Keto–enol tautomers of 1,2-cyclohexanedione in solid, liquid, vapour and a cold inert gas matrix: Infrared spectroscopy and quantum chemistry calculation

Fourier transform infrared (FTIR) spectroscopy of 1,2-cyclohexanedione (CHD) has been studied with the neat sample, low-pressure vapour, CCl4 solution and in cold N2 matrix. The matrix-isolation spectrum has been assigned exclusively in terms of transitions of the enol tautomer, that is stabilized by an intramolecular O⋯HO hydrogen bond (HB). The vibrational fundamentals of the diketo tautomer appear weakly in the spectra of CCl4 solution and vapour. On the other hand, the spectrum of the neat sample shows significant population of the diketo tautomer. Thus the intermolecular interactions, which are dominant in the neat sample, stabilize the diketo form. The predictions of electronic structure calculations by B3LYP/6-311++G(d,p) and MP2/cc-pVTZ methods are found to be consistent with the measured tautomeric distribution in the cold inert gas matrix. The larger dipole moment of the diketo tautomer (5.61 D) compared to enol form (3.60 D) is proposed to be responsible for stability of the former in the neat sample. The vibrational fundamentals predicted by anharmonic calculation at B3LYP/6-311++G(d,p) level display excellent agreement with measured frequencies. The proposed assignments are further corroborated by noting the deuterium isotopic shifts of different bands and their predicted shifts by the same theoretical methods.