Synthesis and spectroscopic properties of pyridones - Experimental and theoretical insight

Cyano pyridone (CPy) and its substituted derivatives OCH₃ and NO₂) were synthesized and their absorption spectra were examined in different solvents. The absorption spectra of CPy in different solvents indicated a dependence on the polarizability of the solvent. NMR studies of the keto and enol equilibrium in CPy revealed temperature dependence, with the keto form being more predominant. The equilibrium constant values were in agreement with the theoretical calculations. The calculated pK_a value for CPy was found to be 9.50 ± 0.57, whereas, for the nitro and methoxy substituted CPy, the values were 8.52 ± 0.36 and 11.04 ± 0.21 respectively. In mixed solvent systems, the preferential solvation was observed in DMSO/CH₂Cl₂ mixtures, whereas in DMSO/i-PrOH, the mixture behaved ideally. The calculations using the PCM model indicated ΔG values favoring the keto form in both dichloromethane and DMSO. The calculated transition energies were found to be higher than those observed experimentally. For each compound, the keto form was predicted to absorb at a lower energy than the enol form. The main transition of both forms of CPy corresponded to a π-π* transition from the HOMO → LUMO of the compound.