We have constructed an infrared electroabsorption spectroscopic system employing the alternating current (AC) coupled dispersive method developed previously in our laboratory. It is highly sensitive, measuring electric-field-induced infrared absorption changes as small as 6 × 10-8 in the mid-infrared region of 4000-820 cm-1. Infrared electroabsorption spectra of liquids and solutions can be measured at room temperature. The system was first applied to liquid acetone to observe the orientational as well as the electronic polarization signals. These signals show distinct behaviors that are exactly predicted by theory. It was then applied to liquid 1,2-dichloroethane, for which electric-field-induced gaucheltrans equilibrium changes were detected as infrared absorption changes. The gaucheltrans ratio in room-temperature liquid 1,2-dichloroethane has been determined for the first time as 1.4 ± 0.2. This equilibrium constant has led to the free energy difference ΔG of 0.9 ± 0.4 kJ mol-1 (ΔG = Gg - Gt) and the entropy difference of ΔS = -3.0 ± 1.4 J K-1 mol-1 with the known enthalpy difference ΔH ∼ 0.0 kJ mol-1. Finally, the system was applied to 1,4-dioxane solutions of N-methylacetamide. The dipole moments of the monomer as well as that of the dimer of N-methylacetamide have been determined separately and a head-to-tail structure of the dimer has been clarified. The instrumentation and the theoretical basis of infrared electroabsorption spectroscopy are described in detail together with the above-mentioned experimental results.
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