PHASE DEPENDENCE OF CONFORMATIONAL MOTIONS IN SOLIDS - THE TERT-BUTYLROTATION IN RISK)-5-TERT-BUTYL-2-HYDROXYCYCLOPENTANECARBOXYLIC ACID

R())-5-tert-Butyl-2-hydroxycyclopentanecarboxylic acid exists in seve ral solid phases, two of which are investigated in this report: a firs t phase (phase I) obtained by crystallization from common solvents and a second phase (phase II) obtained by heating the first phase above c a. 70 degrees C. The second phase appears to be the stable phase at am bient temperature, and the first phase appears to be a metastable phas e. 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 ha ve been measured by CP/MAS NMR techniques. The activation parameters f or the tert-butyl group rotation differ markedly between the two phase s: phase I, Delta H not equal = 59.0 +/- 2.1 kJ mol(-1), Delta S not e qual = 51.6 +/- 8.1 J K-1 mol(-1); and phase II, Delta H not equal = 4 3.9 +/- 2.5 kJ mol(-1), Delta S not equal = 28.3 +/- 9.3 J K-1 mol(-1) . The structure of phase I has been solved by X-ray diffraction techni ques, and this assists in understanding the experimental observations.