Combining ab initio calculations and Fourier-transform infrared (FT-IR) spectroscopy for quantitative analysis of multicomponent systems in solution: Tautomer proportions of ethyl acetoacetate

A quantitative analysis method, combined experimental–computational approach (CECA), has been developed and applied for the detection of tautomer ratios of ethyl acetoacetate (eaa) in three organic solvents (acetonitrile, methanol, and chloroform). In order to obtain the relative concentrations of tautomers of eaa, IR intensities of both tautomers have been calculated at three different calculation levels (B3LYP/6-311++G(2d,2p), MP2/cc-pVDZ, and MP2/cc-pVTZ), augmented by the data obtained using basis set extrapolation technique. Experimental absorption bands were recorded at specific wavenumbers with FT-IR spectrophotometer and combined with calculated IR intensities in Lambert–Beer equation. Though the pure computational approach does not provide accurate values of the tautomers’ ratio, yet the results obtained using the CECA method are very close to the experimental data.