Vapor Pressures of Isotopic Methanes—Evidence for Hindered Rotation

Abstract

The liquid—vapor separation factors of CH3T and 14CH4 from 12CH4 have been determined over the temperature range 90°—120°K by the column-distillation technique. 14CH4 is less volatile than 12CH4 over the measured temperature range; CH3T shows the cross over at 103.9°K. The reduced partition function ratios derived from the elementary separation factors are lnfc/fg(14CH4/12CH4)=231/T2−1.42/T,lnfc/fg(CH3T/12CH4)=503/T2−4.83/T. The reduced partition functions of all the deuteromethanes and 13CH4 in the liquid and solid are derived from known vapor-pressure data. Comparison of the T−2 dependence of species of equal molecular weight but different moments of inertia, i.e., CH3D and 13CH4, and 14CH4, CH3T, and CH2D2, with model calculations leads to the conclusion that the rotation in methane is hindered both in the liquid and the solid.