometries at the minimum energies were fully optimized without any constraints. Since B3LYP method does not include the dispersion term, M06-2X/6-311++G** method is used to refine the energies of the B3LYP optimized geometries, which is known to best suited for non-covalent interactions. The capabilities of all methods to predict the crystallographic structure have been critically examined. Attempts have been made to calculate the intramolecular aromatic π–π interaction energies.