The Entropy of Dimethyl Acetylene from Low Temperature Calorimetric Measurements. Free Rotation in the Dimethyl Acetylene Molecule

The separation of the methyl groups in the dimethyl acetylene molecule, H3C-C≡C-C-CH3, is much greater than it is in ethane, and if the potential barrier [1] of about 3000 cal./mole restricting internal rotation in the latter is due to interactions between the methyl groups, then this barrier should be much smaller in dimethyl acetylene. If, on the other hand, the restricting potential in ethane is largely due to resonance with double bonded structures as proposed by Gorin, Walter, and Eyring [2], that fact that the length of the C-C single bonds in dimethyl acetylene [3] is such as to indicate considerable double bond character might lead one to expect a barrier about as large as in ethane. In order to determine the magnitude of this barrier we have calculated the entropy of dimethyl acetylene from low temperature calorimetric measurements and have compared this experimental value with that computed from molecular data.