Thermodynamic properties of 2-adamantanone in the condensed and ideal gaseous states

Abstract

The results of a comprehensive study of thermodynamic properties of 2-adamantanone in different phase states are reported. The heat capacity of the compound in the condensed state was measured from 5 to 310 K with an adiabatic calorimeter and between 290 and 610 K with a differential scanning calorimeter of the heat-bridge type. Adamantanone undergoes a solid-to-solid phase transition at 216.4 ± 0.1 K with ${\Delta }_{\text{trs}}{H}_{\text{m}}°=7.627\pm 0.014\text{kJ}\text{mo}{\text{l}}^{-1}$ and fusion at 557.5 ± 0.2 K with ${\Delta }_{\text{fus}}{H}_{\text{m}}°=11.77\pm 0.24\text{kJ}\text{mo}{\text{l}}^{-1}$. The solid-phase transition is associated with formation of orientationally disordered (plastic) crystal. The thermodynamic functions of the compound in the crystalline and liquid states were obtained. The saturated vapour pressure for crystalline adamantanone between 280 and 333 K was determined by the Knudsen effusion method, the undersaturation of the vapours in the effusion cell being taken into account. The sublimation enthalpy of the compound was measured with a differential Calvet-type calorimeter. The weighted average value of the sublimation enthalpy was ${\Delta }_{\text{sub}}{H}_{\text{m}}°(298.15\text{K})=66.38\pm 0.25\text{kJ}\text{mo}{\text{l}}^{-1}$. The thermodynamic properties of the compound in the ideal gaseous state were calculated by statistical thermodynamics.

Introduction

Adamantane-based drugs have attracted considerable attention in medicine. Certain of the medications based on 2-substituted adamantanes (kemantan, bromantan, etc.) exhibit characteristics of actoprotectors, adaptogens and even immunostimulants [1]. It was thought earlier [2] that it was complicated to obtain 2-substituted adamantanes because hydrogen attached to a secondary carbon atom in adamantane was much more inert than that of a tertiary carbon atom. However, at present there are a number of fairly selective methods of direct oxidation of adamantane to 2-adamantanone with high yields [2], [3]. That is why 2-adamantanone is widely used in syntheses of some pharmacologically active substances.

Detailed and reliable data on the physical and physicochemical properties of initial substances, intermediates and target compounds are necessary for development of the technology of medicine production. Information on the thermodynamic properties of 2-adamantanone is scanty and, in some cases, ambiguous. For example, Butler et al. [4] investigated the thermal behaviour of the compound by DSC in the range from 120 to 400 K and found only one solid-to-solid transition at 221 K with ${\Delta }_{\text{trs}}{H}_{\text{m}}°=7.93\pm 0.10\text{kJ}\text{mo}{\text{l}}^{-1}$. However, heat capacities in the condensed state as well as the parameters of fusion have not been determined. In addition, some thermochemical properties measured earlier for 2-adamantanone disagree. For example, the values of the sublimation enthalpy, ${\Delta }_{\text{sub}}{H}_{\text{m}}°(298.15\text{K})$, are different: 66.1 ± 1.3 kJ mol⁻¹ (calorimetry) [5], 60.7 ± 0.2 kJ mol⁻¹ (chromatography) [6], 80.3 ± 2.5 kJ mol⁻¹ (from vapour pressures by static method) [7] and 76.1 ± 1.5 kJ mol⁻¹ (the method is not clearly indicated: either by calorimetry or from vapour pressures by Knudsen's method) [8]. Data on the enthalpy of formation, ${\Delta }_{\text{f}}{H}_{\text{m}}°$ (cr, 298.15 K), are also inconsistent: −310.9 ± 3.8 kJ mol⁻¹ [7] and −319.7 ± 1.2 kJ mol⁻¹ [8] (bomb calorimetry in both cases).

Special interest in 2-adamantanone is also connected with the formation of a plastic crystal characterized by positional order but dynamical orientational disorder [9], [10], [11], [12]. Besides the calorimetric investigation by Butler et al. [4] mentioned above, the plastic crystal of the compound has been studied by X-ray diffraction scattering [13], nuclear magnetic resonance (NMR) [14], [15] and incoherent quasi-elastic neutron scattering (IQNS) [16].

A transition from ordered to orientationally disordered crystal is normally accompanied by a substantial increase in entropy of a substance and therefore the melting entropy of plastic crystals is much less (${\Delta }_{\text{fus}}{S}_{\text{m}}°<21\text{J}\text{mo}{\text{l}}^{-1}{\text{K}}^{-1}$). There are also noticeable changes in heat capacity, crystal structure, molar volume, dielectric and optical properties at this transition [10]. Taken together, these suggest potential applications of plastic phases in different devices—heat storage systems, mechanical and optical locks, etc.

The present paper gives the results of an investigation of the thermodynamic properties of 2-adamantanone in the condensed and gaseous states: the heat capacity and the enthalpies of the phase transitions in the condensed state, the sublimation enthalpy, the saturated vapour pressure, and the thermodynamic properties of the compound in the ideal gaseous state. The orientational molecular disorder in its plastic crystal form is considered in terms of the model of the energy states of molecules in plastic crystals described in detail earlier [17], [18], [19].