The enthalpies of vaporization and sublimation of exo- and endo-tetrahydrodicyclopentadienes

The vaporization enthalpies fDglHm (298.15K)} of endo- and exo-tetrahydrodicyclo-pentadiene (THDCPD) have been measured by correlation gas chromatography. Values of ð50:2 2:3ÞkJ mol1 and ð49:1 2:3ÞkJ mol1 have been obtained for the endo and exo isomers, respectively. The sublimation enthalpy of the endo isomer fDgcrHmð298:15KÞ ð51:2 2:4ÞkJ mol1g has been obtained by combining fusion and vaporization enthalpies adjusted as necessary to T 298:15K. Low-temperature d.s.c. studies of both isomers reveal solid-to-solid phase transitions. The endo isomer, which is a plastic crystal at room temperature, exhibits a solid-to-solid phase transition at T 214K fDcrð1Þcrð2ÞHmð214KÞ ð10:7 0:13ÞkJ mol1g and a melting temperature of 356.8KfDlcrð1ÞHmð356:8KÞ ð3:48 0:2ÞkJ mol1gwith some evidence of polymorphism. The exo isomer exhibits a solid phase transition at T 162:1K fDcrð1Þcrð2ÞHmð162:1KÞ ð3:18 0:11ÞkJ mol1g and a melting temperature of 183.2K fDlcrð1ÞHmð183:2KÞ ð1:20 0:04ÞkJ mol1g. Enthalpies of vaporization and sublimation are combined with the respective enthalpies of formation previously reported to yield the following gas-phase values: for endo THDCPD, DfH0m ð61:9 3:2ÞkJ mol1; and for exo-THDCPD