THERMODYNAMIC PROPERTIES, CONFORMATIONAL COMPOSITION, AND PHASE-TRANSITIONS OF CYCLOPENTANOL

Thermodynamic properties of cyclopentanol were studied. The molar heat capacity of c-C5H9OH(cr and 1) in the temperature range T = 5.4 K to 303.0 K was measured by vacuum adiabatic calorimetry. Three solid-to-s olid transitions were found: at T = 176 K with Delta(trs)H(m) = (57 +/ - 5) J . mol(-1); at T = 202.6 K with Delta(trs)H(m) = (3366 +/- 14) J . mol(-1), and at T = 234 K with Delta(trs)H(m) = (55 +/- 6) J . mol( -1). The fusion temperature of c-C5H9OH is 255.6 K, and Delta(fus)H(m) = (1227 +/- 5) J . mol(-1). Basic thermodynamic functions at T = 298. 15 K in the liquid state are C-s,C-m = (182.48 +/- 0.73) J . K-1. mol( -1), S-m = (204.14 +/- 0.90) J . K(-1)mol(-1), and phi(m) = (96.98 +/- 0.40) J . K-1. mol(-1). The enthalpy of vaporization was measured wit h a heat-conducting microcalorimeter: Delta(vap)Hm(298.15 K) = (57.05 +/- 0.65) kJ . mol(-1). Using these and literature data, the standard molar entropy of c-C5H9OH(g) was determined:. S-m(o)(g,340 K) = (362.9 +/- 2.4) J . K- (-1)mol(-1). Conformational analysis was made by the molecular-mechanics method, and statistical calculations of standard m olar thermodynamic functions in the ideal-gas state were carried out o n the basis of molecular parameters and conformational properties. The calculated entropy value at T = 340 K was put into agreement with the experimental one by adjusting the pseudorotational moment of inertia. The standard molar entropy and molar heat capacity of c-C5H9OH in the ideal-gas state at T = 298.15 K are 347.91 J . K(-1)mol(-1) and 105.4 3 J . K(-1)mol(-1), respectively. Thermodynamic analysis of phase tran sitions in the condensed state was made. It was shown that pseudorotat ion in the plastic crystal state of c-C5H9OH is significantly hindered . Thermodynamic quantities allowed us to propose the absence of a non- equilibrium mixture of conformers at T --> 0. An anomalously low entro py difference between liquid and rigid crystal of cyclopentanol in com parison with other cyclopentane derivatives shows a relatively high or dering in the liquid. (C) 1995 Academic Press Limited.