Knowledge Bank

A previous $investigation1$ of the ground-state microwave spectrum of propane yielded a lower limit of 2700 cal/mol for the barrier to internal rotation. We have examined the spectrum of the first excited states of the two torsional vibrations. These transitions are split by internal rotation and appear as symmetrical triplets. Relative intensities of the triplet components were used to assign the observed satellite lines to the vibrational state of proper symmetry. Using a single-top approximation, the barrier to internal rotation has been calculated to be 3.55 kcal/mol.