Conformational analysis, tautomerization, and vibrational spectra of methyl acetoacetate

Abstract

In this study, the molecular structure, tautomerization, conformational stability, and electronic energies of 25 enols and keto forms of methyl acetoacetate (MAA) were examined theoretically using DFT and experimentally by IR, Raman, and UV spectroscopy. According to the theoretical and experimental results, three more stable tautomer’s including an enol (2EU1-I) and two ketos (DK1 and DK2) forms in the gas phase, and solutions have been obtained. Using equilibrium ratios in the gas phase and different solvents at various levels of theory, the abundance of enol was determined. Outcomes indicated that the content of cis-enol or keto forms depended on to phase. In gas phase, cis-enol form is as major form. Conventional and unconventional intramolecular hydrogen bonds for cis-enol and keto forms, respectively, were considered via AIM. Also, using NBO in terms of E(2) values, the enol and keto forms, and the hydrogen bond strength interpreted. By recording the vibrational spectra in various polar and non-polar solvents, deuterated analog, deconvolution, and calculating potential energy distribution of cis-enol for MAA, the strong IR bands were obtained at about 1755–1715 cm−1 regions show the existence of two keto forms in the sample. Also, two medium intensity bands at 1654 and 1630 cm−1 were assigned to the chelated ring of cis-enol form. Finally, the UV spectra in different solvents and frontier molecular orbitals were shown that hydrogen bond strength in the enol form of MAA is weaker than AA. Also, the content of keto forms increased in polar solvents.