Vapor Pressures of the Methylacetylenes, H3CCCH, H3CCCD, D3CCCH, and D3CCCD

The vapor pressures of the four deutero—protio isomers of methylacetylene of C3v symmetry have been measured over the temperature range 167° to 255°K (∼1 mm C3H4 to ∼1000 mm C3H4) by mercury monometry. Methyl-d3-acetylene shows an inverse vapor-pressure isotope effect which amounts to about 1.6% per deuterium atom at 167°K falling to 1.2% per deuterium atom at 255°K, while methylacetylene-d displays a normal effect of about 2.0% at 167°K which falls to an inverse effect of 0.2% at 255°K. The crossover temperature is near 226°K. The effect observed for D3CCCD is slightly less than the sum of the effects of D3CCCH and H3CCCD. The data are interpreted in the light of the statistical theory of isotope effects in condensed systems. A model calculation is made in the framework of this theory in the harmonic-cell approximation using reasonable force fields. It is shown, as in the previous study on ethane, that anharmonicities in the lattice frequencies must be postulated in order to rationalize the vapor-pressure effects observed for the deutero methyl compounds. The vapor pressure of the deutero methynic compound appears unusual and it will be shown that the data indicate considerable molecular association in the condensed phase, and that the amount of this association is temperature dependent. This last point is in agreement with the results of an independent NMR investigation and with spectroscopic studies.