Thermodynamic properties and pseudorotation of chlorocyclopentane

The results of a study of chlorocyclopentane in different phase states are given. The heat capacity of crystalline and liquid chlorocyclopentane was measured by vacuum adiabatic calorimetry ( T = 6 K to 301 K). One solid-to-solid transition cr(II) → cr(I) at T = 169.35 K was discovered. The enthalpy of this transition is Δ trs H o m = (7631±18) J·mol -1. The fusion properties: T fus = 180.0 K and Δ fus H m = (637.3±4.4) J·mol -1 were obtained from the calorimetric investigations. The vaporization enthalpy from T = 299 K to 311 K was determined with a heat-conduction differential microcalorimeter. Δ vap H o m(298.15 K) = (38.79±0.40) kJ·mol -1. The conventional molar entropy of gaseous chlorocyclopentane S o m(g, 298.15 K) = (339.54±1.7) J·K -1·mol -1 was calculated on the base of these calorimetric methods. It was found by recording i.r. spectra that the liquid and crystal I consist of a mixture of the axial and equatorial conformers as a "semi-chair". On the contrary, the crystal II contains a considerably smaller fraction of the equatorial conformer. So, the spectral study shows that the residual entropy at T = 0 caused by the mixing of conformers is negligible. The standard thermodynamic properties of chlorocyclopentane in the gaseous state ( T = 100 K to 1000 K) were calculated using molecular and spectral quantities. The statistical results are in the best accord with experimental values where the conformational transitions are considered as a hindered rotation. The standard molar values C o p. m , Δ T 0 S o m, Δ T 0 H o m/ T, and Φ o m of liquid and gaseous chlorocyclopentane at the temperature 298.15 K are, respectively: {152.43±0.61), (238.05±1.20), (119.63±0.50), (118.41±1.30); 98.86, (339.54±2.80), 63.58, and 275.87} J·K -1. The standard molar thermodynamic functions of formation of liquid and gaseous chlorocyclopentane at T = 298.15 K were determined from our and literature values: Δ f H o m(g) = -(119.7±2.0) kJ·mol -1, Δ f H o m(l) = -(158.5±2.1) kJ·mol -1, Δ f G o m(g) = -(33.82±3.0) kJ·mol -1, and Δ f G o m(l) = -(42.35±3.1) kJ·mol -1.