Knowledge Bank

The microwave spectrum of ethyl methyl ether and deuterated species has been investigated in the region 8 to 30 kMc. For the common isotopic species more than two hundred transitions have been measured. Approximately one hundred of these have been assigned as b-type rotational transitions from the ground state or excited vibrational states of the trans form of ethyl methyl ether. Methyl group $(O-CH3)$ tunnelling causes ground-state transitions to appear as doublets, and analysis of the doublet splittings yields a barrier to internal rotation of 2500 cal/mole. The rotational spectra of five excited vibrational states have been assigned. The most interesting assignments are those for the first, second, and third excited states corresponding to skeletal torsion. Assuming that skeletal torsion and methyl torsion are not coupled, the splittings for these excited states should be identical with ground state splittings. Actually, the observed splittings differ in magnitude and for certain excited states in sign as well. An approximate treatment of coupling of methyl and skeletal torsion reasonably accounts for the observed variations in splitting with torsional state.