Dielectric Relaxation and Equilibrium Thermodynamic Descriptors in Propanol Isomers: Radio-Frequency Measurements and openCOSMO-RS Analysis

Dielectric relaxation of n-propanol and isopropanol was examined by radio-frequency measurements at seven fixed frequencies (0.3–32 MHz) over +20 to −160 °C and by openCOSMO-RS calculations. Apparent characteristic times were assigned from dielectric-loss maxima and analyzed with Arrhenius and Eyring formalisms. Nine temperature scans were available for each sample, obtained with three independent measuring capacitors and three repeat scans per capacitor. Capacitor-specific Arrhenius fits gave apparent activation energies of 21.30 ± 0.16 kJ mol−1 (SD; 95% CI of the mean 20.91–21.69) for n-propanol and 21.99 ± 0.16 kJ mol−1 (95% CI 21.60–22.38) for isopropanol. A conservative Welch comparison of the three independent-capacitor estimates gave ΔEa = 0.687 kJ mol−1 (95% CI 0.332–1.042, p = 0.0058). Although this sub-kJ difference is statistically distinguishable in the cell-level analysis, it is not considered to have mechanistic significance because systematic temperature uncertainty and unresolved spectral overlap are not included in that comparison. Eyring activation enthalpies are 19.60 ± 0.15 and 20.23 ± 0.15 kJ mol−1 (SD), respectively. Entropy and Gibbs-energy values are reported only as apparent quantities under κ = 1. openCOSMO-RS yields liquid-to-n-hexane infinite-dilution transfer enthalpies of 30.5 and 28.0 kJ mol−1. These transfer quantities include hydrogen bonding and nonspecific solvation, dispersion, and packing contributions and are therefore not identified with hydrogen-bond enthalpies. Literature calorimetry gives lower neat-alcohol hydrogen-bonding enthalpy magnitudes of approximately 17 kJ mol−1. Kirkwood factors above unity indicate strong static orientational correlation. Because the seven-frequency window does not independently resolve the Debye and structural α processes, the kinetic parameters are interpreted as effective descriptors of the dominant dielectric-loss process rather than mode-specific barriers.