Spectroscopic, electronic structure and natural bond analysis of 2-aminopyrimidine and 4-aminopyrazolo[3,4-d]pyrimidine: A comparative study

The FTIR and FT-Raman spectra of 2-aminopyrimidine (2-AP) and 4-aminopyrazolo[3,4-d]pyrimidine (4-APP) has been recorded in the region 4000–400 and 3500–100 cm−1, respectively. The tautomeric stability, optimized geometry, frequency and intensity of the vibrational bands of 2-AP and 4-APP were obtained by the DFT level using 6-31G(d) and 6-31G(d,p) basis sets. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FTIR and FT-Raman spectra. A detailed interpretation of the infrared and Raman spectra of 2-AP and 4-APP are also reported based on total energy distribution (TED). The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically simulated spectra. The 1H and 13C NMR spectra have been simulated using the gauge independent atomic orbital (GIAO) method. The theoretical UV–Vis spectrum of the compound using CIS method and the electronic properties, such as HOMO and LUMO energies, were performed by time-dependent DFT (TD-DFT) approach. The calculated HOMO and LUMO energies show that charge transfer occurs within molecule. The first order hyperpolarizability (β0) of these novel molecular system and related properties (β, α0 and Δα) of 2-AP and 4-APP are calculated using DFT/6-31G(d) method on the finite-field approach. The Mulliken charges, the values of electric dipole moment (μ) of the molecule were computed using DFT calculations. The change in electron density (ED) in the σ∗ antibonding orbitals and stabilization energies E(2) have been calculated by natural bond (NBO) analysis to give clear evidence of stabilization originating in the hyper conjugation of hydrogen-bonded interactions.

Graphical abstractThe tautomeric stability, optimized geometry, frequency and intensity of the vibrational bands of 2-aminopyrimidine and 4-aminopyrazolo[3,4-d]pyrimidine were obtained by the DFT level of theory using B3LYP method. In order to have a complete insight into the molecule, the frontier molecular orbitals, the molecular and thermodynamic properties have been discussed.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The complete vibrational assignment and spectroscopic analysis have been carried out. ► The spectral study of 2-AP and 4-APP invoked its internal behavior. ► The comparison of experimental and theoretical data gives confidence on the study performed. ► The thermodynamical and other electronic properties give extra evidence to get insight into the compound.