Abstract
The potential barrier which hinders the internal rotation in ethyl iodide molecule was studied. If the potential energy is assumed to be of the sinusoidal form \fracH2(1−cos 3x), the barrier height is determined as H=3.22±0.5 kcal/mole from the Qb-type transition in the first excited torsional state. The torsional frequency is about 230 cm−1. The effect of internal rotation on the quadrupole hyperfine structure of K=1 levels was analyzed.