Molecular Structure, Theoretical NBO Analysis, Vibrational Spectrum of CO2-Responsive Hydroxyamidine-Based Ionic Liquid: A Combined Computational and Experimental Approach

The utilization and chemical transformation of carbon dioxide remains a pressing problem in modern chemistry. Numerous experimental and theoretical studies have focused on the interaction of CO₂ with amines. In this work, quantum chemical density functional theory (DFT) calculations of equilibrium geometries, energies, electronic and vibrational characteristics of CO₂-sensitive mono-, di-, tris-hydroxyamidines and their associates were carried out by the B3LYP/6-31G(<i>d</i>, <i>p</i>) method. The harmonic vibrational frequencies were scaled and compared with the experimental FTIR spectra for supporting wavenumber assignments. Natural bond orbital (NBO) analysis of the atomic charges and charge delocalization was employed to investigate the nature of hydrogen bonding in hydroxyamidine associates. We also used the intrinsically polarizable continuum model (IEFPCM), and the DFT-D3 method was applied to account for dispersion effects during associate formation. Using the 6-311+G(2<i>d</i>, <i>p</i>) basis set for tris-hydroxyamidine, and its adducts, a comparative analysis of the experimental and calculated ¹H NMR spectra was performed. Here, we considered non-trivial sites of carbon dioxide absorption and hydroxyamidine protonation, which, to our knowledge, have hardly been considered by other authors. Present DFT results agree rather well with the experimental data and support new insight into the formation of the PIL structure.