Dynamics of a conformational change in aqueous 18-crown-6 by an ultrasonic absorption method

A temperature dependence study of the ultrasonic amplitudes, velocities, and relaxation times for a presumed conformational transition of noncomplexed aqueous 18‐crown‐6 (1,4,7,10,13,16‐hexaoxacyclooctadecane) is discussed. At all temperatures a single relaxation was observed within a 15–255‐MHz frequency range. The equilibrium constant for the presumed conformational transition \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CR}_1 \mathop \rightleftarrows\limits^{K_{12} } {\rm CR}_2 $\end{document} was determined to be K21 = (2 ± 2) × 10−2. The activation parameters are ΔH21≠ = 10.2 ± 1.0 kcal/mol, ΔS21≠ = 7.7 ± 0.2 cal/(mol·deg), ΔH12≠ = 7.4 ± 1.0 kcal/mol, and ΔS12≠ = 7.7 ± 0.2 cal/(mol·deg), while the thermodynamic enthalpy and entropy were found to be −2.6 ± 1.0 kcal/mol and 0 ± 0.2 cal/(mol·deg), respectively. The rate constants at 25.0°C for the presumed conformational transition are k21 = (1.0 ± 0.3) × 107 sec−1 and k12 = (6.2 ± 0.2) × 108 sec−1.