For molecules such as ethyl radical that have a small torsional barrier, the interaction between the internal rotation and the other normal modes of the molecule can have a substantial effect on the magnitude of the barrier itself. In ethyl this is experimentally demonstrated by a large change in the torsional barrier on vibrational excitation of the methylene group inversion motion. A simple method is applied to ethyl, involving the use of electronic structure calculations to estimate both the electronic and vibrational contributions to the barrier. Good agreement is found with experimental results from the infrared spectrum of the rocking vibration of the ethyl radical.