Thermodynamic Properties of Trifluorophosphine from 12°K to Its Boiling Point. Entropy from Molecular and Spectroscopic Data

Abstract

The heat capacity of trifluorophosphine has been measured from 12° to 1700K for a sxmple of 99.7 mole % purity. Two solid transitions were observed; one at 83.75°K involving a heat of transition of 59.9 cal/mole and the other at 110.56°K involving a heat of transition of 552.3 cal/mole. The heat of fusion at the triplepoint temperature 121.85°K was 223.9 cal/mole. The heat of vaporization at the normal boiling point 171. 77°K was 3485 cal/mole. The vapor pressure of the liquid to the normal boiling point has been measured, and the data are represented by 10gloPmm = -606.1714/T+2.9857510gloT-0.26322.The density of the liquid is represented by d(g/cc) =2.332-0.004407T.The entropy of trifluorophosphine gas in the standard state at the normal boiling point, 58.48 call (mole OK) , has been calculated from the calorimetric data. This value is in excellent agreement with the value of 58.49 call (mole OK) calculated from the most reliable spectroscopic and molecular data currently available.

A nuclear resonance investigation of molecular motion in solid PF3

“…This large increase, coupled with the decrease in T1 with increasing temperature, is indicative of an increasing translational diffusion rate, in addition to rapid isotropic molecular reorientation. These features are symptomatic of a plastic crystalline state [10] and confirm such an assignment based on the thermodynamic data of Pace and Petrella [4]. An interesting feature of this plastic phase F.I.D.…”

“…For example it has previously been shown to exhibit a low entropy of fusion (7-68 J mole-' K -1) compared with its entropy of transition (20-88 J mole -1 K -x) [4]. In addition, the work reported in this paper on the temperature and frequency dependence of the 19F spin-spin and spin-lattice relaxation rates are clearly indicative of a system undergoing isotropic molecular tumbling plus translational diffusion.…”

“…Although the latter problem could be alleviated by increasing the frequency difference between the measurements, the former requires for its removal a more sophisticated method of T~ measurement than simply observing the F.I.D. 4. CONCLUSI ON Threefold reorientation of the fluorine triangle about its C a axis has proved to be the motional process which dominates both the spectrum and the relaxation measurements in phase III and in phase II of PF a.…”

“…At higher temperatures it was suspected, from the thermodynamic work of Pace and Pctrclla [4], that more substantial motion would take place. T h e basis for this suspicion was their observation of a heat of transition which was twice that of fusion.…”