Structure and vibrational assignment of tautomerism of 4-hydroxyquinazoline in gaseous and aqueous phases

The tautomerizm and conformation of the 4-hydroxyquinazoline molecule were investigated, and its three tautomers (4-HQ1, 4-HQ2, 4-HQ3) and conformer of 4-HQ2 (4-HQ2A) were considered. The tautomeric and conformational equilibrium geometry of 4-hydroxyquinazoline have been studied using density functional theory (DFT) and Møller–Plesset second-order (MP2) methods in the gas phase and in water. The tautomerization processes in water (ε = 78.39) were studied by using self-consistent reaction field theory with the polarized continuum model (PCM). Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and water. In agreement with experimental results, our calculations show that the 4-HQ1 was the most stable form in the gas phase and in water. Solvent effect induced slight changes in geometry of rings and the order of stability changed. The harmonic wavenumbers of 4-HQ1 were calculated in the gas phase and in water by the B3LYP/6-311++G∗∗ DFT method and the wavenumbers were given. The infrared spectra of 4-hydroxyquinazoline were recorded in the regions 400–4000 cm−1. The variation of dipole moments and charges on atoms were studied in the gas phase and in water.