Determination of ΔH and ΔS between syn- and anti-Isomers of CH3COOH in CH3CN by a Temperature-Dependent 2D IR Study.

Temperature-dependent Fourier-transform infrared (FTIR) and two-dimensional infrared (2D IR) spectroscopic measurements are performed on CH3COOH dissolved in CH3CN over the temperature range of 4-62 °C. CH3COOH exists as two conformers, the syn- and anti-isomers, whose relative populations vary with temperature. At each temperature, their relative populations are determined by FTIR and 2D IR spectroscopy, supported by numerical simulations of the corresponding spectra. Accurate determination of the temperature-dependent populations enables the enthalpy difference (ΔH) and entropy difference (ΔS) between the two isomers to be obtained from a van 't Hoff analysis. The resulting thermodynamic parameters for the syn ⇄ anti isomerization are ΔH = 6.34 ± 0.27 kJ·mol-1 and ΔS = -0.64 ± 0.88 J·mol-1·K-1, indicating that the conformational equilibrium is dominated by the enthalpic contribution.