The molecular structure and relative stabilities of the six possible isomers of 6-hydroxy-3(2H)-pyridazinone (DHP) in the gas phase and in solutions of different polarities are predicted using the B3LYP/6-311++G(d,p) method. The oxo-hydroxo isomer is the most stable form in the gas phase and in solution. These results agree with our reported X-ray structure. The effect of solvents on the spectroscopic properties of the most stable isomer has been studied using the polarized continuum method (PCM) at the same level of theory. The vibrational spectra of the compound studied are calculated and compared with the experimentally measured FTIR spectra. The electronic spectra in gas phase and in solution were calculated using the TD-DFT method. The most intense absorption band is predicted at 312.4 nm and belongs mainly to a π→π(*) transition. In polar solvents, this spectral band undergoes a hypsochromic shift. Two stable dimer forms were calculated at same level of theory. Dimer A is more stable than dimer B, by 6.66 kcal mol(-1). The former is stabilized by stronger O-H⋯O H-bonds compared to the weaker N-H⋯O interactions in the latter. The effect of these H-bonding interactions on the molecular structure and vibrational spectra of these compounds are predicted. NBO analyses were carried out to investigate the stabilization energy of various inter- and intramolecular charge transfer interactions within the systems studied.
Keywords: H-bonding; NBO; Pyridazine; TD-DFT; Tautomerism.
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