(1R*,2S*,5R*)-5-tert-Butyl-2-hydroxycyclopentanecarboxylic acid exists in several solid phases, two of which are investigated in this report: a first phase (phase I) obtained by crystallization from common solvents and a second phase (phase II) obtained by heating the first phase above ca. 70 °C. The second phase appears to be the stable phase at ambient temperature, and the first phase appears to be a metastable phase. A third phase (phase III), which reverts to phase II on standing at room temperature, is obtained by cooling the melt to room temperature. The rates of rotation of the tert-butyl groups in phases I and II have been measured by CP/MAS NMR techniques. The activation parameters for the tert-butyl group rotation differ markedly between the two phases: phase I, ΔH≠ = 59.0 ± 2.1 kJ mol–1, ΔS≠ = 51.6 ± 8.1 J K–1 mol–1; and phase II, ΔH≠ = 43.9 ± 2.5 kJ mol–1, ΔS≠ = 28.3 ± 9.3 J K–1 mol–1. The structure of phase I has been solved by X-ray diffraction techniques, and this assists in understanding the experimental observations.
ASJC Scopus subject areas
- Colloid and Surface Chemistry