Knowledge Bank

“The mierowave spectrum of 2-fluoropropene has been studied in order to determine the height of the potential barrier hindering internal rotation of the methyl group. Coupling of internal and overall rotation causes the rotational lines to appear as doublets with strongly barrier dependent separations. About 70 ground torsional state rotational transitions up to $J=36$ have been measured. The barrier is high enough so that doublets could only be resolved for high J transitions $(J=8)$. The doublet splittings were analyzed using the theory of Wilson and co-workers and lead to a barrier, $V(a)=V3(1-\cos 3a)/2$, of $Va=2650\pm 200$ cal/mole. Some 30 rotational transitions for the first excited torsional state have also been measured. The splittings here are roughly 50 times greater than the corresponding ones for the ground state, and they are consistent with a barrier of $2440\pm 25$ cal/mole. The reasons for the apparent discrepancy between the ground and first exeited state barrier determinations will be discussed.”