Synthesis, structure, spectral, thermal analyses and DFT calculation of a hydrogen bonded crystal: 2-Aminopyrimidinium dihydrogenphosphate monohydrate

Abstract

A proton transfer complex of 2-aminopyrimidine with phosphoric acid was synthesized and crystallized. Single crystal X-ray studies, the vibrational spectral analysis using Laser Raman and FT-IR spectroscopy in the range of 4000–400cm−1, UV–Vis-NIR studies and thermogravimetric analyses were carried out in the solid crystalline form. The single crystal X-ray studies shows that the crystal packing is dominated by NH⋯O and OH⋯O hydrogen bonds leading to a hydrogen bonded ensemble. The two dimensional cationic layers, connected through the centrosymmetric anionic dimer of R22(8) motif, is extending along ab plane of the crystal leading to zig-zag infinite chain C21(6) and C22(6) motifs. To investigate the strength of the hydrogen bonds, vibrational spectral studies were adopted and the shifting of bands due to the intermolecular interactions were analyzed. Density Functional Theory (DFT) using the B3LYP function with the 6-311++G(d,p) basis set was applied to the solid state molecular geometry obtained from single crystal X-ray studies. The optimized molecular geometry and computed vibrational spectra are compared with experimental results which shows appreciable agreement. NBO analysis has been carried out by DFT level. In this study explains charge delocalization of the present molecule which shows the possible biological/pharmaceutical activity of the molecule. The number of normal modes were also attempted by the factor group analysis method. It is evident that the influence of extensive intermolecular hydrogen bonds reduces the Td symmetry of the phosphate anion to the lower C2v symmetry. The existence of exothermic peaks in DTA iterate the breaking of intermolecular hydrogen bonds and the phase change of the crystal. The presence of water molecule is also confirmed in the thermal analyses.