Heat capacity, heats of transitions, fusion, and vaporization, and vapor pressure of octafluorocyclobutane

Th e heat capacity of octafluorocyclobutane o f 99.98,-mole-percent purity was de te rmined in an adiabatic calorimeter from lr to 270 ° J\:.Th e heat of fu sion is 2768 .2± 2.0 abs j mole-1 at the triple-point temperature of 232.96 ± 0.05° K .The three heat-of-vapori• zation experiments at t he vapor press ure of 590.9 mm H g and 261.25°K gave an a,'erage value of 23721 ± 20 abs j mole-I.Four solid-solid transition s \\'ere observed wi th th e heatcapaci ty peaks at 141.3 ± 0.2°, 174.6 ± 0.2°, 214 .84± 0.05°, and 216.99 ± 0.05° K .The anomalous behavior in the heat capacity at about 97 0 K , " ' here t he hea t capacity chan ges its slope abruptly , is attributed to g lass t ransform a tion.Th e vapor press ure \\'a s measu red from 177 0 to 274 0 K .The res ults of this mea surement from the triple-po in t te mperat ure to 274 0 K ca n be represe n ted within ± 0.5 mm H g in mos t cases by the equation 10glOP(mm H g) = 6.70267 1 3 1 ~906 +8 .778482 X 10-3'1'-I.7:3969 1 X I 0-5'1'2.Th e e ntropy of octa(tu oro cyclobu ta ll e in the id eal ga s state at 26 1.25 0 ]\: and 1 atm was compu ted from th e experime ntal data to be 380.52 ± 0.71 abs j deg-1 mole-lor 90.95 ± O.J 7 cal jeg-1 mole-1 ( 1 cal = 4. 1840 abs j) .The entropy compu ted from the spectroscop ic as,ignments bv Claa;;se n a nd the mo lec ul ar co nstan ts based on t he D4h model amounteri to 87.,:3 cal deg-'I mol e-I.