A complete scheme of tautomerism on diacetyl monoxime in the gas and solution phases. A comparative DFT study between B3LYP and M06-2X functionals

A detailed study on tautomerism of diacetyl monoxime (DAMO) was performed using M06-2X/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels of theory. Kinetic, thermodynamic, structural and also spectral data of the most stable tautomers of DAMO were calculated. The results showed that the tautomer 1 was more stable than the other possible tautomers at both levels of theory. Then, interconversion of most stable tautomers to each other was surveyed using internal rotation and proton transfer (PT) routes through pathway A–G. Energetic data were used to calculate the energy barriers of tautomeric interconversions and by this way, higher values of energy barriers obtained for PT steps. Moreover, solvent effects were explored for all of the paths A–G. Results showed that tautomerism could be facilitates in the presence of solvents especially in water. All of the tautomers involving in intramolecular hydrogen bonding represented red-shifting and it was found that stronger H-bonds showed more red-shifting. Finally, NBO analysis has been performed in order to elucidate donor–acceptor interactions in tautomers 1–6.