On the Phase Transition of Hexamethyldisilane

Measurements of the heat capacity (200∼300°K) and the vapor pressure (274∼310°K) were carried out covering the transition and melting points of hexamethyldisilane. The following results were obtained; Ttr 221.8°K, ΔH(transition) 2330 cal./mol., ΔS(transition) 10.5 e. u.; Tf 287.72°K, ΔH(fusion) 721 cal./mol, ΔS(fusion) 2.51 e. u.; vapor pressure equations, & logP(mmHg)=-2105.4/T+8.5503 & (274.5∼286.6°K) & logP(mmHg)=-1953.1/T+8.0105 & (287.6∼310.4°K); ΔH sub 9640±25 cal./mol., ΔSsub 26.0±0.17 e.u., ΔG°298.16(sub.) 1900±76 cal./mol.; ΔHvap 8943±16 cal./mol., ΔSvap 23.5±0.05 e. u., ΔG°298.16(vap.) 1940±31 cal./mol. These data showed that the present crystal belongs to the class of plastic crystal. We have discussed the qualitative nature of the phase transition in the present crystal from the point of view of entropy, referring to the X-ray and the nuclear magnetic resonance studies. The remarkable “vapor snake” phenomenon was also observed on this material.