The molecular structures of ethane, ethene, and ethyne

The term hydrocarbon covers compounds formed from hydrogen and carbon atoms only. In addition to ethane, there are two more hydrocarbons containing of two carbon atoms, C2H4 (ethene) and C2H2 (ethyne). The molecular structures of these compounds are shown in Fig. 14.1. 

The coordination geometries of the carbon atoms in ethane are distorted tetrahedral, and the molecule has a threefold symmetry axis along the C-C bond. In the equilibrium structure the relative orientation of the two methyl groups is such that distances between C-H bond vectors is at a maximum as indicated in Fig. 14.1. This conformation is referred 

A careful study of the infrared absorption spectrum of the tri-deuterated molecule CH3CD3 has shown that the potential energy of the molecule as a function of a dihedral angle / (h1 CCH ) to a high degree of accuracy is given by 

At room temperature the energy associated with internal rotation is equal to RT = 2.4 kJ mol i and most of the molecules in the gas will be restricted to small twisting oscillations about one of the three minima. But a certain fraction of the molecules will have sufficient energy for the methyl groups to pass over the barrier and continue the internal rotation without change of direction. The number of such molecules will, of course, increase with increasing temperature. 

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