The Gaseous Heat Capacity and Restricted Internal Rotation of Diborane

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Abstract
The heat capacity of gaseous diborane has been measured from 95° to 324°K with the low pressure thermal conductivity apparatus described earlier. The bracketing method was applied over most of this temperature range, with acetylene and ethylene as comparison gases. At 95°K, ethylene and ethane were used as comparison gases and the accommodation coefficient ratios were assumed to be unity. The heat capacity at low temperatures indicates that internal rotation in the diborane molecule is restricted by a potential barrier whose height is probably between 4000 and 6000 cal./mole if the barrier is assumed to be a sinusoidal function of the internal rotational angle.